- Email: [email protected]
Bubble detectors—A maturing technology
Radiation Measurements, Vol. 27, No. 1, pp. 1-1 I. 1997 © 1997 ElsevierScienceLtd. All fights reserved Printed in Great Britain P I h S1350-4487(96)00156-4 1350-4487/97 $17.00+ 0.00
Pergamon
INVITED PAPER BUBBLE DETECTORS--A M A T U R I N G TECHNOLOGY H. ING, R. A. NOULTY and T. D. McLEAN Bubble Technology Industries Inc., Highway 17, Chalk River, Ontario, Canada K0J 1JO (Received 29 September 1996; accepted 25 November 1996)
A~traet--Since the development of the first bubble detector over 10 yr ago, there has been continuing expectation that it would solve the well-known problems of personal neutron dosimetry. Research in the intervening years has led to a much better understanding of this interesting technology and to the development of a variety of radiation detectors that have found diverse applications in radiation physics. In recent years, a bubble detector has been developed for personal neutron dosimetry that is increasingly being adopted by many groups world-wide. Although this dosimeter has improved significantly the status of neutron dosimetry, there is continuing research to improve the properties of bubble detectors, not only in this application, but for general radiation detection. © 1997 Elsevier Science Ltd
I. BACKGROUND The development of the bubble detector for radiation detection was driven by the need in the nuclear industry for a personal neutron dosimeter that would meet the requirements for safe radiation monitoring of personnel. Historically (Cheka, 1954), personal neutron dosimetry has relied heavily on the use of nuclear emulsions, such as Nuclear Film Type A (NTA) by Kodak. Although the inadequacies of NTA film for such an application were well recognized (see for instance, Hofert and Piesch, 1985), film continued to be used by certain groups even to the present day. The main reason for its popularity is the lack of a completely satisfactory alternative. Attempts to improve personal neutron dosimetry led the U.S. Department of Energy (DOE) to implement a formal program with the goal of finding a suitable alternative to NTA in 1969 (see for instance, Vallario, 1971). The results of this program are summarized in a series of DOE workshops spanning a period of almost 20 yr (see for instance, Vallario, 1984). In retrospect, this program can be identified with the accelerated development mainly of two new dosimeters: the damage track detector and the albedo neutron dosimeter. Both of these dosimeters have certain technical advantages over NTA. However, neither is able to completely meet the requirements of personal neutron dosimetry (see for instance, Luszik-Bhadra et al., 1993; Hankins et al., 1989). Although the bubble detector is not a direct product of the DOE program, the world-wide scientific interest stimulated by the program had some influence on its development (Cross and Ing,
1984). One of the technical limitations of the damage track detector identified in the early 1980s was the relatively high energy threshold for fast neutrons, which was approximately 1 MeV. This made the damage track detector insensitive to a large fraction of the lower energy neutrons encountered in power reactor environments. Much effort by groups world-wide was spent in trying to solve this limitation. Subsequently, the identification of a very sensitive plastic---called Columbia resin 39 or CR-39--along with improvements in track etching techniques (see for instance, Tommasino, 1981) led to some improvements in the lowering of the energy threshold. In the search for more sensitive plastic for damage track detectors, one of the authors (H.I.) began to experiment with the feasibility of pre-loading plastic with sources of stored energy in order to amplify the effect of neutron interactions in the medium. Among several approaches that were examined, the concept of stored mechanical energy in the form of superheated liquid dispersed throughout the medium appeared most promising. Since the work of Glaser (1952) on the bubble chamber, many studies have been done on the response of superheated liquid to ionizing radiation (see for instance, Kahn and Peacock, 1963; West and Howlett, 1968). Of particular interest were studies on the response of droplets of superheated liquid to radiation (Skripov, 1974; Bell et ai., 1974; Apfel, 1979). Most of these studies had the droplets suspended in a liquid or a viscous medium. The focus of the bubble detector development was to have microscopic droplets of superheated liquid dispersed throughout a plastic medium in order to
2
H. ING et al.
enhance its response. The first attempts to make detectors in a plastic medium utilized Freon-12 as the superheated liquid and a water-based crosslinked polymer (polyacrylamide) as the plastic medium. Initial tests on the proof-of-concept prototype showed that such detectors had an extremely high detection efficiency relative to any existing passive neutron detector. The reporting (Ing and Birnboim, 1984) of these preliminary results generated enormous optimism throughout the international dosimetric community, not only because of the high detection sensitivity of the novel bubble detector, but also because of its low neutron energy threshold. 2. OVERVIEW OF BUBBLE DETECTOR
Over a decade of research has since been carried out on this novel approach to detect radiation. This work, of course, has led to a much better understanding of the physics of the bubble detector and has resulted in a variety of products for various applications in radiation physics. One of the most advanced products which is often identified with the term "bubble detector" is the BD PND (Bubble Detector Personal Neutron Dosimeter), shown in Fig. 1. This dosimeter consists of 104-105 droplets of superheated liquid ( ~ 20/2m dia) dispersed throughout 8 cm 3 of clear, elastic polymer. This dosimeter has built-in compensation for temperature effects on the response of bubble
detectors and is used for personal neutron dosimetry by many groups world-wide. The BD PND is activated by unscrewing the cap on the top of the dosimeter. Neutrons striking the loaded plastic produce small visible bubbles which appear instantly in the dosimeter. The bubbles are fixed in the elastic medium and can be subsequently counted, visually or by use of automatic readers employing image analysis techniques. The bubbles can be recompressed by screwing the cap assembly back on the dosimeter. By varying the formulation of the detector liquid and the polymeric medium, the radiation detection properties of the bubble detector can be varied to meet different requirements. Thus, a range of bubble detectors can and have been produced over the years. There is considerable confusion over the term "bubble detector" and other radiation detectors based on superheated liquids such as the superheated drop detector or SDD (Apfel, 1979). While the radiation detection physics is common to all detectors using superheated liquid (including the bubble chamber), there are major differences in design and operational properties between the bubble detector and others. In fact, the term bubble detector should be restricted to detectors which use superheated droplets but where the bubbles are trapped in a firm polymer at the sites of formation. This requirement carries with it a set of unique problems a.nd advantages, one of the latter being the reusability of
Fig. 1. Bubble Detector Personal Neutron Dosimeter before (on the right) and after irradiation (on the left).
BUBBLE DETECTORS P©
--???_ ? 27 - - - - - - - - - - - . . . . . . . . . . .
.
- - - - - - - - - - - - . - - - - - - - - - - - - . . . . . . . . . . . . . . . . . . . . . . . .
/ Vapour bubble Fig. 2. Schematic diagram showing a vapour bubble of radius r~ in a liquid medium. External pressure Pe plus pressure due to bubble surface tension tend to crush the bubble while the pressure Pi of the vapour associated with the liquid acts to expand the bubble. the detector by merely recompressing the bubbles back into droplets. Much research on various aspects of the bubble detector by various groups has been made over the last decade. This work has established (Portal and Dietze, 1992) that the bubble detector is the only personal neutron dosimeter which has adequate sensitivity to meet the implications of the ICRP 60 (ICRP, 1991) recommendations for neutron dosimetry. The work has also spawned a new field of activity, often included as a sub-discipline in international scientific meetings (see for example, Goldfinch et al., 1993) and an increasing number of open literature publications. 3. THEORETICAL ASPECTS A theory to account for the behaviour of the bubble detector must encompass classical cavitation theory in liquids, the physics of metastable states in liquid-gas systems and the effect of radiation in matter. All these disciplines have been extensively studied in the past (Skripov, 1974), although a complete understanding of the physical processes involved is lacking. When a liquid continues to exist in the liquid state above its normal boiling point, it is said to be superheated. Theoretically, boiling or nucleation can be retarded until the temperature of the liquid reaches its so-called superheat limit. The maximum attainable superheat, at atmospheric pressure, can be predicted on thermodynamic and kinetic grounds to be
approximately 90% of the liquid's critical temperature (Eberhart et al., 1975). Depending on the degree of superheat, the phase transition when it does occur can be explosive in nature. Generally, boiling in liquids is the result of "heterogeneous nucleation" where impurities in the liquid or liquid/solid interfaces facilitate the phase transition. But if the liquid is surrounded by a second immiscible phase with which it has a zero contact angle then normal boiling is suppressed and the liquid can be heated as high as its superheat limit at which point, it undergoes "homogeneous nucleation". A superheated medium to detect ionizing radiation was first used by Glaser (Glaser, 1952). The so-called bubble chamber consisted of a bulk superheated liquid (e.g. propane). When sufficiently energetic particles entered the chamber, tracks could be observed as a trail of vapour bubbles. It was Seitz (Seitz, 1958) who first proposed a reasonable mechanism by which nucleation takes place in superheated liquids. Known as the "thermal spike" theory, it is still widely accepted today. This theory postulates that the ionizing radiation produces highly localized hot regions or "temperature spikes" within the liquid which literally explodes into bubbles through the evaporation of the superheated liquid. The physical processes responsible for the production of bubbles are viewed to be similar to those responsible for producing radiation damage in solids. In order to determine quantitatively the relationship between energy deposition and bubble,
4
H. ING et al.
formation, Seitz postulated that the formation of a visible bubble involved two critical steps: the formation of a critical-size vapour bubble and the growth of this bubble to a visible or macroscopic bubble. To calculate the size of the critical bubble, he assumed that classical macroscopic concepts of pressure and surface tension were applicable down to the dimensions of a critical bubble and considered a static vapour bubble in a superheated liquid medium under external pressure po (see Fig. 2). The pressure Pe along with the surface tension of the bubble tend to compress the bubble. The pressure p~ acting on the inner surface of the bubble is associated with the vapour of the superheated liquid. Since the effective pressure from surface tension of the bubble is 2a/r, the equilibrium condition for a critical-size bubble of radius r~ implies that the pressures acting inward are balanced by the outward pressure: Pe
2or +
-rc
=
P,
(1)
where ¢r is the surface tension of the liquid. Rearrangement of equation (1) yields rc
2a = ~pp
(2)
where Ap = p~ - p, and is a measure of the degree of superheat and is defined as the pressure difference between the equilibrium vapour pressure of the liquid and the externally applied pressure (generally 1 atmosphere). Typically, the value of rc is a few tens ofnanometers. In comparison, droplet radii in bubble detectors are on the order of 10 microns. It can be shown (Gibbs, 1961; Blander and Katz, 1975) that the existence of a vapour bubble of radius re represents a maximum in the free energy potential of the system and is therefore unstable to slight perturbation. Vapour cavities of smaller radii will collapse under the effects of surface tension. On the other hand if rc is exceeded then the cavity will spontaneously expand to form a macroscopic-sized bubble. The validity of the concept of ro has since been demonstrated by many workers (see for instance, Ward et al., 1970). Seitz (1958) showed, using classical thermodynamic arguments under the assumption of reversibility, that the minimum amount of energy required to form a vapour cavity of radius rc is approximated by: Emi, = 41ttrr 2 + 4 / 3 n r ~ p v H / M
(3)
or in terms of equation (1): E , in = 16na3/(Ap)2[1 + 2/3p~H/(MAp)]
where P, is the density of the vapour while H and M represent the molar heat of vaporization and molecular weight respectively. The first part of equation (3) represents the energy required to form a bubble of radius r, against the forces of surface tension while the second term gives the energy required to vapourize the liquid to produce the
bubble. With increasing temperature, the surface energy, vapour density and heat of vapourization will decrease while, of course, Ap will increase. The overall effect is to lower the value of re and the minimum energy required to form the initial critical bubble. Seitz postulated that the energy required to nucleate the superheated liquid was the result of energetic recoil ions stopping in the liquid. A portion of the energy loss is degraded to thermal energy allowing localized vapour formation. Nucleation occurs if the local value of re is exceeded or, stated equivalently, the local superheat limit is surpassed. Although Seitz's theory was developed for the bubble chamber, it also applies to bubble detectors as each superheated droplet is, in effect, an isolated bubble chamber. Later investigators (EI-Nagdy and Harris, 1971; Norman and Spiegler, 1963), building on Seitz's work, postulated that the effective length (L) over which a particle must deposit sufficient energy to cause nucleation must be linearly related to the value of re. If L is assumed to be much shorter than the total track length of the particle then the energy deposited is given by: E = L(dE/dx)avr = krc(dE/dx)aw
(4)
where (dE/dX)avr is the average stopping power over the interaction length L and k is a proportional constant. Widely varying estimates (from 2-13) for k have been given in the literature (Bell et al., 1974; E1-Nagdy and Harris, 1971; Norman and Spiegler, 1963). Recently, Harper (Harper, 1991) has proposed that the parameter k should be expressed in terms of the effective radius (r0) of the liquid volume evaporated to form a critically sized vapour bubble. Equating equations (3) and (4), it is clear that at a given temperature there is a threshold value of (dE/ dX)a~r below which nucleation is not possible. This observation forms the basis for using several bubble detectors with unique thresholds as a crude neutron spectrometer, as will be discussed in the following section. Attempts have been made to model the energy and temperature response of superheated droplets with respect to neutron irradiation (Harper, 1991; Lo and Apfel, 1988; Lim and Wang, 1993). Detector response (R) for monoenergetic neutrons can be described in terms of the incident energy (E,) and temperature (T) as follows: n
m
R ( E . , T ) = ()(E.)V£.N,~,o,jFo(E,,T)
(5)
where ~b(E.) is the neutron flux and V is the total volume of superheated droplets. The number of isotopes in the superheated material is given by n and Ni represents the atomic density of each isotope. The number of considered reaction cross-sections is given by m and o Uis the relevant cross-section for the i, j interaction. The relevant nuclear reactions which are usually taken into account include elastic and
BUBBLE DETECTORS inelastic scattering as well as charged particle production, e.g. (n,p). F~j is a factor which describes the probability of the i, j interaction leading to nucleation. In the case of elastic scattering, the F U factor is often calculated in the following manner. First, energy distributions for all the potential product ions are calculated as a function of incident monoenergetic neutron energy. Then for each ion, the stopping power in the droplet material is calculated as a function of energy. The fraction of recoil ions with stopping powers greater than the minimum average value necessary to cause nucleation [equation (3)] gives directly the probability of nucleation for each combination of recoil ion and incident neutron energy. This approach of modeling detector response has successfully predicted threshold neutron energies for a number of superheated liquids (Apfel et al., 1985). It has shown that for fast neutrons the predominant mechanism for nucleation is elastic scattering of recoil ions. These calculations have also clearly demonstrated (Apfel et al., 1985) that the C135(n,p)Sa5 reaction is responsible for the thermal neutron sensitivity of certain detectors (other than bubble detectors) which also use superheated liquid drops. In the case of continuous neutron sources, detector response is calculated by integrating equation (5) with respect to neutron energy or by summing over adjacent energy intervals. Harper (1991) has had reasonable success in predicting detector response to bare and moderated Cf sources. He found that the best fit to experimental data was obtained with k = 4.3, i.e. the effective pathlength of energy transfer is 4.3r0. Taking into account the relative liquid and gas densities of typical detector liquids, this implies L ~ 2re. It is helpful to summarize qualitatively the various physical processes leading to the formation of visible bubbles from exposure of the bubble detector to neutrons. Out of the neutrons which traverse the bubble detector, a certain fraction of them will interact with the detector. The probability of interaction is determined by the specific composition of the medium in the detector. Some of these interactions will occur "far" from the sites of the superheated liquid droplets and others will occur "close" to these sites and in fact, a small fraction will interact with the superheated liquid droplets themselves. These interactions give rise to a variety of secondary charged particles (including recoil ions) in terms of energy and charge. These charged particles will slow down in the medium in accordance with the stopping power of the specific ions in the immediate environment of the interaction site. The energy dissipation during this process produces the "thermal spike" as envisaged by Seitz. Many of the secondary charged particles may not interact with a superheated droplet ( ~ 20 #m dia) and will therefore be "wasted". Some of the secondary charged particles will be intercepted by a
5
droplet and, depending on their energy, will be stopped by the droplet or will be forced by the droplet to lose some of their energy. The passage of the particle within the superheated droplet will give rise to a trail of microscopic bubbles. Some of these may coalesce to form somewhat bigger bubbles. If any one of these bubbles exceed the size of the critical bubble [equation (2)], the bubble will grow, if none of these is as large as the critical bubble, they will be recompressed by the forces of surface tension and no evidence of their formation will be apparent. The minimum amount of energy for the formation of a single critical bubble is given by equation (3). Simplistically speaking, this energy must be deposited over a distance less than approximately 100 nm within the superheated liquid droplet, although the energy deposition process is probably much more complicated including other energy contributing processes such as through thermal conductivity, high-energy delta rays, acoustic propagation, etc. This energy deposition process is basic to the function of the bubble detector and occurs over a time period of < 10-,3 s. Once a bubble greater than the critical bubble is formed, the remaining liquid in the superheated droplet vapourizes into the bubble forcing it to grow very quickly (in the order of microseconds). This vapourization of the droplet emits a characteristic (typical of nucleation "crackling") acoustic signal which can be used for the identification of bubble formation. It is interesting to note that the size of the resulting visible bubble is determined by the size of the droplet within which the energy deposition occurs. The physics of the neutron interaction process have already all taken place before the formation of the visible bubble and so the size of the visible bubble is not related to the property of the incident neutron. In the bubble detector, the visible bubbles formed by neutron interactions remain fixed at the initial droplet sites. After exposure to radiation, the number of these bubbles can be used to provide a measure of the neutron field. There is a slow growth (over periods of many days) of the visible bubbles after their formation. This is associated with migration of vapour molecules from the polymer medium into the bubbles. Thus, it is recommended that the bubbles be recompressed as soon as convenient if one wishes to prevent bubbles from growing to a size large enough to damage the polymer medium. Once a site is damaged by a large bubble, the site can no longer be reused because the vapour bubble can no longer be recompressed into a superheated droplet.
4. APPLICATIONS OF BUBBLE DETECTORS
4.1. Neutron dosimetry Many groups worldwide are using bubble detectors for routine monitoring of personal neutron doses. Monitoring is especially important in neutron
H. ING et al. I
I
I
I I II
'
'
'
' ''"I
'
'
'
' ''"I
'
I
I--
(Detector sensitivity = !.0 bubblelmrem)
'7 .o 10-5 da e~ .-s d~
d~ v
to L) z to .l ///$
to eL
N
m eL 2.0 m o~ Z O 1.0 e L to
(lcm DEPTHDOSE EQUIV:LENT)
10-6
z 0
eL
0.5
to
0.2 10-7
I
I
I
I I II
I
I
I
I I IIII
0.1
I
I
l
I I IIII
1.0
I
I
t
0.1
10
NEUTRON ENERGY (MeV) Fig. 3. Bubble Detector Personal Neutron Dosimeter normalized response per unit fluence (O) and normalized response per unit dose equivalent (0). Conversion from fluence to dose equivalent based on NCRP Report No. 38. "streaming" through shielding where fields may be high and localized. One version of the bubble detector the BD PND, is most popular for such applications and has been extensively tested by several groups (Hoffman et al., 1992; Chemtob et al., 1995; Shannon, 1996) in terms of energy dependence, temperature dependence, repeated use, reliability, reproducibility, dynamic range, etc. Figure 3 shows the response curve with neutron energy for the BD PND. This detector has an approximate 100 keV threshold with a reasonably flat tissue equivalent dose response from about 200 keV to greater than 15 MeV. These detectors have extremely high neutron sensitivity (at least an order of magnitude more sensitive than any other passive device), are isotropic in response, provide an immediate visual dose record and are completely insensitive to gamma radiation. These detectors incorporate an integral recompression ability (a pressure cap) allowing the bubbles to be recompressed in situ and essentially reused indefinitely. A technical limitation of the bubble detector that was identified early in its development was solved with the introduction of the BD PND. Figure 4 compares the response of a bubble detector without compensation and with temperature compensation. The use of temperature compensation allows for constant sensitivity over the temperature range 15--40°C. Temperature compensation is accomplished by utilizing the expansion properties of a special
proprietary material. This material is placed in a small chamber located just above the detector and sealed with a thin rubber diaphragm. The expansion of this material with temperature exerts appropriate pressure on the detector to compensate for the increase in superheat. Recently, a new personal bubble detector, the BDT (Bubble Detector Thermal), has been introduced for monitoring of thermal neutrons. This detector uses a 6Li compound dispersed throughout the polymer
3.0
-/0
~2.5 O eL Z0
e-
m
S < 1.o z
0.5 ~
0.0 I0
•
I
I
I
I
I
I
15
20
25
30
35
40
TEMPERATURE (C) Fig. 4. Bubble Detector Personal Neutron Dosimeter ( I ) normalized response to A m - B e vs temperature. For comparison, uncompensated neutron bubble detector ( O ) is shown.
BUBBLE DETECTORS I I t rrit I
~
t
i i lilt
I
t
I
I
f. . . .III
II /,"
,
(Detector sensitivity = 1.0 bubble/mrem)
•
.
!
E
.=
/
d~ dD
i I.
' BDS-1001
/
10 -5
I
7-'-~ / /.~"
/
i
/
Ct~ Z ©
t
r¢ 10-6
,.d <
!
/
/
t BDS-100
I
/
BDS-10,000
~E
}DS-2500/10
BDS-600
0~ ©
I
z
i I I 10-7 0.01
t
I
t
t
t JJtl
I
1
I
I I IIII
0.1
I
I
t
~ t ttt[
10
1.0 N E U T R O N E N E R G Y (MeV)
Fig. 5. Normalized response per unit fluence for Bubble Detector Spectrometer: 10 keV threshold (A); 100 keV threshold (rq); 600 keV threshold ( 0 ) ; 1000 keV threshold (A); 2500 keV threshold ( i ) ; 10,000 keV threshold (0)-
medium
and
a
special
formulation
to
detect
preferentially ~ particles from the 6Li(n,~)T reaction. The thermal to fast neutron response is approximately 10:1. A combination type dosimeter using both
the B D P N D
and
the B D T
is n o w
being
implemented in some facilities to measure both thermal and fast neutron doses simultaneously.
that the LET associated with electrons cannot provide sufficient energy to create bubbles larger than the critical bubble and thus, even intense gamma fields have no effect on the detector response. Thus, it can measure a weak neutron field in the presence of a strong gamma background which is often the case in many radiation environments. Figure 6 shows
4.2. Spectrometry
By varying the formulation, bubble detectors having different neutron energy thresholds have been m a d e (Buckner et al., 1994). Figure 5 s h o w s the response functions of a set of neutron bubble detectors having thresholds of 10, 100, 600, 1000, 2500 a n d 10,000 keV. T h i s set w a s m a d e for crude
neutron spectroscopy, to give some indication of the energies o f the n e u t r o n s that m a y be e n c o u n t e r e d in
certain radiation environments. This knowledge of the general spectral distribution assists the health physicist in the proper choice of dosimeter to be worn by personnel exposed to these fields. The bubble detector spectrometer or the "BDS" has been used by various groups (see for instance, R o s e n s t o c k et al., 1995) for c h a r a c t e r i z a t i o n o f neutron fields in nuclear utilities, handling of fissile materials and accelerator laboratories. One particular advantage of the BDS over many other spectrometers is its complete insensitivity to gamma rays. All the components of the spectrometer are formulated such
~ , 7.00E+05 -i t~ 6.00E+05
m
¢-i"
~E 5.00E+05 --
m 4.00E+05 L) Z 3.00E+05 ,.d tL 2.00E+05 Z O 1.00E+05 m Z
- _
.......
EXPT. -THEORy
] .......
!
O.00E+00
10
100
1000
i__j 10,000
I tllnl
100,000
N E U T R O N E N E R G Y (keV)
Fig. 6. Neutron energy spectrum of a bare Cf m source measured with the Bubble Detector Spectrometer ( ) For comparison, a theoretical Ctn52 spectrum ( - - - ) is shown.
H. ING et al. 1.8 m 1.6 1.4 1,2 1.0 0.8 0.6 0.4
BION-9
--
-------
Doserate = 8.0 m r e m / d 156%!
I
0.2 - - , 0 2.5 - -
nli
,,.,1
I I t IIIIII
I I I III IlL
'
I. I Illlll
BION-10 Doserate = 14.0 m r e m / d
2.0
I I
55%1
1.51.0 - ~0,5
[
0 4.0
IIIIll
I
I I IIII11
I
I I Jill
llllll
MIR
-
D o s e r a t e = 9.2 mrern/d I 3.0 - -
I 151%I
I
2.0 t 1.0 I 0 0.01
'
ililll
II I
0.1
I i liliil
i
,: i i llliil
1.0
I
l0
i
lilill
100
En (MeV) Fig. 7. Neutron spectra measured in space for Russian BIOCOSMOS (BION) #9 and #10 and Russian space station MIR using bubble detectors. a measurement of a bare Cf 252source using the BDS. There is good agreement with the theoretical spectrum.
high energy protons in the South Atlantic Anomaly striking the aluminum shell of the satellite and the station and producing neutrons via the (p,n) reaction. Recently, bubble detectors were flown aboard the space shuttle (STS-78) as part of an educational experiment. The bubble detectors were activated by an astronaut and were viewed with a video camera as the shuttle orbited the earth. Visual observation by the astronaut and the video recording showed that very few bubbles formed during much of the orbit but many formed when the shuttle crossed the South Atlantic Anomaly. This seems to confirm our original hypothesis. Further experiments aboard the space shuttle and MIR are being planned. Bubble detectors have also been popular in the monitoring of commercial and military aviation crews (Spumy et al., 1993; Lewis et al., 1994; Tume et al., 1996). In these experiments, several bubble detectors were activated and often carried in the hand-luggage (or pocket) of the passenger or crew member. After the trip, the detectors were read-usually by eye--to determine the transit neutron dose. Often, TLDs are also carried in many of these studies to yield the gamma dose. For altitudes used by aviation, the neutron dose is an important component of the total radiation exposure. Studies are still on going with air crews being monitored by bubble detectors in Canada, Europe, Japan and Russia. Studies involving high altitude flights using bubble detectors (NASA's ER-2 program) are expected to commence shortly. 4.4. Other applications
4.3. Space and high altitude research An interesting application of the BDS was the measurement of the neutron spectrum in space. Such a measurement had been difficult to perform because of the high background of charged particles-especially high-energy protons. The protons create extraneous signals in most other detectors, but the neutron bubble detectors had been shown to be quite insensitive to high-energy protons in experiments using the Orsay cyclotron. The importance of knowing the neutron spectrum in space had been discussed earlier by Benton and Parnell (1988). Three separate measurements of the neutron spectrum in space have been made (Ing et al., 1993, 1994a; Ing and Mortimer, 1994). Figure 7 shows the measured spectrums in the Russian satellites BIOCOSMOS (BION) #9 and BIOCOSMOS (BION) #10 and the Russian space station MIR. Despite the different orbital parameters and times, the neutron levels were quite consistent--in the order of 100/~Sv per day. There appeared to be a large number of neutrons above 10 MeV, which were originally unexpected by some theorists and neutrons around a few MeV which were expected because they were the "evaporation neutrons". It was postulated that these high energy neutrons were produced by
Bubble detectors have been made in a variety of sizes and formulated for various applications. Detectors have been made for very special applications such as anthropomorphic phantom work (Cousins et al., 1990a). Bubble detectors have a particular advantage for measurements inside a phantom because they have isotropic response. Since neutron scattering inside the body produce multidirectional fields, the use of other detectors such as CR-39 gives readings which cannot be interpreted because of the unknown directional distributions impinging the detector. Furthermore, the high sensitivity of bubble detectors and ease of reading make such experiments much simpler to perform. Studies have been made using bubble detectors inside and on the surface of phantoms to determine the relationship between badge reading and dose to the gut and to determine the best position to wear dosimeters. This knowledge is especially important, e.g. for dosimetry on-board nuclear submarines where biological shielding is limited by space and weight considerations. Special bubble detectors are used extensively by NATO scientists and the success of the detector has been recognized by its use as the standard dosimeter for free field and anthropomorphic phantom work.
BUBBLE DETECTORS Detectors have been made as small as 5/8" in length for applications where the size of the conventional bubble detectors was not appropriate. Not only were these detectors small, but they were also able to detect up to 10 Sv. Detectors of this type were used by military scientists working with intense neutron fluences and energy spectra associated with weapon simulators employed in electronic irradiations (Cousins et al., 1990b). A version of these small detectors (with much higher sensitivities), called the PM 100R (Personal Monitor 100 keV Reuseable), are used by various groups to monitor finger tip doses in fuel processing plants not unlike finger tip TLDs. Detectors have been made as big as 6" in dia and 1/2" thick. These detectors were used in work involving identification of radioactive "hot spots" in soil samples (Zeissler, 1992). Here various soil samples were placed on top of the slab bubble detector for a period of time. Only contaminated samples produced bubbles in the detector. Another group has examined the feasibility of incorporating a small amount of the waste itself into the bubble detector before manufacture. The radiation level of the waste could then be internally measured by storing the "spiked" detector in a low background environment and comparing the bubble formation to a non-doped sample. Bubble detectors have also been formulated for the detection of radon. The formulation of these detectors took into consideration the superheat conditions for detection of or-particles combined with a polymer medium which facilitated gas migration. These detectors had no difficulty detecting a few pCi/1 over a period of a few days. One could actually see the bubble front move down the detector slowly with time as the radon diffused into the detector medium. Medical related research has used bubble detectors for mapping of the neutron field in newly-installed medical linacs (Nelson and Gordon, 1993) and other neutron sources used in medical treatments. Neutrons are produced by the (~,n) reaction in medical linacs and since the BD PND is insensitive to gammas, it is ideally suited for neutron measurements in the intense gamma field. Other past applications of bubble detector include the search for "cold fusion" (Sawicki, 1993), fast neutron radiology (Ilic et al., 1989) and non-ionizing radiation detection/protection [laser and microwave (Olsen, 1990)].
9
tissue equivalent and have a response fairly constant down to a few keV. We have also successfully applied temperature compensation to the gamma detector similar to that done for the neutron detector as shown in Fig. 8 . One technical appeal of the gamma bubble detector is its extremely high sensitivity. Sensitivities as high as 1000 bubbles per /zSv are easily obtained and this allows for measurements of very low fields using a passive device. We have also started to make electronic dosimeters that use the bubble detector as the active element. Two devices are presently being manufactured: RAISA (Radiation Assessment In Space Applications) for use in aircraft or the space shuttle cockpit as an area monitor and Mini-RAISA, a miniaturized version, for use by astronauts during EVA operations. RAISA uses two bubble detectors operated under microprocessor control. At any time, only one of the detectors is used as the radiation sensor. When this detector has accumulated a fixed number of bubbles (or after being activated for a pre-determined time period), the microprocessor switches the radiation sensor to the second detector and instructs a tiny motor to recompress the bubbles of the first detector. The counting of the bubbles is done electronically as the bubbles are formed. By switching the sensor back and forth between the two detectors, RAISA has an enormous dose and dose-rate range and can operate, in principle, continuously for many years without intervention. An interesting potential application of bubble detectors pertains to the detection of "dark matter" as proposed by Zacek (Zacek, 1994). Interest in dark matter results from current models of the evolution of the universe which predicts the existence of a significant mass of non-luminous, non-baryonic matter made up of relativistic light particles and non-relativistic, massive particles. The latter are known as Cold Dark Matter and their properties in terms of interactions (which occurs infrequently) with 7 /
~ 6
--
D
/ /
--
/
r~
/
4
_
,~
[ooompoos.,o"I ,'
s3
/ /
.1
5. CONTINUING DEVELOPMENTS Over the last few years, research has been continuing in our laboratory to fabricate gammasensitive bubble detectors (BD GAMMA). We had not achieved a completely satisfactory gamma detector based on the bubble detector because of imperfect energy and temperature dependence. However, there is continued progress and we have recently made bubble gamma detectors that are more
-
15
~ tf 20
I 2.5
Ic°mpe"sated I I I 30
35
I
40
TEMPERATURE (C) Fig. 8. Temperature compensated gamma bubble detectors (0) normalized response to Csm v s temperature. For comparison, uncompensated gamma bubble detector (I-1) is shown.
H. I N G et al.
10
ordinary matter have been predicted by theory. One particular interaction property which has been recognized as a good indicator of dark matter is to measure the recoil energy of an atomic nucleus which has been struck by dark matter. By appropriate formulation of the bubble detector, it is possible to limit the response essentially solely to dark matter, which makes the detector very attractive for such an application. On the theoretical side, there is continuing research (Ing et al., 1994b, 1996) to assess the capability of the bubble detector to mimic the effect of radiation on biological systems-especially for space radiation. Currently, there is growing recognition (Bond et al., 1996) that the concept of "dose equivalent" may be inadequate as an indication of biological detriment due to radiation. As indicated earlier, the bubble detector provides a method of measuring energy deposition over distances of tens of nanometers and is therefore, a nanodosimeter. If energy deposition over such distances is truly the physical process which causes D N A damage, the response of the bubble detector may provide a better indicator of biological response than the concept of dose equivalent. This hypothesis will be tested shortly using high energy particles from accelerators in D u b n a , Russia. Bubble detectors with different formulations will be irradiated alongside a variety of biological specimens and the response of the latter will be compared to the response of the bubble detectors in an attempt to arrive at an effective biological "critical volume" most representative of biological damage.
REFERENCES Apfel, R. E., Roy, S. C. and Lo, Y.-C. (1985) Prediction of the minimum neutron energy to nucleate vapor bubbles in superheated liquids. Physics Review A 31, 3194. Apfel, R. E. (1979) The superheat drop detector. Nuclear Instruments and Methods 162, 603. Bell, C. R., Oberle, N. P., Rohsenow, W., Todreas, N. and Tso, C. (1974) Radiation-induced boiling in superheated water and organic liquids. Nuclear Science and Engineering 53, 458. Benton, E. V. and Parnell, T. A. (1988) Space radiation dosimetry on U.S. and Soviet manned missions. In Terrestrial Space Radiation and Its Biological Effects (Edited by P. D. McCormack, C. E. Swenberg and H. Bucker), NATO ASI Series, Series A: Life Sciences, Vol. 154, p. 729. Plenum Press, New York. Blander, M. and Katz, J. L. (1975) Bubble nucleation in liquids. A.I.Ch.E.J. 25, 833. Bond, V. P., Wielopolski, L. and Shani, G. (1996) Current misinterpretation of the linear no-threshold hypothesis. Health Physics 70, 877. Buckner, M. A., Noulty, R. A. and Cousins, T. (1994) The effect of temperature on the neutron energy thresholds of bubble technology industries' bubble detector spectrometry. Radiation Protection Dosimetry 55, 23. Cheka, J. S. (1954) Recent developments in film monitoring of fast neutrons. Nucleonics 12, 40. Chemtob, M., Dollo, R., Coquema, C., Chary, J. and Ginisty, C. (1995) Essais de dosimetres neutrons a bulles, modele BD 100R-PND et modele BDT.
Radioprotection 30, 61. Cousins, T., Hoffarth, B. E., lng, H. and Tremblay, K. (1990) Recent re-measurement of the neutron and gamma ray fields at large distances from a prompt critical facility, Defence Research Establishment Ottawa, Report No. 1031, April 1990. Cousins, T., Tremblay, K. and Ing, H. (1990b) The application of the bubble spectrometer to the measurement of intense neutron fluences and energy spectra. IEEE Transactions Nuclear Science 37, 1769. Cross, W. G. and Ing, H. (1984) Overview of neutron dosimetry in Canada. In Tenth DOE Workshop On Personnel Neutron Dosimetry, Pacific Northwest Laboratory Report PNL-SA-12352, p. 13. Eberhart, J. G., Kemsner, W. and Blander, M. (1975) Metastability limits of superheated liquids: bubble nucleation of hydrocarbons and their mixtures. Journal of Colloid and Interface Science 50, 369. EI-Nagdy, M. M. and Harris, M. J. (1971) Experimental study of radiation induced boiling on superheated liquids. Journal of the British Nuclear Engineering Society 10, 131. Gibbs, J. W. (1961) The Scientific Papers of Willard Gibbs, Vol. 1. Dover, New York. Glaser, D. A. (1952) Some effects of ionizing radiation on the formation of bubbles in liquids. Physics Review 87, 665. Goldfinch, E. P., McKeever, S. W. S. and Scharmann, A. (1993) Proceedings of the 10th International Conference on Solid State Dosimetry, Washington, D.C., 1317 July 1992. Radiation Protection Dosimetry 47, 535. Hankins, D. E., Homann, S. G. and Buddemeier, B. (1989) Personnel neutron dosimetry using electrochemically etched CR-39 foils. UCRL Report UCRL-53833 Rev. 1. Harper, M. J. (1991) A theoretical model of a superheated liquid droplet neutron detector. Ph.D thesis, Univ. of Maryland. Hofert, M. and Piesch, E. (1985) Neutron dosimetry with nuclear emulsion. Radiation Protection Dosimetry 10, 189. Hoffman, J. M., Harvey, W. F. and Foltyn, E. M. (1992) Bubble dosimetry experience at Los Alamos National Laboratory. Report for Los Alamos National Lab (LA-UR-92-3700), TLD User Symposium, Dosimetry: Users, Results and Trends, San Antonio, Tex. ICRP (1991) The 1990-1991 Recommendations of the International Commission of Radiological Prot. Publication 60, Ann. ICRP 21(1-3). Oxford, Pergamon Press, 1991. Ilic, R., Rant, J. and Sutej, T. (1989) Fast Neutron Radiography with a Bubble Damage Polymer Detector. 3rd World Conference on Neutron Radiography, Osaka, Japan. Ing, H. and Birnboim, H. C. (1984) A bubble-damage polymer detector for neutrons. Nuclear Tracks and Radiation Measurements 8, 285. Ing, H., McLean, T. D., Noulty, R. A. and Mortimer, A. (1993) Measurements of space radiation using bubble detectors. Proceedings of Spacebound '93, 16--18 May 1993, Ottawa. CSA Publication No. SB 93-001, p. 9. Ing, H., McLean, T., Noulty, R., Akatov, Y., Archangelsky, V., Mortimer, A., Lone A. and Wong, P. (1994) Measurements of Neutron Spectra and Dose Equivalent Inside Spacecrafts. Proceedings of 30th COSPAR, Scientific Assembly and Associated Events, Hamburg, Germany. Ing, H. and Mortimer, A. (1994) Space Radiation Dosimetry Using Bubble Detectors. Advances in Space Research 14, 73. Ing, H., Mclean, T. D., Nouity, R. A. and Mortimer, A. (1994) Assessment of Biological Detriment from Space Radiation and the Bubble Detector, Proceedings of
BUBBLE DETECTORS Spacebound '94, May 19-20 1994, Montreal, Quebec p. 35. lng, H., McLean, T., Noulty, R. and Mortimer, A. (1996) Bubble detectors and the assessment of biological risk from space radiation. Radiation Protetection Dosimetry 65, 421. Kahn, B. and Peacock, R. N. (1963) Ultrasonic cavitation induced by neutrons. Nuovo Cimento 28, 334. Lewis, B. J.. Kosierb, R., Cousins, T., Hudson, D. F. and Guery, G. (1994) Measurements of neutron radiation exposure of commercial airline pilots using bubble detectors. Nuclear Technology 106, 373. Lim, W. and Wang, C, K. (1993) Computational studies of neutron response function for a neutron spectrometer which uses Freon -12, -22 and -115 superheated liquids. Nuclear Instruments and Methods. Physics Research A 335, 243. Lo, Y.-C. and Apfel, R. E. (1988) Prediction and experimental confirmation of the response function for neutron detection using superheated droplets. Physical Review A 38, 5260. Luszik-Bhadra, M., Alberts, W. G., Dietz, E. and Guldbakke, S. (1993) Aspects of combining albedo and etched track techniques for use in individual neutron monitoring. Radiation Protection Dosimetry 46, 31. Nelson, M. E. and Gordon, R. (1993) Comparison of neutron measurement at Linacs using bubble detectors to other neutron detectors. Radiation Protection Dosimetry 47, 547. Norman, A. and Spiegler, P. (1963) Radiation nucleation of bubbles in water. Nuclear Science Engineering 16, 213. Olsen, R. G. (1990) Radiofrequency detection by bubble dosimeter technology. Radiation Protection Dosimetry 34, 381. Portal, G. and Dietze, G. (1992) Implications of ICRP60 for neutron dosimetry. Radiation Protection Dosimetry 44, 165. Rosenstock, W., Schulze, J., Koble, T., Kruzinski, G., Thesing, P., Jaunich, G. and Krozholz, H.-L. (1995) Estimation of neutron energy spectra with bubble detectors: potentials and limitations. Radiation Protection Dosimetry 61, 133. Sawicki, J. A. (1993) Longevity tests and background response of bubble neutron detectors. Nuclear Instruments and Methods. Physics Research A 336, 215.
11
Scitz, F. (1958) On the theory of the bubble chamber. Physics and Fluids 1, 2. Shannon, D. J. (1996) Operation of the neutron bubble detector from 70 to 200 °F. HP 8501 Special Problem Report, George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology. Skripov, V. P. (1974) Metastable Liquids, p. 182. Halsted Press, Wiley, New York. Spurny, F., Obraz, O., Pernicka, F., Votockova, I., Turek, K., Nguyen, V. D., Vojtisek, O., Starostova, V. and Kolin, O. (1993) Dosimetric characteristics of radiation fields on board Czechoslovak Airlines' aircraft as measured with different active and passive detectors, Radiation Protection Dosimetry 48, 73. Tommasino, L. (1981) Nuclear track detection by avalanche-type processes and electrochemical etching, spark and breakdown counter. In Proc. 11th Int. Conf. on Solid State Nuclear Track Detectors, Bristol, p. 199. Pergamon Press, Oxford. Tume, P., Lewis, B. J , Bennett, L. G. I., Cousins, T., Jones, T. A., Hoffarth, B. E., Brisson, J. R., Goldhagen, P., Cavallo, A., Van Steveninck, W., Reginatto, M., Shebel, P., Hajnal, F., Jamieson, T. J. and LeMay, F. J. (1996) Assessment of the cosmic radiation field at jet altitudes. Proc. 1996 Topical Meet. Radiat. Protection and Shielding, Vol. 1, p. 68. Falmouth, Mass. Vallario, E. J. (1984) Introductory comments. In Tenth DOE Workshop on Personnel Neutron Dosimetry, Pacific Northwest Laboratory Report PNL-SA-12352, p. 1. Vallario, E. J. (Meeting Co-ordinator) (1971) Second ACE Workshop on Personnel Neutron Dosimetry, Pacific Northwest Laboratory Report BNWL-1616, National Technical Information Service, Springfield, VA. Ward, C. A., Balakrishman, A. and Hooper, F. C. (1970) On the thermodynamics of nucleation in weak gas-liquid systems. Transactions of the American Society of Mechanical Engineers 92, 695. West, C. and Howlett, R. (1968) Some experiments on ultrasonic cavitation using a pulsed neutron source. British Journal of Applied Physics 2, 247. Zacek, V. (1994) Search for dark matter with moderately superheated liquids. 1l Nuovo Cimento 107A, 291. Zeissler, C. J. (1992) Imaging with Bubble Detectors. Presented at lOth International Conference on Solid State Dosimetry, Georgetown Univ., Washington, D.C.
Pergamon
INVITED PAPER BUBBLE DETECTORS--A M A T U R I N G TECHNOLOGY H. ING, R. A. NOULTY and T. D. McLEAN Bubble Technology Industries Inc., Highway 17, Chalk River, Ontario, Canada K0J 1JO (Received 29 September 1996; accepted 25 November 1996)
A~traet--Since the development of the first bubble detector over 10 yr ago, there has been continuing expectation that it would solve the well-known problems of personal neutron dosimetry. Research in the intervening years has led to a much better understanding of this interesting technology and to the development of a variety of radiation detectors that have found diverse applications in radiation physics. In recent years, a bubble detector has been developed for personal neutron dosimetry that is increasingly being adopted by many groups world-wide. Although this dosimeter has improved significantly the status of neutron dosimetry, there is continuing research to improve the properties of bubble detectors, not only in this application, but for general radiation detection. © 1997 Elsevier Science Ltd
I. BACKGROUND The development of the bubble detector for radiation detection was driven by the need in the nuclear industry for a personal neutron dosimeter that would meet the requirements for safe radiation monitoring of personnel. Historically (Cheka, 1954), personal neutron dosimetry has relied heavily on the use of nuclear emulsions, such as Nuclear Film Type A (NTA) by Kodak. Although the inadequacies of NTA film for such an application were well recognized (see for instance, Hofert and Piesch, 1985), film continued to be used by certain groups even to the present day. The main reason for its popularity is the lack of a completely satisfactory alternative. Attempts to improve personal neutron dosimetry led the U.S. Department of Energy (DOE) to implement a formal program with the goal of finding a suitable alternative to NTA in 1969 (see for instance, Vallario, 1971). The results of this program are summarized in a series of DOE workshops spanning a period of almost 20 yr (see for instance, Vallario, 1984). In retrospect, this program can be identified with the accelerated development mainly of two new dosimeters: the damage track detector and the albedo neutron dosimeter. Both of these dosimeters have certain technical advantages over NTA. However, neither is able to completely meet the requirements of personal neutron dosimetry (see for instance, Luszik-Bhadra et al., 1993; Hankins et al., 1989). Although the bubble detector is not a direct product of the DOE program, the world-wide scientific interest stimulated by the program had some influence on its development (Cross and Ing,
1984). One of the technical limitations of the damage track detector identified in the early 1980s was the relatively high energy threshold for fast neutrons, which was approximately 1 MeV. This made the damage track detector insensitive to a large fraction of the lower energy neutrons encountered in power reactor environments. Much effort by groups world-wide was spent in trying to solve this limitation. Subsequently, the identification of a very sensitive plastic---called Columbia resin 39 or CR-39--along with improvements in track etching techniques (see for instance, Tommasino, 1981) led to some improvements in the lowering of the energy threshold. In the search for more sensitive plastic for damage track detectors, one of the authors (H.I.) began to experiment with the feasibility of pre-loading plastic with sources of stored energy in order to amplify the effect of neutron interactions in the medium. Among several approaches that were examined, the concept of stored mechanical energy in the form of superheated liquid dispersed throughout the medium appeared most promising. Since the work of Glaser (1952) on the bubble chamber, many studies have been done on the response of superheated liquid to ionizing radiation (see for instance, Kahn and Peacock, 1963; West and Howlett, 1968). Of particular interest were studies on the response of droplets of superheated liquid to radiation (Skripov, 1974; Bell et ai., 1974; Apfel, 1979). Most of these studies had the droplets suspended in a liquid or a viscous medium. The focus of the bubble detector development was to have microscopic droplets of superheated liquid dispersed throughout a plastic medium in order to
2
H. ING et al.
enhance its response. The first attempts to make detectors in a plastic medium utilized Freon-12 as the superheated liquid and a water-based crosslinked polymer (polyacrylamide) as the plastic medium. Initial tests on the proof-of-concept prototype showed that such detectors had an extremely high detection efficiency relative to any existing passive neutron detector. The reporting (Ing and Birnboim, 1984) of these preliminary results generated enormous optimism throughout the international dosimetric community, not only because of the high detection sensitivity of the novel bubble detector, but also because of its low neutron energy threshold. 2. OVERVIEW OF BUBBLE DETECTOR
Over a decade of research has since been carried out on this novel approach to detect radiation. This work, of course, has led to a much better understanding of the physics of the bubble detector and has resulted in a variety of products for various applications in radiation physics. One of the most advanced products which is often identified with the term "bubble detector" is the BD PND (Bubble Detector Personal Neutron Dosimeter), shown in Fig. 1. This dosimeter consists of 104-105 droplets of superheated liquid ( ~ 20/2m dia) dispersed throughout 8 cm 3 of clear, elastic polymer. This dosimeter has built-in compensation for temperature effects on the response of bubble
detectors and is used for personal neutron dosimetry by many groups world-wide. The BD PND is activated by unscrewing the cap on the top of the dosimeter. Neutrons striking the loaded plastic produce small visible bubbles which appear instantly in the dosimeter. The bubbles are fixed in the elastic medium and can be subsequently counted, visually or by use of automatic readers employing image analysis techniques. The bubbles can be recompressed by screwing the cap assembly back on the dosimeter. By varying the formulation of the detector liquid and the polymeric medium, the radiation detection properties of the bubble detector can be varied to meet different requirements. Thus, a range of bubble detectors can and have been produced over the years. There is considerable confusion over the term "bubble detector" and other radiation detectors based on superheated liquids such as the superheated drop detector or SDD (Apfel, 1979). While the radiation detection physics is common to all detectors using superheated liquid (including the bubble chamber), there are major differences in design and operational properties between the bubble detector and others. In fact, the term bubble detector should be restricted to detectors which use superheated droplets but where the bubbles are trapped in a firm polymer at the sites of formation. This requirement carries with it a set of unique problems a.nd advantages, one of the latter being the reusability of
Fig. 1. Bubble Detector Personal Neutron Dosimeter before (on the right) and after irradiation (on the left).
BUBBLE DETECTORS P©
--???_ ? 27 - - - - - - - - - - - . . . . . . . . . . .
.
- - - - - - - - - - - - . - - - - - - - - - - - - . . . . . . . . . . . . . . . . . . . . . . . .
/ Vapour bubble Fig. 2. Schematic diagram showing a vapour bubble of radius r~ in a liquid medium. External pressure Pe plus pressure due to bubble surface tension tend to crush the bubble while the pressure Pi of the vapour associated with the liquid acts to expand the bubble. the detector by merely recompressing the bubbles back into droplets. Much research on various aspects of the bubble detector by various groups has been made over the last decade. This work has established (Portal and Dietze, 1992) that the bubble detector is the only personal neutron dosimeter which has adequate sensitivity to meet the implications of the ICRP 60 (ICRP, 1991) recommendations for neutron dosimetry. The work has also spawned a new field of activity, often included as a sub-discipline in international scientific meetings (see for example, Goldfinch et al., 1993) and an increasing number of open literature publications. 3. THEORETICAL ASPECTS A theory to account for the behaviour of the bubble detector must encompass classical cavitation theory in liquids, the physics of metastable states in liquid-gas systems and the effect of radiation in matter. All these disciplines have been extensively studied in the past (Skripov, 1974), although a complete understanding of the physical processes involved is lacking. When a liquid continues to exist in the liquid state above its normal boiling point, it is said to be superheated. Theoretically, boiling or nucleation can be retarded until the temperature of the liquid reaches its so-called superheat limit. The maximum attainable superheat, at atmospheric pressure, can be predicted on thermodynamic and kinetic grounds to be
approximately 90% of the liquid's critical temperature (Eberhart et al., 1975). Depending on the degree of superheat, the phase transition when it does occur can be explosive in nature. Generally, boiling in liquids is the result of "heterogeneous nucleation" where impurities in the liquid or liquid/solid interfaces facilitate the phase transition. But if the liquid is surrounded by a second immiscible phase with which it has a zero contact angle then normal boiling is suppressed and the liquid can be heated as high as its superheat limit at which point, it undergoes "homogeneous nucleation". A superheated medium to detect ionizing radiation was first used by Glaser (Glaser, 1952). The so-called bubble chamber consisted of a bulk superheated liquid (e.g. propane). When sufficiently energetic particles entered the chamber, tracks could be observed as a trail of vapour bubbles. It was Seitz (Seitz, 1958) who first proposed a reasonable mechanism by which nucleation takes place in superheated liquids. Known as the "thermal spike" theory, it is still widely accepted today. This theory postulates that the ionizing radiation produces highly localized hot regions or "temperature spikes" within the liquid which literally explodes into bubbles through the evaporation of the superheated liquid. The physical processes responsible for the production of bubbles are viewed to be similar to those responsible for producing radiation damage in solids. In order to determine quantitatively the relationship between energy deposition and bubble,
4
H. ING et al.
formation, Seitz postulated that the formation of a visible bubble involved two critical steps: the formation of a critical-size vapour bubble and the growth of this bubble to a visible or macroscopic bubble. To calculate the size of the critical bubble, he assumed that classical macroscopic concepts of pressure and surface tension were applicable down to the dimensions of a critical bubble and considered a static vapour bubble in a superheated liquid medium under external pressure po (see Fig. 2). The pressure Pe along with the surface tension of the bubble tend to compress the bubble. The pressure p~ acting on the inner surface of the bubble is associated with the vapour of the superheated liquid. Since the effective pressure from surface tension of the bubble is 2a/r, the equilibrium condition for a critical-size bubble of radius r~ implies that the pressures acting inward are balanced by the outward pressure: Pe
2or +
-rc
=
P,
(1)
where ¢r is the surface tension of the liquid. Rearrangement of equation (1) yields rc
2a = ~pp
(2)
where Ap = p~ - p, and is a measure of the degree of superheat and is defined as the pressure difference between the equilibrium vapour pressure of the liquid and the externally applied pressure (generally 1 atmosphere). Typically, the value of rc is a few tens ofnanometers. In comparison, droplet radii in bubble detectors are on the order of 10 microns. It can be shown (Gibbs, 1961; Blander and Katz, 1975) that the existence of a vapour bubble of radius re represents a maximum in the free energy potential of the system and is therefore unstable to slight perturbation. Vapour cavities of smaller radii will collapse under the effects of surface tension. On the other hand if rc is exceeded then the cavity will spontaneously expand to form a macroscopic-sized bubble. The validity of the concept of ro has since been demonstrated by many workers (see for instance, Ward et al., 1970). Seitz (1958) showed, using classical thermodynamic arguments under the assumption of reversibility, that the minimum amount of energy required to form a vapour cavity of radius rc is approximated by: Emi, = 41ttrr 2 + 4 / 3 n r ~ p v H / M
(3)
or in terms of equation (1): E , in = 16na3/(Ap)2[1 + 2/3p~H/(MAp)]
where P, is the density of the vapour while H and M represent the molar heat of vaporization and molecular weight respectively. The first part of equation (3) represents the energy required to form a bubble of radius r, against the forces of surface tension while the second term gives the energy required to vapourize the liquid to produce the
bubble. With increasing temperature, the surface energy, vapour density and heat of vapourization will decrease while, of course, Ap will increase. The overall effect is to lower the value of re and the minimum energy required to form the initial critical bubble. Seitz postulated that the energy required to nucleate the superheated liquid was the result of energetic recoil ions stopping in the liquid. A portion of the energy loss is degraded to thermal energy allowing localized vapour formation. Nucleation occurs if the local value of re is exceeded or, stated equivalently, the local superheat limit is surpassed. Although Seitz's theory was developed for the bubble chamber, it also applies to bubble detectors as each superheated droplet is, in effect, an isolated bubble chamber. Later investigators (EI-Nagdy and Harris, 1971; Norman and Spiegler, 1963), building on Seitz's work, postulated that the effective length (L) over which a particle must deposit sufficient energy to cause nucleation must be linearly related to the value of re. If L is assumed to be much shorter than the total track length of the particle then the energy deposited is given by: E = L(dE/dx)avr = krc(dE/dx)aw
(4)
where (dE/dX)avr is the average stopping power over the interaction length L and k is a proportional constant. Widely varying estimates (from 2-13) for k have been given in the literature (Bell et al., 1974; E1-Nagdy and Harris, 1971; Norman and Spiegler, 1963). Recently, Harper (Harper, 1991) has proposed that the parameter k should be expressed in terms of the effective radius (r0) of the liquid volume evaporated to form a critically sized vapour bubble. Equating equations (3) and (4), it is clear that at a given temperature there is a threshold value of (dE/ dX)a~r below which nucleation is not possible. This observation forms the basis for using several bubble detectors with unique thresholds as a crude neutron spectrometer, as will be discussed in the following section. Attempts have been made to model the energy and temperature response of superheated droplets with respect to neutron irradiation (Harper, 1991; Lo and Apfel, 1988; Lim and Wang, 1993). Detector response (R) for monoenergetic neutrons can be described in terms of the incident energy (E,) and temperature (T) as follows: n
m
R ( E . , T ) = ()(E.)V£.N,~,o,jFo(E,,T)
(5)
where ~b(E.) is the neutron flux and V is the total volume of superheated droplets. The number of isotopes in the superheated material is given by n and Ni represents the atomic density of each isotope. The number of considered reaction cross-sections is given by m and o Uis the relevant cross-section for the i, j interaction. The relevant nuclear reactions which are usually taken into account include elastic and
BUBBLE DETECTORS inelastic scattering as well as charged particle production, e.g. (n,p). F~j is a factor which describes the probability of the i, j interaction leading to nucleation. In the case of elastic scattering, the F U factor is often calculated in the following manner. First, energy distributions for all the potential product ions are calculated as a function of incident monoenergetic neutron energy. Then for each ion, the stopping power in the droplet material is calculated as a function of energy. The fraction of recoil ions with stopping powers greater than the minimum average value necessary to cause nucleation [equation (3)] gives directly the probability of nucleation for each combination of recoil ion and incident neutron energy. This approach of modeling detector response has successfully predicted threshold neutron energies for a number of superheated liquids (Apfel et al., 1985). It has shown that for fast neutrons the predominant mechanism for nucleation is elastic scattering of recoil ions. These calculations have also clearly demonstrated (Apfel et al., 1985) that the C135(n,p)Sa5 reaction is responsible for the thermal neutron sensitivity of certain detectors (other than bubble detectors) which also use superheated liquid drops. In the case of continuous neutron sources, detector response is calculated by integrating equation (5) with respect to neutron energy or by summing over adjacent energy intervals. Harper (1991) has had reasonable success in predicting detector response to bare and moderated Cf sources. He found that the best fit to experimental data was obtained with k = 4.3, i.e. the effective pathlength of energy transfer is 4.3r0. Taking into account the relative liquid and gas densities of typical detector liquids, this implies L ~ 2re. It is helpful to summarize qualitatively the various physical processes leading to the formation of visible bubbles from exposure of the bubble detector to neutrons. Out of the neutrons which traverse the bubble detector, a certain fraction of them will interact with the detector. The probability of interaction is determined by the specific composition of the medium in the detector. Some of these interactions will occur "far" from the sites of the superheated liquid droplets and others will occur "close" to these sites and in fact, a small fraction will interact with the superheated liquid droplets themselves. These interactions give rise to a variety of secondary charged particles (including recoil ions) in terms of energy and charge. These charged particles will slow down in the medium in accordance with the stopping power of the specific ions in the immediate environment of the interaction site. The energy dissipation during this process produces the "thermal spike" as envisaged by Seitz. Many of the secondary charged particles may not interact with a superheated droplet ( ~ 20 #m dia) and will therefore be "wasted". Some of the secondary charged particles will be intercepted by a
5
droplet and, depending on their energy, will be stopped by the droplet or will be forced by the droplet to lose some of their energy. The passage of the particle within the superheated droplet will give rise to a trail of microscopic bubbles. Some of these may coalesce to form somewhat bigger bubbles. If any one of these bubbles exceed the size of the critical bubble [equation (2)], the bubble will grow, if none of these is as large as the critical bubble, they will be recompressed by the forces of surface tension and no evidence of their formation will be apparent. The minimum amount of energy for the formation of a single critical bubble is given by equation (3). Simplistically speaking, this energy must be deposited over a distance less than approximately 100 nm within the superheated liquid droplet, although the energy deposition process is probably much more complicated including other energy contributing processes such as through thermal conductivity, high-energy delta rays, acoustic propagation, etc. This energy deposition process is basic to the function of the bubble detector and occurs over a time period of < 10-,3 s. Once a bubble greater than the critical bubble is formed, the remaining liquid in the superheated droplet vapourizes into the bubble forcing it to grow very quickly (in the order of microseconds). This vapourization of the droplet emits a characteristic (typical of nucleation "crackling") acoustic signal which can be used for the identification of bubble formation. It is interesting to note that the size of the resulting visible bubble is determined by the size of the droplet within which the energy deposition occurs. The physics of the neutron interaction process have already all taken place before the formation of the visible bubble and so the size of the visible bubble is not related to the property of the incident neutron. In the bubble detector, the visible bubbles formed by neutron interactions remain fixed at the initial droplet sites. After exposure to radiation, the number of these bubbles can be used to provide a measure of the neutron field. There is a slow growth (over periods of many days) of the visible bubbles after their formation. This is associated with migration of vapour molecules from the polymer medium into the bubbles. Thus, it is recommended that the bubbles be recompressed as soon as convenient if one wishes to prevent bubbles from growing to a size large enough to damage the polymer medium. Once a site is damaged by a large bubble, the site can no longer be reused because the vapour bubble can no longer be recompressed into a superheated droplet.
4. APPLICATIONS OF BUBBLE DETECTORS
4.1. Neutron dosimetry Many groups worldwide are using bubble detectors for routine monitoring of personal neutron doses. Monitoring is especially important in neutron
H. ING et al. I
I
I
I I II
'
'
'
' ''"I
'
'
'
' ''"I
'
I
I--
(Detector sensitivity = !.0 bubblelmrem)
'7 .o 10-5 da e~ .-s d~
d~ v
to L) z to .l ///$
to eL
N
m eL 2.0 m o~ Z O 1.0 e L to
(lcm DEPTHDOSE EQUIV:LENT)
10-6
z 0
eL
0.5
to
0.2 10-7
I
I
I
I I II
I
I
I
I I IIII
0.1
I
I
l
I I IIII
1.0
I
I
t
0.1
10
NEUTRON ENERGY (MeV) Fig. 3. Bubble Detector Personal Neutron Dosimeter normalized response per unit fluence (O) and normalized response per unit dose equivalent (0). Conversion from fluence to dose equivalent based on NCRP Report No. 38. "streaming" through shielding where fields may be high and localized. One version of the bubble detector the BD PND, is most popular for such applications and has been extensively tested by several groups (Hoffman et al., 1992; Chemtob et al., 1995; Shannon, 1996) in terms of energy dependence, temperature dependence, repeated use, reliability, reproducibility, dynamic range, etc. Figure 3 shows the response curve with neutron energy for the BD PND. This detector has an approximate 100 keV threshold with a reasonably flat tissue equivalent dose response from about 200 keV to greater than 15 MeV. These detectors have extremely high neutron sensitivity (at least an order of magnitude more sensitive than any other passive device), are isotropic in response, provide an immediate visual dose record and are completely insensitive to gamma radiation. These detectors incorporate an integral recompression ability (a pressure cap) allowing the bubbles to be recompressed in situ and essentially reused indefinitely. A technical limitation of the bubble detector that was identified early in its development was solved with the introduction of the BD PND. Figure 4 compares the response of a bubble detector without compensation and with temperature compensation. The use of temperature compensation allows for constant sensitivity over the temperature range 15--40°C. Temperature compensation is accomplished by utilizing the expansion properties of a special
proprietary material. This material is placed in a small chamber located just above the detector and sealed with a thin rubber diaphragm. The expansion of this material with temperature exerts appropriate pressure on the detector to compensate for the increase in superheat. Recently, a new personal bubble detector, the BDT (Bubble Detector Thermal), has been introduced for monitoring of thermal neutrons. This detector uses a 6Li compound dispersed throughout the polymer
3.0
-/0
~2.5 O eL Z0
e-
m
S < 1.o z
0.5 ~
0.0 I0
•
I
I
I
I
I
I
15
20
25
30
35
40
TEMPERATURE (C) Fig. 4. Bubble Detector Personal Neutron Dosimeter ( I ) normalized response to A m - B e vs temperature. For comparison, uncompensated neutron bubble detector ( O ) is shown.
BUBBLE DETECTORS I I t rrit I
~
t
i i lilt
I
t
I
I
f. . . .III
II /,"
,
(Detector sensitivity = 1.0 bubble/mrem)
•
.
!
E
.=
/
d~ dD
i I.
' BDS-1001
/
10 -5
I
7-'-~ / /.~"
/
i
/
Ct~ Z ©
t
r¢ 10-6
,.d <
!
/
/
t BDS-100
I
/
BDS-10,000
~E
}DS-2500/10
BDS-600
0~ ©
I
z
i I I 10-7 0.01
t
I
t
t
t JJtl
I
1
I
I I IIII
0.1
I
I
t
~ t ttt[
10
1.0 N E U T R O N E N E R G Y (MeV)
Fig. 5. Normalized response per unit fluence for Bubble Detector Spectrometer: 10 keV threshold (A); 100 keV threshold (rq); 600 keV threshold ( 0 ) ; 1000 keV threshold (A); 2500 keV threshold ( i ) ; 10,000 keV threshold (0)-
medium
and
a
special
formulation
to
detect
preferentially ~ particles from the 6Li(n,~)T reaction. The thermal to fast neutron response is approximately 10:1. A combination type dosimeter using both
the B D P N D
and
the B D T
is n o w
being
implemented in some facilities to measure both thermal and fast neutron doses simultaneously.
that the LET associated with electrons cannot provide sufficient energy to create bubbles larger than the critical bubble and thus, even intense gamma fields have no effect on the detector response. Thus, it can measure a weak neutron field in the presence of a strong gamma background which is often the case in many radiation environments. Figure 6 shows
4.2. Spectrometry
By varying the formulation, bubble detectors having different neutron energy thresholds have been m a d e (Buckner et al., 1994). Figure 5 s h o w s the response functions of a set of neutron bubble detectors having thresholds of 10, 100, 600, 1000, 2500 a n d 10,000 keV. T h i s set w a s m a d e for crude
neutron spectroscopy, to give some indication of the energies o f the n e u t r o n s that m a y be e n c o u n t e r e d in
certain radiation environments. This knowledge of the general spectral distribution assists the health physicist in the proper choice of dosimeter to be worn by personnel exposed to these fields. The bubble detector spectrometer or the "BDS" has been used by various groups (see for instance, R o s e n s t o c k et al., 1995) for c h a r a c t e r i z a t i o n o f neutron fields in nuclear utilities, handling of fissile materials and accelerator laboratories. One particular advantage of the BDS over many other spectrometers is its complete insensitivity to gamma rays. All the components of the spectrometer are formulated such
~ , 7.00E+05 -i t~ 6.00E+05
m
¢-i"
~E 5.00E+05 --
m 4.00E+05 L) Z 3.00E+05 ,.d tL 2.00E+05 Z O 1.00E+05 m Z
- _
.......
EXPT. -THEORy
] .......
!
O.00E+00
10
100
1000
i__j 10,000
I tllnl
100,000
N E U T R O N E N E R G Y (keV)
Fig. 6. Neutron energy spectrum of a bare Cf m source measured with the Bubble Detector Spectrometer ( ) For comparison, a theoretical Ctn52 spectrum ( - - - ) is shown.
H. ING et al. 1.8 m 1.6 1.4 1,2 1.0 0.8 0.6 0.4
BION-9
--
-------
Doserate = 8.0 m r e m / d 156%!
I
0.2 - - , 0 2.5 - -
nli
,,.,1
I I t IIIIII
I I I III IlL
'
I. I Illlll
BION-10 Doserate = 14.0 m r e m / d
2.0
I I
55%1
1.51.0 - ~0,5
[
0 4.0
IIIIll
I
I I IIII11
I
I I Jill
llllll
MIR
-
D o s e r a t e = 9.2 mrern/d I 3.0 - -
I 151%I
I
2.0 t 1.0 I 0 0.01
'
ililll
II I
0.1
I i liliil
i
,: i i llliil
1.0
I
l0
i
lilill
100
En (MeV) Fig. 7. Neutron spectra measured in space for Russian BIOCOSMOS (BION) #9 and #10 and Russian space station MIR using bubble detectors. a measurement of a bare Cf 252source using the BDS. There is good agreement with the theoretical spectrum.
high energy protons in the South Atlantic Anomaly striking the aluminum shell of the satellite and the station and producing neutrons via the (p,n) reaction. Recently, bubble detectors were flown aboard the space shuttle (STS-78) as part of an educational experiment. The bubble detectors were activated by an astronaut and were viewed with a video camera as the shuttle orbited the earth. Visual observation by the astronaut and the video recording showed that very few bubbles formed during much of the orbit but many formed when the shuttle crossed the South Atlantic Anomaly. This seems to confirm our original hypothesis. Further experiments aboard the space shuttle and MIR are being planned. Bubble detectors have also been popular in the monitoring of commercial and military aviation crews (Spumy et al., 1993; Lewis et al., 1994; Tume et al., 1996). In these experiments, several bubble detectors were activated and often carried in the hand-luggage (or pocket) of the passenger or crew member. After the trip, the detectors were read-usually by eye--to determine the transit neutron dose. Often, TLDs are also carried in many of these studies to yield the gamma dose. For altitudes used by aviation, the neutron dose is an important component of the total radiation exposure. Studies are still on going with air crews being monitored by bubble detectors in Canada, Europe, Japan and Russia. Studies involving high altitude flights using bubble detectors (NASA's ER-2 program) are expected to commence shortly. 4.4. Other applications
4.3. Space and high altitude research An interesting application of the BDS was the measurement of the neutron spectrum in space. Such a measurement had been difficult to perform because of the high background of charged particles-especially high-energy protons. The protons create extraneous signals in most other detectors, but the neutron bubble detectors had been shown to be quite insensitive to high-energy protons in experiments using the Orsay cyclotron. The importance of knowing the neutron spectrum in space had been discussed earlier by Benton and Parnell (1988). Three separate measurements of the neutron spectrum in space have been made (Ing et al., 1993, 1994a; Ing and Mortimer, 1994). Figure 7 shows the measured spectrums in the Russian satellites BIOCOSMOS (BION) #9 and BIOCOSMOS (BION) #10 and the Russian space station MIR. Despite the different orbital parameters and times, the neutron levels were quite consistent--in the order of 100/~Sv per day. There appeared to be a large number of neutrons above 10 MeV, which were originally unexpected by some theorists and neutrons around a few MeV which were expected because they were the "evaporation neutrons". It was postulated that these high energy neutrons were produced by
Bubble detectors have been made in a variety of sizes and formulated for various applications. Detectors have been made for very special applications such as anthropomorphic phantom work (Cousins et al., 1990a). Bubble detectors have a particular advantage for measurements inside a phantom because they have isotropic response. Since neutron scattering inside the body produce multidirectional fields, the use of other detectors such as CR-39 gives readings which cannot be interpreted because of the unknown directional distributions impinging the detector. Furthermore, the high sensitivity of bubble detectors and ease of reading make such experiments much simpler to perform. Studies have been made using bubble detectors inside and on the surface of phantoms to determine the relationship between badge reading and dose to the gut and to determine the best position to wear dosimeters. This knowledge is especially important, e.g. for dosimetry on-board nuclear submarines where biological shielding is limited by space and weight considerations. Special bubble detectors are used extensively by NATO scientists and the success of the detector has been recognized by its use as the standard dosimeter for free field and anthropomorphic phantom work.
BUBBLE DETECTORS Detectors have been made as small as 5/8" in length for applications where the size of the conventional bubble detectors was not appropriate. Not only were these detectors small, but they were also able to detect up to 10 Sv. Detectors of this type were used by military scientists working with intense neutron fluences and energy spectra associated with weapon simulators employed in electronic irradiations (Cousins et al., 1990b). A version of these small detectors (with much higher sensitivities), called the PM 100R (Personal Monitor 100 keV Reuseable), are used by various groups to monitor finger tip doses in fuel processing plants not unlike finger tip TLDs. Detectors have been made as big as 6" in dia and 1/2" thick. These detectors were used in work involving identification of radioactive "hot spots" in soil samples (Zeissler, 1992). Here various soil samples were placed on top of the slab bubble detector for a period of time. Only contaminated samples produced bubbles in the detector. Another group has examined the feasibility of incorporating a small amount of the waste itself into the bubble detector before manufacture. The radiation level of the waste could then be internally measured by storing the "spiked" detector in a low background environment and comparing the bubble formation to a non-doped sample. Bubble detectors have also been formulated for the detection of radon. The formulation of these detectors took into consideration the superheat conditions for detection of or-particles combined with a polymer medium which facilitated gas migration. These detectors had no difficulty detecting a few pCi/1 over a period of a few days. One could actually see the bubble front move down the detector slowly with time as the radon diffused into the detector medium. Medical related research has used bubble detectors for mapping of the neutron field in newly-installed medical linacs (Nelson and Gordon, 1993) and other neutron sources used in medical treatments. Neutrons are produced by the (~,n) reaction in medical linacs and since the BD PND is insensitive to gammas, it is ideally suited for neutron measurements in the intense gamma field. Other past applications of bubble detector include the search for "cold fusion" (Sawicki, 1993), fast neutron radiology (Ilic et al., 1989) and non-ionizing radiation detection/protection [laser and microwave (Olsen, 1990)].
9
tissue equivalent and have a response fairly constant down to a few keV. We have also successfully applied temperature compensation to the gamma detector similar to that done for the neutron detector as shown in Fig. 8 . One technical appeal of the gamma bubble detector is its extremely high sensitivity. Sensitivities as high as 1000 bubbles per /zSv are easily obtained and this allows for measurements of very low fields using a passive device. We have also started to make electronic dosimeters that use the bubble detector as the active element. Two devices are presently being manufactured: RAISA (Radiation Assessment In Space Applications) for use in aircraft or the space shuttle cockpit as an area monitor and Mini-RAISA, a miniaturized version, for use by astronauts during EVA operations. RAISA uses two bubble detectors operated under microprocessor control. At any time, only one of the detectors is used as the radiation sensor. When this detector has accumulated a fixed number of bubbles (or after being activated for a pre-determined time period), the microprocessor switches the radiation sensor to the second detector and instructs a tiny motor to recompress the bubbles of the first detector. The counting of the bubbles is done electronically as the bubbles are formed. By switching the sensor back and forth between the two detectors, RAISA has an enormous dose and dose-rate range and can operate, in principle, continuously for many years without intervention. An interesting potential application of bubble detectors pertains to the detection of "dark matter" as proposed by Zacek (Zacek, 1994). Interest in dark matter results from current models of the evolution of the universe which predicts the existence of a significant mass of non-luminous, non-baryonic matter made up of relativistic light particles and non-relativistic, massive particles. The latter are known as Cold Dark Matter and their properties in terms of interactions (which occurs infrequently) with 7 /
~ 6
--
D
/ /
--
/
r~
/
4
_
,~
[ooompoos.,o"I ,'
s3
/ /
.1
5. CONTINUING DEVELOPMENTS Over the last few years, research has been continuing in our laboratory to fabricate gammasensitive bubble detectors (BD GAMMA). We had not achieved a completely satisfactory gamma detector based on the bubble detector because of imperfect energy and temperature dependence. However, there is continued progress and we have recently made bubble gamma detectors that are more
-
15
~ tf 20
I 2.5
Ic°mpe"sated I I I 30
35
I
40
TEMPERATURE (C) Fig. 8. Temperature compensated gamma bubble detectors (0) normalized response to Csm v s temperature. For comparison, uncompensated gamma bubble detector (I-1) is shown.
H. I N G et al.
10
ordinary matter have been predicted by theory. One particular interaction property which has been recognized as a good indicator of dark matter is to measure the recoil energy of an atomic nucleus which has been struck by dark matter. By appropriate formulation of the bubble detector, it is possible to limit the response essentially solely to dark matter, which makes the detector very attractive for such an application. On the theoretical side, there is continuing research (Ing et al., 1994b, 1996) to assess the capability of the bubble detector to mimic the effect of radiation on biological systems-especially for space radiation. Currently, there is growing recognition (Bond et al., 1996) that the concept of "dose equivalent" may be inadequate as an indication of biological detriment due to radiation. As indicated earlier, the bubble detector provides a method of measuring energy deposition over distances of tens of nanometers and is therefore, a nanodosimeter. If energy deposition over such distances is truly the physical process which causes D N A damage, the response of the bubble detector may provide a better indicator of biological response than the concept of dose equivalent. This hypothesis will be tested shortly using high energy particles from accelerators in D u b n a , Russia. Bubble detectors with different formulations will be irradiated alongside a variety of biological specimens and the response of the latter will be compared to the response of the bubble detectors in an attempt to arrive at an effective biological "critical volume" most representative of biological damage.
REFERENCES Apfel, R. E., Roy, S. C. and Lo, Y.-C. (1985) Prediction of the minimum neutron energy to nucleate vapor bubbles in superheated liquids. Physics Review A 31, 3194. Apfel, R. E. (1979) The superheat drop detector. Nuclear Instruments and Methods 162, 603. Bell, C. R., Oberle, N. P., Rohsenow, W., Todreas, N. and Tso, C. (1974) Radiation-induced boiling in superheated water and organic liquids. Nuclear Science and Engineering 53, 458. Benton, E. V. and Parnell, T. A. (1988) Space radiation dosimetry on U.S. and Soviet manned missions. In Terrestrial Space Radiation and Its Biological Effects (Edited by P. D. McCormack, C. E. Swenberg and H. Bucker), NATO ASI Series, Series A: Life Sciences, Vol. 154, p. 729. Plenum Press, New York. Blander, M. and Katz, J. L. (1975) Bubble nucleation in liquids. A.I.Ch.E.J. 25, 833. Bond, V. P., Wielopolski, L. and Shani, G. (1996) Current misinterpretation of the linear no-threshold hypothesis. Health Physics 70, 877. Buckner, M. A., Noulty, R. A. and Cousins, T. (1994) The effect of temperature on the neutron energy thresholds of bubble technology industries' bubble detector spectrometry. Radiation Protection Dosimetry 55, 23. Cheka, J. S. (1954) Recent developments in film monitoring of fast neutrons. Nucleonics 12, 40. Chemtob, M., Dollo, R., Coquema, C., Chary, J. and Ginisty, C. (1995) Essais de dosimetres neutrons a bulles, modele BD 100R-PND et modele BDT.
Radioprotection 30, 61. Cousins, T., Hoffarth, B. E., lng, H. and Tremblay, K. (1990) Recent re-measurement of the neutron and gamma ray fields at large distances from a prompt critical facility, Defence Research Establishment Ottawa, Report No. 1031, April 1990. Cousins, T., Tremblay, K. and Ing, H. (1990b) The application of the bubble spectrometer to the measurement of intense neutron fluences and energy spectra. IEEE Transactions Nuclear Science 37, 1769. Cross, W. G. and Ing, H. (1984) Overview of neutron dosimetry in Canada. In Tenth DOE Workshop On Personnel Neutron Dosimetry, Pacific Northwest Laboratory Report PNL-SA-12352, p. 13. Eberhart, J. G., Kemsner, W. and Blander, M. (1975) Metastability limits of superheated liquids: bubble nucleation of hydrocarbons and their mixtures. Journal of Colloid and Interface Science 50, 369. EI-Nagdy, M. M. and Harris, M. J. (1971) Experimental study of radiation induced boiling on superheated liquids. Journal of the British Nuclear Engineering Society 10, 131. Gibbs, J. W. (1961) The Scientific Papers of Willard Gibbs, Vol. 1. Dover, New York. Glaser, D. A. (1952) Some effects of ionizing radiation on the formation of bubbles in liquids. Physics Review 87, 665. Goldfinch, E. P., McKeever, S. W. S. and Scharmann, A. (1993) Proceedings of the 10th International Conference on Solid State Dosimetry, Washington, D.C., 1317 July 1992. Radiation Protection Dosimetry 47, 535. Hankins, D. E., Homann, S. G. and Buddemeier, B. (1989) Personnel neutron dosimetry using electrochemically etched CR-39 foils. UCRL Report UCRL-53833 Rev. 1. Harper, M. J. (1991) A theoretical model of a superheated liquid droplet neutron detector. Ph.D thesis, Univ. of Maryland. Hofert, M. and Piesch, E. (1985) Neutron dosimetry with nuclear emulsion. Radiation Protection Dosimetry 10, 189. Hoffman, J. M., Harvey, W. F. and Foltyn, E. M. (1992) Bubble dosimetry experience at Los Alamos National Laboratory. Report for Los Alamos National Lab (LA-UR-92-3700), TLD User Symposium, Dosimetry: Users, Results and Trends, San Antonio, Tex. ICRP (1991) The 1990-1991 Recommendations of the International Commission of Radiological Prot. Publication 60, Ann. ICRP 21(1-3). Oxford, Pergamon Press, 1991. Ilic, R., Rant, J. and Sutej, T. (1989) Fast Neutron Radiography with a Bubble Damage Polymer Detector. 3rd World Conference on Neutron Radiography, Osaka, Japan. Ing, H. and Birnboim, H. C. (1984) A bubble-damage polymer detector for neutrons. Nuclear Tracks and Radiation Measurements 8, 285. Ing, H., McLean, T. D., Noulty, R. A. and Mortimer, A. (1993) Measurements of space radiation using bubble detectors. Proceedings of Spacebound '93, 16--18 May 1993, Ottawa. CSA Publication No. SB 93-001, p. 9. Ing, H., McLean, T., Noulty, R., Akatov, Y., Archangelsky, V., Mortimer, A., Lone A. and Wong, P. (1994) Measurements of Neutron Spectra and Dose Equivalent Inside Spacecrafts. Proceedings of 30th COSPAR, Scientific Assembly and Associated Events, Hamburg, Germany. Ing, H. and Mortimer, A. (1994) Space Radiation Dosimetry Using Bubble Detectors. Advances in Space Research 14, 73. Ing, H., Mclean, T. D., Nouity, R. A. and Mortimer, A. (1994) Assessment of Biological Detriment from Space Radiation and the Bubble Detector, Proceedings of
BUBBLE DETECTORS Spacebound '94, May 19-20 1994, Montreal, Quebec p. 35. lng, H., McLean, T., Noulty, R. and Mortimer, A. (1996) Bubble detectors and the assessment of biological risk from space radiation. Radiation Protetection Dosimetry 65, 421. Kahn, B. and Peacock, R. N. (1963) Ultrasonic cavitation induced by neutrons. Nuovo Cimento 28, 334. Lewis, B. J.. Kosierb, R., Cousins, T., Hudson, D. F. and Guery, G. (1994) Measurements of neutron radiation exposure of commercial airline pilots using bubble detectors. Nuclear Technology 106, 373. Lim, W. and Wang, C, K. (1993) Computational studies of neutron response function for a neutron spectrometer which uses Freon -12, -22 and -115 superheated liquids. Nuclear Instruments and Methods. Physics Research A 335, 243. Lo, Y.-C. and Apfel, R. E. (1988) Prediction and experimental confirmation of the response function for neutron detection using superheated droplets. Physical Review A 38, 5260. Luszik-Bhadra, M., Alberts, W. G., Dietz, E. and Guldbakke, S. (1993) Aspects of combining albedo and etched track techniques for use in individual neutron monitoring. Radiation Protection Dosimetry 46, 31. Nelson, M. E. and Gordon, R. (1993) Comparison of neutron measurement at Linacs using bubble detectors to other neutron detectors. Radiation Protection Dosimetry 47, 547. Norman, A. and Spiegler, P. (1963) Radiation nucleation of bubbles in water. Nuclear Science Engineering 16, 213. Olsen, R. G. (1990) Radiofrequency detection by bubble dosimeter technology. Radiation Protection Dosimetry 34, 381. Portal, G. and Dietze, G. (1992) Implications of ICRP60 for neutron dosimetry. Radiation Protection Dosimetry 44, 165. Rosenstock, W., Schulze, J., Koble, T., Kruzinski, G., Thesing, P., Jaunich, G. and Krozholz, H.-L. (1995) Estimation of neutron energy spectra with bubble detectors: potentials and limitations. Radiation Protection Dosimetry 61, 133. Sawicki, J. A. (1993) Longevity tests and background response of bubble neutron detectors. Nuclear Instruments and Methods. Physics Research A 336, 215.
11
Scitz, F. (1958) On the theory of the bubble chamber. Physics and Fluids 1, 2. Shannon, D. J. (1996) Operation of the neutron bubble detector from 70 to 200 °F. HP 8501 Special Problem Report, George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology. Skripov, V. P. (1974) Metastable Liquids, p. 182. Halsted Press, Wiley, New York. Spurny, F., Obraz, O., Pernicka, F., Votockova, I., Turek, K., Nguyen, V. D., Vojtisek, O., Starostova, V. and Kolin, O. (1993) Dosimetric characteristics of radiation fields on board Czechoslovak Airlines' aircraft as measured with different active and passive detectors, Radiation Protection Dosimetry 48, 73. Tommasino, L. (1981) Nuclear track detection by avalanche-type processes and electrochemical etching, spark and breakdown counter. In Proc. 11th Int. Conf. on Solid State Nuclear Track Detectors, Bristol, p. 199. Pergamon Press, Oxford. Tume, P., Lewis, B. J , Bennett, L. G. I., Cousins, T., Jones, T. A., Hoffarth, B. E., Brisson, J. R., Goldhagen, P., Cavallo, A., Van Steveninck, W., Reginatto, M., Shebel, P., Hajnal, F., Jamieson, T. J. and LeMay, F. J. (1996) Assessment of the cosmic radiation field at jet altitudes. Proc. 1996 Topical Meet. Radiat. Protection and Shielding, Vol. 1, p. 68. Falmouth, Mass. Vallario, E. J. (1984) Introductory comments. In Tenth DOE Workshop on Personnel Neutron Dosimetry, Pacific Northwest Laboratory Report PNL-SA-12352, p. 1. Vallario, E. J. (Meeting Co-ordinator) (1971) Second ACE Workshop on Personnel Neutron Dosimetry, Pacific Northwest Laboratory Report BNWL-1616, National Technical Information Service, Springfield, VA. Ward, C. A., Balakrishman, A. and Hooper, F. C. (1970) On the thermodynamics of nucleation in weak gas-liquid systems. Transactions of the American Society of Mechanical Engineers 92, 695. West, C. and Howlett, R. (1968) Some experiments on ultrasonic cavitation using a pulsed neutron source. British Journal of Applied Physics 2, 247. Zacek, V. (1994) Search for dark matter with moderately superheated liquids. 1l Nuovo Cimento 107A, 291. Zeissler, C. J. (1992) Imaging with Bubble Detectors. Presented at lOth International Conference on Solid State Dosimetry, Georgetown Univ., Washington, D.C.