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Fast-neutron response characteristics of TSEE dosimeters
NUCLEAR
INSTRUMENTS
AND
METHODS
113
(I973) 611-614;
©
NORTH-HOLLAND
PUBLISHING
CO.
F A S T - N E U T R O N R E S P O N S E C H A R A C T E R I S T I C S OF TSEE D O S I M E T E R S * K L A U S B E C K E R and M. A B D - E L
RAZEK**
Health Physics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, U.S.A. Received 2 July 1973 Further applications are described of the previously reported technique of fast neutron dosimetry with combinations of hydrogenous radiators and ceramic BeO, to be evaluated by thermally stimulated exoelectron emission (TSEE). For example, a pronounced directional response has been minimized with a
dosimeter-radiator-dosimeter sandwich; and information on the effective neutron energy can be obtained by varying either the radiator thickness, or the thickness of an absorber layer between radiator and TSEE dosimeter.
Integrating solid-state dosimeters based on the effect of stimulated exoelectron emission have an extremely thin effective sensitive layer (for recent reviews, see refs. 1,2). They are, therefore, eminently suitable for the measurement of short-range types of radiation (such as tritium beta radiation), or low energy ionizing particles, and measurements of steep radiation gradients, for example at interfaces. Unfortunately, organic materials with a high hydrogen content, for which a relatively high fast neutron response could be expected, exhibit little if any exoelectron emissiona'4). However, as we reported previouslyS'6), use can be made of the response of inorganic TSEE dosimeters with a low inherent fast neutron sensitivity to fastneutron induced recoil protons from external "radiators" for neutron dosimetry in mixed radiation fields. This has, for example, been accomplished by covering the front face of one detector with a polyethylene layer, while that of a second one is covered with a nonhydrogenous low-Z material such as graphite or Teflon. Submersion in hydrogenous liquids and gases 'would serve the same purpose. If thermally stimulated exoelectron emission (TSEE) from the surface of ceramic BeO is used as a detector system, the difference between the reading of the two detectors increases slightly with increasing neutron energy from 18% of the g a m m a radiation response on a tissue rad/rad basis at 0.1 MeV, to 28% at 17 MeV 7). This surprisingly flat response indicated that the thin sensitive layer of the TSEE detector responds, in essence, only to the constant-LET "stern" of the recoil proton tracks, fairly independent of their total range iin the detector material. The lower practicable energy
limit of the system has not yet been established, but we estimate it to be less than 10-50 keV. We are currently investigating the possiblility of extending the range to lower energies by covering a small fraction of the TSEE dosimeter with a material such as LiF (TED-100) or LizB407, whose constituents undergo (n,e) reactions with the thermalized and backscattered neutrons from the human body. The signal from the neutron-induced c~ particles is accumulated along with that from the recoil protons. By proper balancing the respective contributions of recoil and albedo mechanisms to the total measured TSEE signal, it is hoped to obtain a smooth energy response over a large part of the neutron spectrum at least for
*
Research sponsored by the U.S. Atomic Energy Commission under contract with the Union Carbide Corporation. ** I A E A Fellow, on leave from Egyptian Atomic Energy Authority.
611
Gh ABSORBER P
(n,a) MATERIAl
THORAX ~BeO
z
TEFLOn
2 co bJ rr
F--
"'•BEDO
(n, ~) RESPONSE ~
RECOIL PROTON RESPONSE
)% /
!
/
~-,~
/
LOG NEUTRON ENERGY
Fig. I. Schematical diagram of TSEE neutron dosimeter in which fast neutrons are detected via recoil proton registration, and low-energy neutrons via (n,~) reactions in an albedo thermal neutron detector covering a small fraction of the sensitive surface.
612
K. B E C K E R A N D M. A B D - E L R A Z E K
frontal neutron incidence on a detector worn on the surface of the thorax. The thermal neutron sensitivity of Ei or B covered detectors exceeds the fast neutron sensitivity by about two orders of magnitude. The principle is illustrated in fig. 1. There is apparently no decrease in sensitivity with increasing neutron energy, if the radiator thickness is sufficient for establishing recoil proton equilibrium. Fast neutron doses of less than one mrad can be measured with detectors having 1 cmz effective surface area. With its interesting energy response and high sensitivity, the technique has also been studied in other laboratories in recent years (see, for example, refs. 8-10). Ceramic BeO discs 12.5mm in diameter and ~0.16 m m thick, manufactured by the Brush Beryllium Company in Elmore, Ohio (USA) under the trade name Thermalox 995, have been used in these experiments. They were sensitized and stabilized by a two-day 1450°C heat treatment, followed by exposure to H 2 0 as described earlier11), and evaluated in a modified version lz) of the O R N L read-out instrument. The exposure levels of the detectors (~0.01-0.3 rad), and the heating-rate (2°C/sec) were chosen to minimize errors due to counting statistics, background, or deadtime losses. Typical integral counts were about 104 tO 105 in the main BeO peak area (~250-350°C), maximum count-rates ~ 103 cps. With frequent calibration of the detectors and corrections for differences in their individual sensitivities, the accuracy of the individual measurement was about cr = 3-5%. Each data point in the graphs represents the average of 2 6 dosimeter readings. Pairs of detectors covered with ~1.5 mm thick Teflon and polyethylene, respectively, were used unless specified otherwise. Care was taken to press the detector surface only gently against those materials which had been cleaned carefully to prevent contamination and spurious signals. Also, the detectors were handled with great caution to avoid tribo-signals. Because the fast neutron effect is obtained as a difference between two readings, good precision and sensitivity of the measurement, as well as a high proton to gamma radiation (or high LET to low LET) response ratio of the detector material are desirable. Although some materials are known 5' lo) to exhibit a ratio higher than that of BeO, this material was selected because it is relatively reliable standard material for TSEE dosimetry which has been well studied in recent years. If exposed to 14 MeV neutrons from a typical (d,T) source, the low gamma background results in a response ratio of the polyethylene to the Teflon-covered detector which is about 3-4, depending somewhat on the irradiation geometry. For the fast fission spectrum
of the Health Physics Research Reactor (HPRR) with a n/7 tissue dose ratio of ~ 7, this ratio is about two. Consequently, with its present limitations of LET response and the accuracy of individual measurements, careful attention to detector reproducibility is required when using this method in mixed radiation fields in which the n/7 dose ratio (in tissue rads) becomes one or less. Another complicating factor is the angular response of single, radiator-covered detectors. As can be seen in fig. 2, the sensitivity drops rapidly when approaching or exceeding an angle of neutron incidence of 90 ° (frontal exposure = 0°). This directional dependence becomes more pronounced with decreasing neutron energy. A simple modification, however, reduces this undesirable directional response. The hydrogenous radiator is sandwiched by two BeO disks. Both disks are both evaluated by reading out the side facing the radiator, and the differences between each detector reading and that of the Teflon-covered one are added. As can be seen in fig. 3, the resulting fast neutron response is quite directionally independent for higher neutron energies, but still exhibits a dip around 90 ° for fission neutrons. The response ratio between the two detectors gives an indication of the primary direction of non-isotropic neutron incidence as illustrated schematically in
Fig. 2. Directional response of a ceramic BeO disk TSEE dosimeter, covered with a thick polyethylene recoil proton radiator, for three different neutron energies.
CHARACTERISTICS
fig. 4. Naturally, the effective directional response of these detectors becomes more pronounced if they are attached to a thorax phantom. In case of 14 MeV neutrons, the response is reduced to 55% if the neutron beam has to traverse 20 cm of water (free-air exposure = 100%), and for H P R R fission neutrons traversing 20 cm of water, the detector at the back of the phantom indicates only ,-,6% of the frontal response (the response is ~ 7 0 % for the detector on the side of the phantom). A TSEE fast neutron dosimeter can also be easily
\
transformed into a "poor man's fast neutron spectrometer". The method somewhat resembles the well known filter-analytical evaluation of photon radiation in photographic film dosimetry. It is based on the strong neutron energy dependence of the recoil proton range and consequently of the radiator thickness which is required to establish recoil proton equilibrium. With 14 MeV neutrons, for example, more than 2 m m of polyethylene are required to establish maximum recoil proton response, while only ~ 0 . 3 mm are necessary for the H P R R fission spectrum (fig. 5). If a RADIATOR THICKNESS (#m)
40° 225 °
~oo
<>S "o~
613
OF TSEE D O S I M E T E R S
1000
0
~
2000
3000
t00
)35:
LJ co Z
o
8o
g
6o
z
40
d
2o
,
/
/
HPRR.F]SSION NELITRONS
;PE-
40--
270 °
/~/
c~ 0
TEFLON"
"
O
IOO 200 RADIATOR THICKNESS (~.ml
"
3OO
15OO
Fig. 5. Relative fast neutron response of T S E E dosimeters as a function of the thickness of the hydrogenous radiator (polyethylene) layer for two different neutron spectra.
/ 315 °
/
I
Fig. 3. Directional response of a composite T S E E fast neutron dosimeter, consisting of two detectors sandwiching a thick radiator. °z
100
\
PEj
~-BeO
/ I co
PE [~BeO
n
o o ' ~ V
50
B/' l u uJ cd
'A
;-
I
90 °
.........
j~ ill 0
0
90 DIRECTION OF NEUTRON INCIDENCE (deg)
t80
Fig. 4. Schematical presentation of the directional dependence of the recoil proton response of the two detectors indicated in fig. 3.
O
500 IOOO ABSORBER THICKNESS {H.m)
t5OO
Fig. 6. Relative response of T S E E dosimeters to fast neutrons as a function of the thickness of Teflon or aluminum absorber layers between a thick hydrogenous radiator and the detector.
614
K. B E C K E R A N D M. A B D - E L
sequence of radiators with different thicknesses are placed between the detector and a non-hydrogenous medium, the shape of the resulting curve also indicates whether the neutrons are monoenergetic or not, a wide energy distribution such as in the fission spectrum resulting in a less steep rise of the curve (fig. 5). A slight modification of this method consists of an arrangement in which absorbers of varying thickness are placed between a thick recoil proton radiator and the detector. As an illustration, it can be seen in fig. 6 that either Teflon or aluminum serve this purpose (high Z absorber foils would have advantages for relativistic neutron energies). Although directiondependent, the effective absorber thickness increasing, of course, as the angle of incidence deviates substantially from normal, either method should be of interest whenever a fast, easy and inexpensive determination of effective fast neutron energies is desirable, for instance around accelerators. Further results of our group concerning high dose-level neutron measurements, a more complex system for the determination of the direction of neutron incidence13), dosimetric studies at neutron-irradiated interfaces such as bonetissueS4), etc., will lze reported elsewhere.
RAZEK
The authors appreciate the assistance of R. L. Shoup in the 14 MeV irradiations, and discussions on the subject of TSEE fast neutron dosimetry with F. H. Attix, R. B. Gammage, and F. F. Haywood. References 1) K. Becker, Crit. Rev. Solid-State Sc. 3, (1972) 39. ~) K. Becker, Chapter 3 in "'Solid-State D o s i m e t r y " , C R C Press (1973). :~) (3. Arabin, W. Kriegseis, and_ A. S c h a r m a n n , Z. Naturf. 27a (1972) 1378. 4) D. D. Peterson, pers. c o m m . (1972). 5) K. Becker, Proceed. Sec. lnternat. Conf. Luminescence Dosimetry (J. A. Auxier, K. Becker, a n d E. M. Robinson. ed.), CONF-680920, p. 205 (1968). 6) K. Becker, U.S. Patent 3.484.601 (filed 1968). 7) K. Becker and. K. W. Crase, Nucl. Instr. Meth. 82 (1970) 297. s) W. Rasp and V. Siegel, Proceed. First Symp. Neutron dosimetry in biology and medicine, EUR-4986, Vol. 1, (1972) p. 383. '~) R. F. Laitano, a n d E. Rotondi, ibid. (1972) p. 395. J0) V. H. Ritz a n d F. H. Anix, Appl. Phys. Lett. to be published (1973). 11) K. Becket et al., in: Advances in Physical a n d Biologic. Radiat. Detect., IAEA Vienna, (1970) p. 25. 1~) R. B. G a m m a g e and J. S. Cheka, to be published. I3) M. H. Lee et al., to be published. 14) K. Becker, to be published.
INSTRUMENTS
AND
METHODS
113
(I973) 611-614;
©
NORTH-HOLLAND
PUBLISHING
CO.
F A S T - N E U T R O N R E S P O N S E C H A R A C T E R I S T I C S OF TSEE D O S I M E T E R S * K L A U S B E C K E R and M. A B D - E L
RAZEK**
Health Physics Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37830, U.S.A. Received 2 July 1973 Further applications are described of the previously reported technique of fast neutron dosimetry with combinations of hydrogenous radiators and ceramic BeO, to be evaluated by thermally stimulated exoelectron emission (TSEE). For example, a pronounced directional response has been minimized with a
dosimeter-radiator-dosimeter sandwich; and information on the effective neutron energy can be obtained by varying either the radiator thickness, or the thickness of an absorber layer between radiator and TSEE dosimeter.
Integrating solid-state dosimeters based on the effect of stimulated exoelectron emission have an extremely thin effective sensitive layer (for recent reviews, see refs. 1,2). They are, therefore, eminently suitable for the measurement of short-range types of radiation (such as tritium beta radiation), or low energy ionizing particles, and measurements of steep radiation gradients, for example at interfaces. Unfortunately, organic materials with a high hydrogen content, for which a relatively high fast neutron response could be expected, exhibit little if any exoelectron emissiona'4). However, as we reported previouslyS'6), use can be made of the response of inorganic TSEE dosimeters with a low inherent fast neutron sensitivity to fastneutron induced recoil protons from external "radiators" for neutron dosimetry in mixed radiation fields. This has, for example, been accomplished by covering the front face of one detector with a polyethylene layer, while that of a second one is covered with a nonhydrogenous low-Z material such as graphite or Teflon. Submersion in hydrogenous liquids and gases 'would serve the same purpose. If thermally stimulated exoelectron emission (TSEE) from the surface of ceramic BeO is used as a detector system, the difference between the reading of the two detectors increases slightly with increasing neutron energy from 18% of the g a m m a radiation response on a tissue rad/rad basis at 0.1 MeV, to 28% at 17 MeV 7). This surprisingly flat response indicated that the thin sensitive layer of the TSEE detector responds, in essence, only to the constant-LET "stern" of the recoil proton tracks, fairly independent of their total range iin the detector material. The lower practicable energy
limit of the system has not yet been established, but we estimate it to be less than 10-50 keV. We are currently investigating the possiblility of extending the range to lower energies by covering a small fraction of the TSEE dosimeter with a material such as LiF (TED-100) or LizB407, whose constituents undergo (n,e) reactions with the thermalized and backscattered neutrons from the human body. The signal from the neutron-induced c~ particles is accumulated along with that from the recoil protons. By proper balancing the respective contributions of recoil and albedo mechanisms to the total measured TSEE signal, it is hoped to obtain a smooth energy response over a large part of the neutron spectrum at least for
*
Research sponsored by the U.S. Atomic Energy Commission under contract with the Union Carbide Corporation. ** I A E A Fellow, on leave from Egyptian Atomic Energy Authority.
611
Gh ABSORBER P
(n,a) MATERIAl
THORAX ~BeO
z
TEFLOn
2 co bJ rr
F--
"'•BEDO
(n, ~) RESPONSE ~
RECOIL PROTON RESPONSE
)% /
!
/
~-,~
/
LOG NEUTRON ENERGY
Fig. I. Schematical diagram of TSEE neutron dosimeter in which fast neutrons are detected via recoil proton registration, and low-energy neutrons via (n,~) reactions in an albedo thermal neutron detector covering a small fraction of the sensitive surface.
612
K. B E C K E R A N D M. A B D - E L R A Z E K
frontal neutron incidence on a detector worn on the surface of the thorax. The thermal neutron sensitivity of Ei or B covered detectors exceeds the fast neutron sensitivity by about two orders of magnitude. The principle is illustrated in fig. 1. There is apparently no decrease in sensitivity with increasing neutron energy, if the radiator thickness is sufficient for establishing recoil proton equilibrium. Fast neutron doses of less than one mrad can be measured with detectors having 1 cmz effective surface area. With its interesting energy response and high sensitivity, the technique has also been studied in other laboratories in recent years (see, for example, refs. 8-10). Ceramic BeO discs 12.5mm in diameter and ~0.16 m m thick, manufactured by the Brush Beryllium Company in Elmore, Ohio (USA) under the trade name Thermalox 995, have been used in these experiments. They were sensitized and stabilized by a two-day 1450°C heat treatment, followed by exposure to H 2 0 as described earlier11), and evaluated in a modified version lz) of the O R N L read-out instrument. The exposure levels of the detectors (~0.01-0.3 rad), and the heating-rate (2°C/sec) were chosen to minimize errors due to counting statistics, background, or deadtime losses. Typical integral counts were about 104 tO 105 in the main BeO peak area (~250-350°C), maximum count-rates ~ 103 cps. With frequent calibration of the detectors and corrections for differences in their individual sensitivities, the accuracy of the individual measurement was about cr = 3-5%. Each data point in the graphs represents the average of 2 6 dosimeter readings. Pairs of detectors covered with ~1.5 mm thick Teflon and polyethylene, respectively, were used unless specified otherwise. Care was taken to press the detector surface only gently against those materials which had been cleaned carefully to prevent contamination and spurious signals. Also, the detectors were handled with great caution to avoid tribo-signals. Because the fast neutron effect is obtained as a difference between two readings, good precision and sensitivity of the measurement, as well as a high proton to gamma radiation (or high LET to low LET) response ratio of the detector material are desirable. Although some materials are known 5' lo) to exhibit a ratio higher than that of BeO, this material was selected because it is relatively reliable standard material for TSEE dosimetry which has been well studied in recent years. If exposed to 14 MeV neutrons from a typical (d,T) source, the low gamma background results in a response ratio of the polyethylene to the Teflon-covered detector which is about 3-4, depending somewhat on the irradiation geometry. For the fast fission spectrum
of the Health Physics Research Reactor (HPRR) with a n/7 tissue dose ratio of ~ 7, this ratio is about two. Consequently, with its present limitations of LET response and the accuracy of individual measurements, careful attention to detector reproducibility is required when using this method in mixed radiation fields in which the n/7 dose ratio (in tissue rads) becomes one or less. Another complicating factor is the angular response of single, radiator-covered detectors. As can be seen in fig. 2, the sensitivity drops rapidly when approaching or exceeding an angle of neutron incidence of 90 ° (frontal exposure = 0°). This directional dependence becomes more pronounced with decreasing neutron energy. A simple modification, however, reduces this undesirable directional response. The hydrogenous radiator is sandwiched by two BeO disks. Both disks are both evaluated by reading out the side facing the radiator, and the differences between each detector reading and that of the Teflon-covered one are added. As can be seen in fig. 3, the resulting fast neutron response is quite directionally independent for higher neutron energies, but still exhibits a dip around 90 ° for fission neutrons. The response ratio between the two detectors gives an indication of the primary direction of non-isotropic neutron incidence as illustrated schematically in
Fig. 2. Directional response of a ceramic BeO disk TSEE dosimeter, covered with a thick polyethylene recoil proton radiator, for three different neutron energies.
CHARACTERISTICS
fig. 4. Naturally, the effective directional response of these detectors becomes more pronounced if they are attached to a thorax phantom. In case of 14 MeV neutrons, the response is reduced to 55% if the neutron beam has to traverse 20 cm of water (free-air exposure = 100%), and for H P R R fission neutrons traversing 20 cm of water, the detector at the back of the phantom indicates only ,-,6% of the frontal response (the response is ~ 7 0 % for the detector on the side of the phantom). A TSEE fast neutron dosimeter can also be easily
\
transformed into a "poor man's fast neutron spectrometer". The method somewhat resembles the well known filter-analytical evaluation of photon radiation in photographic film dosimetry. It is based on the strong neutron energy dependence of the recoil proton range and consequently of the radiator thickness which is required to establish recoil proton equilibrium. With 14 MeV neutrons, for example, more than 2 m m of polyethylene are required to establish maximum recoil proton response, while only ~ 0 . 3 mm are necessary for the H P R R fission spectrum (fig. 5). If a RADIATOR THICKNESS (#m)
40° 225 °
~oo
<>S "o~
613
OF TSEE D O S I M E T E R S
1000
0
~
2000
3000
t00
)35:
LJ co Z
o
8o
g
6o
z
40
d
2o
,
/
/
HPRR.F]SSION NELITRONS
;PE-
40--
270 °
/~/
c~ 0
TEFLON"
"
O
IOO 200 RADIATOR THICKNESS (~.ml
"
3OO
15OO
Fig. 5. Relative fast neutron response of T S E E dosimeters as a function of the thickness of the hydrogenous radiator (polyethylene) layer for two different neutron spectra.
/ 315 °
/
I
Fig. 3. Directional response of a composite T S E E fast neutron dosimeter, consisting of two detectors sandwiching a thick radiator. °z
100
\
PEj
~-BeO
/ I co
PE [~BeO
n
o o ' ~ V
50
B/' l u uJ cd
'A
;-
I
90 °
.........
j~ ill 0
0
90 DIRECTION OF NEUTRON INCIDENCE (deg)
t80
Fig. 4. Schematical presentation of the directional dependence of the recoil proton response of the two detectors indicated in fig. 3.
O
500 IOOO ABSORBER THICKNESS {H.m)
t5OO
Fig. 6. Relative response of T S E E dosimeters to fast neutrons as a function of the thickness of Teflon or aluminum absorber layers between a thick hydrogenous radiator and the detector.
614
K. B E C K E R A N D M. A B D - E L
sequence of radiators with different thicknesses are placed between the detector and a non-hydrogenous medium, the shape of the resulting curve also indicates whether the neutrons are monoenergetic or not, a wide energy distribution such as in the fission spectrum resulting in a less steep rise of the curve (fig. 5). A slight modification of this method consists of an arrangement in which absorbers of varying thickness are placed between a thick recoil proton radiator and the detector. As an illustration, it can be seen in fig. 6 that either Teflon or aluminum serve this purpose (high Z absorber foils would have advantages for relativistic neutron energies). Although directiondependent, the effective absorber thickness increasing, of course, as the angle of incidence deviates substantially from normal, either method should be of interest whenever a fast, easy and inexpensive determination of effective fast neutron energies is desirable, for instance around accelerators. Further results of our group concerning high dose-level neutron measurements, a more complex system for the determination of the direction of neutron incidence13), dosimetric studies at neutron-irradiated interfaces such as bonetissueS4), etc., will lze reported elsewhere.
RAZEK
The authors appreciate the assistance of R. L. Shoup in the 14 MeV irradiations, and discussions on the subject of TSEE fast neutron dosimetry with F. H. Attix, R. B. Gammage, and F. F. Haywood. References 1) K. Becker, Crit. Rev. Solid-State Sc. 3, (1972) 39. ~) K. Becker, Chapter 3 in "'Solid-State D o s i m e t r y " , C R C Press (1973). :~) (3. Arabin, W. Kriegseis, and_ A. S c h a r m a n n , Z. Naturf. 27a (1972) 1378. 4) D. D. Peterson, pers. c o m m . (1972). 5) K. Becker, Proceed. Sec. lnternat. Conf. Luminescence Dosimetry (J. A. Auxier, K. Becker, a n d E. M. Robinson. ed.), CONF-680920, p. 205 (1968). 6) K. Becker, U.S. Patent 3.484.601 (filed 1968). 7) K. Becker and. K. W. Crase, Nucl. Instr. Meth. 82 (1970) 297. s) W. Rasp and V. Siegel, Proceed. First Symp. Neutron dosimetry in biology and medicine, EUR-4986, Vol. 1, (1972) p. 383. '~) R. F. Laitano, a n d E. Rotondi, ibid. (1972) p. 395. J0) V. H. Ritz a n d F. H. Anix, Appl. Phys. Lett. to be published (1973). 11) K. Becket et al., in: Advances in Physical a n d Biologic. Radiat. Detect., IAEA Vienna, (1970) p. 25. 1~) R. B. G a m m a g e and J. S. Cheka, to be published. I3) M. H. Lee et al., to be published. 14) K. Becker, to be published.