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Method for radioassay of migratory alpha-particle emitters
Nuclear Instruments & Methods in Physics Research
Nuclear Instruments and Methods in Physics Research B66 (1992) 379-387 North-Holland
Section B
Method for radioassay of migratory alpha-particle emitters A.J . Howard
Department of Physics, Trinity College, Hartford, CT 06106, USA
Received 5 September 1991 and in revised form 22 November 1991 An electrostatic precipitation method is described for the individual alpha-particle activities associated with radionuclidic decays that trace their origin to sites in the surface layers of solid matter . The energy resolution is sufficient to resolve all of the detected, major alpha-particle groups of the identifying member(s) of the uranium, thorium, and actinium series. The 12% absolute efficiency associated with the device allows determination of individual migration rates from surface layers of solid materials of 10 -4 atom/s, which corresponds to detection of one alpha-particle per day, to be made with negligible background contributions: This efficiency is independent of the material's surface area over a wide range. The background in the detector, presumably produced by cosmic rays, is not found to be measurably dependent upon the orientation of the detector with respect to the cosmic-ray flux . Ambiguities -ssociated with the birthplaces of migratory radon atoms are discussed . 1 . Introduction
studies and are a major limit on the sensitivities associated with current experiments . Focussing on the DBD experiments, a prime nuclear species particularly amenable to experimental scrutiny is 1a6Xe, which chemically is the heaviest stable noble gas . The combined target/detector systems associated with these experiments involve a confinement chamber, and so the (metal) chamber and its detection components are necessary sources of background caused by heavy radionuclides, which are members of the so-called thorium, neptunium, uranium,
There are several experimental areas in fundamental nuclear physics that involve searches for rare nuclear events. Two examples of these involve studies on the possible existence of superheavy elements (SHE) [1,21 and on the strength of the weak interaction as regards double beta decay (DBD) [3-61. In both of these cases, background events caused by naturally occurring radionuclides, particularly those with atomic mass A >_ 208, have proven to be problematic in these zzz
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URANIUM THORIUM ACTINIUM SERIES Fig . 1 . Portions of the uranium, thorium, and actinium series of particular relevance to the present study. The nuclear half-life associated with each species is located at the bottom of its box . The relative alpha (a) and beta (0) branch intensities for 211 13i are given in parentheses as percentages . Alpha-particle transition energies are given in keV. 0168-583X/92/$05 .00 0 1992 - Elsevier Science Publishers B.V . All rights reserved
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A .J. li(nvarlI ,l Radioassn3~ of nrigratoKy a-parti,1, emitter.,
and actinium series [7] . These series arc sources of well-identified alpha-, beta-, and gamma-ray transitions [7,8], and members of the three series which are of particular pertinence to the present study arc illustrated in the partial series diagrams shown in fig . 1 . In the construction, execution, and interpretation of these temporally long experiments, it is crucial to understand the nature of this background source in detail, and it is the purpose of this paper to describe a method which should prove of diagnostic utility in current and future SHE and DBD experiments as well as other applications where a detailed quantitative knowledge of heavy radionuclidic content is desired . The method described in this paper evolved from previous measurements involving determinations of air-borne === ltn concentrations [9] and neutralization rates of certain heavy ions in air, argon, and helium [10,11]. Although the system employed is simple, achievable results arc characterized by high sensitivity and high energy resolution . The technique can be applied to identify background contributions from individual components of the detection chamber as well as the assembled device, the latter being the case when the active clement of the current chamber (a surfacebarrier detector) is integrated into the design of future devices for in situ observations . The method involves electrostatic precipitation of radionuclidic ions that arc formed in a hydrostatic carrier gas (helium at atmospheric pressure in the current study) contained in a chamber (a hollow aluminum cylinder with dimensions comparable to those employed in typical DBD experiments [4,5] involving 1 'r'Xe). The precipitation is made onto the surface of a surface-barrier detector, and decay alpha particles are concurrently energy analyzed with an 80-kcV energy resolution that is sufficient to isolate virtually all of the observed alpha-particle transitions associated with those displayed in fig. 1 . Because the nuclear decay schemes associated with members of these radioactive series are very well known [8,12], the observed alphaparticle spectra may also be employed to deduce individual beta- and gamma-ray fluxes within the interior of the chamber as generated by radionuclidic decay originating in the detection volume itself. The time-dependence (field-on-field-off) inherent to the electrostatic precipitation process onto the surface-barrier detector also allows isolation of bacl . ground in the active volume of the detector itself, including that induced by thermal neutrons and/or cosmic rays. The cosmic-ray contribution involving the detector is scrutinized by a variation of the orientation of the 100-Run thickness, 4 .5-cm 2 area active silicon volume with respect to the vertical direction . Measurements of this type (with zero electric field) allow determination of the ultimate sensitivity inherent to the system.
The apparatus employed and the procedural details are presented and explained in section 2. The measurements themselves are performed to ascertain the information as described above and are summarized in section 3 . The experimental findings arc presented in section 4, which is followed by absolute calibration and sensitivity information in sections 5 and 6, respectively. Conclusions drawn from this study are then highlighted in section 7 . 2. Apparatus and procedures The cylindrical chamber employed for the current measurements has been described previously [9] and is shown schematically in fig . 2. Of particular importance to the present considerations are the composition and dimensions of this chamber. The chamber itself is composed of standard grade aluminum and has 0.93-cm wall thickness for the 76.5-cm length cylindrical shell portion and 1 .0-cm thickness for each of the two 32-cm diameter cylindrical end plates. The gas volume enclosed is 38 .2 I, and the total internal area is - 7200 cm` . A silicon surface-barrier detector having a thickness of 100 Wm and an active area of 4 .5 CMZ is employed as the combined collector/detector : It is electrically isolated at the location shown in fig . 2 so that both bias ac :nss the active volume and (negative) precipitation voltage may be concurrently applied. This device was previously calibrated [9,10] by measure-
Fig. 2. Cross-sectional view of the aluminum chamber . During electrostatic precipitation, the combined collector/detector is constant at -1000 V inside the grounded chamber. O-ring assemblies at both ends of the cylindrical shell are not shown.
A.J. Howard / Radioassay ofmigratory a-particle emitters
ments involving 222Rn of known activity contained in various carrier gases, including helium . Such calibrations involved passage of the carrier gas through one of two liquid bubbler sources containing 3 .24- and 3.76nCi 221Ra activities. The emerging gas passed through a filtration tube containing CaS04 crystals (a desiccant) and glass wool into the chamber, which was initially evacuated and also contained -0.1 kg of CaS04 crystals for continuous desiccant action during the calibration measurements. Determinations of equilibrium counting rates for the 6003-keV 2 '"Po and 7687-keV 214Po alpha-particle transitions shown in fig. 1 were made as a function of the applied collection voltage . The respective absolute efficiencies associated with the aforementioned transitions, namely e, and e 2 , represent 100% multiplied by the ratio of the pertinent observed equilibrium counting rate to the known activity of the 222 Rn dispersed throughout the carrier gas in the chamber . Numerical values for e, and e 2 for the current situation as obtained from the results of these previous studies [9,10] are presented in section 5 . It was during these numerous previous calibration procedures that an unknown amount of 226Ra may have inadvertently been transferred from a bubbler into the chamber . Immediately prior to the present study, this chamber was completely disassembled and all internal surfaces were cleaned by sanding with emery paper and wiping with ethanol . This rudimentary procedure was carried out primarily to remove any -'Ra atoms which may have been transferred into the chamber from 222 Rn bubblers employed in the previous investigations [9-11] : The chamber had not been exposed to equivalent 2'9Rn or 22"Rn sources. The gas-handling system and entry ports of the chamber were entirely replaced with larger bore (2 .54-cm diameter) components throughout, which resulted in a radioactively clean system and greatly reduced pumping resistance as compared to the original version [9] : A new molecular drag pump station was employed for the evacuation procedure. Prior to reassembly of the chamber, electrical feedthroughs and connections involving the surface-barrier detector (see fig . 2) were replaced . Thus conventional (not extraordinary) steps were taken to reduce the radionuclidic content associated with all the surfaces involved in the operation of the experiments described herein with the exception of the surface-barrier detector, whose external surfaces were cleaned by a helium jet . This Si(SB) detector used in the current study [13] had been employed in one previous study [11] . It was reemployed (rather than replaced) because its previous exposure to 222Rn decay products resulted in the presence of a 210Pb source (see fig. 1) located within the surface layers that produces 5305-keV alpha-particles with ideal activity for a continuously operative calibration source during the measurements to be described : This alpha-particle en-
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ergy is just below the major region of interest involved in the present study. Having reassembled the system, the chamber was immediately evacuated to a pressure p - 10 -fi atm and flushed twice with helium of unmeasured radionuclidic content . It was then filled with helium to p = 1 .1 atm . No drying agents (such as CaS0 4 crystals or PROs powder employed in previous measurements [9-11]) were employed because of their potential for radionuclidic contamination of the helium gas. The selection of helium as the carrier gas was primarily based on the relatively high mobility of atomic ions in this particular inert gas [14] and the associated high collection efficiency observed for it [10]. In particular, previous results [9,10] indicate that the ion-collection efficiency is fully saturated in this monopole device when the precipitation voltage has the value (-1000 V) employed in the current study. As discussed in section 6 of this paper, the current results indicate that the helium gas is not the major source of heavy radionuclides detected in the present study. The orientation and location of the detector within the chamber is shown in fig . 2 : The axes of revolution for the detector and chamber are colinear, with the exposed active area of the former facing downward and at a 12 .5-cm distance from the inner surface of the chamber's upper end . The detector is fixed inside the chamber, and so its orientation is determined by the orientation of the entire chamber. Bias for the electrically-isolated detector is applied via an externally located 45-V battery in series with a 22-Mil current-limiting resistor . This underbiasing of the detector (maximum depletion occurs for - 100-V bias) reduces spurious electrical bursts to a negligible level and energy depositions by penetrating high-energy (cosmic-ray) particles . The design of the detector is such that the entire external area (active area and mount) are at the same potential . The precipitating electric field is controlled by a standard nuclear instrumentation module high-voltage power supply whose output is fed through the preamplifier line as indicated in fig. 2. Conventional electronics were employed to carry out the pulse-height analyses described in section 3. Microphonic noise caused by sporadic mechanical vibrations in the building were observed to generate spurious electronic signals during preliminary measurements, and these extraneous pulses were eliminated by mounting the entire system shown in fig . 2 and also the preamplifier on 5-cm thickness foam padding . 3 . Measurements Having filled the chamber with helium gas to p = 1 .1 atm at time t --- 0, this device was located in a quasibasement room (approximately half of which is below
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A.J. Howard / Radioassay of migratory a-particle emitters
the earth's surface) at 41°N latitude with orientation such that the exposed active area of the detector faced directly towards the center of the earth. Other than the shielding provided by the building and the segment of surrounding earth, no shielding materials were employed in the ensuing measurements. The sequence of measurements; and associated alpha-particle activities are summarized in table 1 . It started with a six-day continuous run commenced at time t( ,=24 h during which the precipitation voltage, HV, was held constant
at -1000 V. Signals from the surface-barrier detector were sent through a simple, standard preamplifier-lincar-amplifier-multichannel-analyzer (MCA) series arrang^ .ment: No pulse-shape discrimination was performed. The MCA was operated in the pulse-height analysis mode, the dispersion being45 keV/channel so that the energy range covered in the 2048-channel memory segment was -0.5 to 23 MeV. Intermediate readouts were performed during this first measurement for the purpose of establishing any time dependences associated either with growth behavior intrinsic to the radioactive series involved or changes in count rates due to (unwanted) sources such as inward leakage of air. A spectrum illustrating the data obtained over the final 70-h segment of this 141-h measurement is shown in fig. 3, where identifications of individual alpha-particle groups is made with the information summarized in fig. 1.
The second measurement in the sequence was identical to that just described except that HV =0 was involved, which results in the condition of no electrostatic precipitation . This measurement was started at t = 166 h and was carried out continuously (except for
100 He HV = -1,000V At=70h
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tated in the first measurement have decayed, is illustrated in fig . 4, where it is clear that the only definitely discernible radionuclidic activity is that associated with 21oPo
the long-lived activity that serves as a continuously operative calibration source that was introduced into the surface layers of the detector during previous
Table I Summary of alpha-particle counting rates observed in the present study ID ` 1 1 1 2 2 2 3 3 3 3 4 4 4 5 5 °) b)
HV (V) -11000 -11000 - IIOw 0 0 0 -11000 -11000 -11000 -11000 0 0 0 -11000 -11000
9
Fig. 3. Semilogarithmic plot of a 70-h spectrum obtained with HV = - 1000 V commencing at t() = 45 h. Energy identifications are in keV and correspond to transitions illustrated in fig. 1. The 5?1,15-keV line results from a previous ion implantation involving the uranium series, and locations of the unob served radon transitions are indicated with arrows. Orientation of the detector is as shown in fig . 2.
t (1 (h)
At (h)
Counts of alpha particles /At (h -1 ) for four transitions 5305( 2 .0 po) h) 6003(2'"Po) 7687(21° Po)
8784( 212 Po)
24 45 116 166 190 213 266 284 308 331 379 427 502 549 597
21 70 50 23 22 48 18 24 23 48 47 49 46 47 52
1 .37±0.26 0.92±0.11 1 .08±0.15 1 .15±0.22 0.85+0.20 1 .02±0.14 0.91 ±0 .23 1 .27+0.26 0.26 1 .25±0.26 1 .24±0.17 1 .20±0.19 1 .65+0.18 1 .67+0.19 1 .43+0.17 1 .47±0.17
0.14±0.08 0.20+0r5 0.l8±0 .05 0 .18±0 .09 0 0 0.28±0.12 0.21±0 .10 0.22±0 .10 0.19±0.06 0.06±0.04 0 0 0.11±0.05 0.21±0.06
0.74±0.19 0.73±0.10 0.98±0.14 0 0 0 1.36±0.35 1.56±0.26 1.55±0.26 1 .39±0.17 0.06±0.04 0 0 1 .24±0.16 1 .47±0.17
Measurement identification number. Alpha-particle energy in keV and between parentheses the associated radionucleus .
0.64±0.18 0.90±0.11 1 .32±0.16 0 0 0 2.05±0 .33 2.15±0 .31 2.20±0 .31 1 .70±0.19 0 .02±0.02 0 0 1 .79±0.19 1 .59±0.17
A.J. Howard / Radioassay of migratory a-particle emitters 100
383
transitions which represent those for the uranium series in transient equilibrium. The fourth measurement was commenced immedi-
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ately before the measurement just described and was intended to probe the orientational effects associated with background events in the surface-barrier detector .
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4. Experimental results
Fig. 4. A spectrum similar to that shown in fig. 3, except the illustrated segment was taken with HV = 0 24 h after its establishment at t = 166 h.
The counting rates summarized in table 1 during each of the growth periods (the first and third mea-
measurements [11] : the energy region of interest for the current method (- 6 to 9 MeV) is essentially
clidic series illustrated in fig . I are all consistent with the assignments made in fig. 3, as are observations
devoid of events . The results of the above two measurements as summarized in figs . 3 and 4 as well as table 1 establish the
taken over finer time grids (not recorded in table 1) at the outset of these four individual measurements. It therefore appears that all of these identifications are
that follow migration of members of the associated series from the surface layers of the chamber material
being within ±20 keV of the quoted energies for an assumed linear calibration based on the known [9] 5305-, 6003-, and 7687-keV transition locations in the spectrum displayed in fig . 3. It is noted here that the only unresolved transitions in this spectrum involve the
surements reported in section 3) as well as those for ensuing decay periods (the second and fourth measurements) associated with the portions of the radionu-
complete dominance of identified radionuclidic events
into the helium gas, a situation to be quantitatively considered in section 4. Entries in table 1 give crude,
but nonetheless clearly identifiable, information about the half-life effects associated with the time-depen-
certain, the individual locations of alpha-particle groups
6003-keV
= '"po transition and the 6051 + 6090-keV
dence of the various detected activities. However, the possible presence of time dependences due to any
`1= Bi transitions, the latter of which have 54% of the strength observed for the 8784-keV =' =po case (see fig .
operation of the uranium series involve the 3.82-d of 222 half-life 11n: Any significant residual 222 Rn in the chamber immediately after the evacuation-flush-fill cy-
previously observed [9-11] and are accounted for by
longer-term nonequilibrium situations with respect to the system requires measurements taken over a longer time span. In particular, the activities involved with
cle described in section 2 would manifest itself via a decrease in the observed activities for the 6003- as well as the 7687-keV alpha-particle transitions, whereas air leakage into the chamber would be manifested by an increase in these activities, relative to the anticipated
growth curve [7] associated with 222Rn . In order to further investigate such possible long-term time-dependent effects, the third measurement involved a repeat of the first measurement MV = -1000 V) and was
commenced at t = 266 h. Upon completion of the fourth measurement, which is described in the next paragraph, the fifth (and final) measurement involved HV = -1000 V again: It was commenced at t = 549 h, which corresponds to 6 T, 12 for 222 Rn, and so provides counting rates for the 6003- and 7687-keV
1) . Thus the peak in fig. 3 near 6000 keV is composed primarily (87%) of the lower-energy transition, the weaker higher-energy peak indeed appearing as a shoulder in that spectrum . The low-energy tails observed for all of the alpha-particle groups have been decays which occur along the epoxy seal that forms the perimeter of the exposed active area . Intercomparison of figs. 3 and 4, which involve
identical detection intervals, indicates the vast predominance of radionuclidic-decay transitions in the spectrum associated with transient equilibrium. A similar
intercomparison involving the two zero-field spectra establishes no measured difference in the nonradionuclidic background for the two orientations studied.
Because the incident cosmic-ray flux is strongly peaked in the downward direction [15], the passage of high-en-
ergy cosmic rays down through the large area (4 .5 cm2) but small thickness (100 pm) results in very small energy deposition in the detector when oriented in the position employed for the first three measurements
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A.J. Howard /Radioassay of inigratory a-particle emitters
(i.e., the orientation depicted in fig. 2). In contrast, the orientation employed for the fourth measurement offers a smaller area (- 0.02 cm') but much larger thickness ( - 2 cm) of the active area to a downwardly-directed cosmic ray. It is apparent that the region of interest in the current method involving particle energies between 6003- and 8784-keV (see fig. 3) is therefore devoid of contributions from primary cosmic rays (see fig . 4), independent of detector orientation. In further considerations of the present experimental results, especially those summarized in table 1, it is important to bear in mind the various time depcndences associated with the current study . A major attribute of the method described is the simultaneous nature of the ion-collection and detection processes, which causes the growth (and decay) of activities to be analyzed in straightforward fashion [7]. When this first time dependence is coupled with the natural ones associated with the operative members of the three radionuclidic series as summarized in fig. 1, it is apparent that a situation of tiansicnt equilibrium has been established in most of the later time intervals summarized in table 1 . In consequence, even the slight ( 20%) differences in efficiencies for some members of the same radionuclidic series as discussed in section 5 below are both understood and quantifiable . The third time dependence involves various diffusion processes, and these manifest themselves collectively as time variations in counting rates that are not manifestations of the two natural time dependences just identified . The three counting rates of major interest with respect to this third time dependence arc displayed in fig. 5 : They arc the summed values for the 6003-keV 21sPo and 7687-keV 21 ; Po members of the uranium series, the constant-activity 5305-keV 2111Po transition caused by previous [11] implantation, and the individual counting rate for the 8784-keV member of the thorium series (see ûg . 1) . In the following discussions, it is crucial to bear in mind the unique (inert-gas) character of radon that is being exploited in the experimental method, the very different lifetimes for the three pertinent radon isotopes (T1i2 = 3.82 d, 55.6 s, and 3 .96 s for 222Rn, 22° Rn and 21"kn, respectively), and also the very different lifetimes for their radium parents (T 1 2 = 1600 y, 3 .67 d, and 111 .4 d for 226 Ra, 22° Ra, and --- Ra, respecti"ely) . An extremely significant consequence of these time-dcpendenccs is associated with the observation, within statistical uncertainties, of a constant counting rate for the 8784-keV 212Po transition in the thorium series illustrated in fig. 5 . This finding implies that no significant amount of 224Ra was transferred into the chamber via the helium gas during its introduction (from the helium storage tank) at time t = 0: Any such a 224 Ra transfer would have manifested itself as a decrease in 2112 Po activity characterized by the 3.67-d
10 -6003 , 7687 o5305 .8784
rr 0
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~-HV=0-1
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01
0
100
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TIME
200 300 (HOURS)
400
Fig. 5. Semilogarithmic plot of observed alpha-particle counting rates versus time for the 400-h period commencing at time t = 0. Numeric labels in the legend correspond to alpha-particle transition energies in keV that are fully identified in table I . The delimited segment labelled HV=0 involved the non-precipitative mode of operation, and the counting rate was zero for each of the five datum points within that segment which are absent. The dashed curve represents the growth rate I'llRn predicted for 222 Rn in an isolated system: A similar plot for would indicate a constant counting rate beyond t1) =0.
half-life of the former radionuclide . Coupled with the short 220 Rn half-life and also the previous history of the chamber regarding radon exposures as given in section 2, this result gives strong evidence that the present observations involving the thorium series are associated with emanations from radionuclides that were present inside the chamber prior to t() = 0 . The time dependence observed in fig. 5 as regards members of the uranium series is consistent with the growth curve [7] shown which is characteristic of the comparatively long 222 Rn half-life (91 .8 h). The evacuation-flush-fill cycle that occurred immediately prior to time t,) = 0 of course removes all radon from the hollow region of the chamber at t, ) __ 0, and so this counting-rate behavior is expected, provided longer-term exhalation phenomena are relatively small . The results indicate an equilibrium 222Rn concentration corresponding to - 2 counts/h for the 7687-keV 214Po transition (see the appropriate entries for t >_ 266 h in table 1). Because of the long half-life for 226 Ra (1600 y), it is not possible to rule out any extraneous radon sources (such as might be present from the history of the chamber as described in section 2) by the present consideration . In consequence, the observed counting rate in this case is interpreted as representing an upper limit on the activity of the uranium series that is independent of this previous potential exposure to 22t, Ra .
A.J. Howard / Radioassay of migratory a-partie%emitters
5 . Absolute detection efficiencies The lack of alpha-particles corresponding to decay of 222 Rn in fig . 3 follows from its inert gas nature: Any such ions which are precipitated by the electric field onto the exposed active area of the detector are immediately neutralized there and return into the helium gas . Thus the activities observed in the current method are in fact only those associated with the major alphaparticle transitions as depicted after the radon transitions summarized in fig . 1 . In consequence, previous measurements [10] concerning the migration of ions formed by the alpha-particle decay of 222 Rn in helium allow a determination of the combined collection and detection efficiencies associated with the present device . The calibration procedures have been outlined in section 2 and are fully described elsewhere [9,10] . Using results [f0] for dry helium at p = 1 atm and with radon concentration [ 222 Rn] = 0, it is found that the electric fields generated in the chamber for HV = - 1000 V lead to the saturated value for heavy-ion collection and that the absolute efficiency for collection and subsequent detection of the 21 "Po ions formed by 222 Rn decay in the helium as defined in section 2 is e, --(10± 1)% . This result is about one-fifth the maximum value associated with a surface-barrier detector (when a point source is located at distance r = 0 from the surface), or one-third the value for the ideal geometry in which all of the 2"Po ions are collected onto the active area of the detector. The higher efficiency associated with the integrated = ' 4 Po activity, e 2 -- (12 ± 1)%, is consistent with the previous radon-in-air results [9] for the saturated monopole configuration employed in that as well as the present studies: It is attributed to the additional contribution of =1 "Po atoms that were neutral (or neutralized) before precipitation and subsequently reemerge into the helium as ionized decay products. The shorter half-life of =2 "Rn (T,,_ = 55 .6 s) compared to 222 Rn (T, I , = 3.82 d) is important in considerations involving the physical site of origin for the radon . Any exhalation effects [16] involving diffusion to the material's surface that contribute to the == "Rn content of the helium must occur within seconds of birth, which is not the case for 222 Rn. Nevertheless, the growth (or decay) of the radionuclides under surveillance on surfaces of materials, which is the objective of this methodology as regards the cited experiments [3-5] and similar applications, is possible for both the uranium- and thorium-series cases with the values of e, and e, presented for 222 Rn being also assumed re =="Rn. The comparative limitations presented above for the == "Rn case are even more pronounced for the = ""Rn member of the actinium series (T, /_ = 3 .96 s). The weak evidence discerned herein for its detection (the 6623-kcV alpha-particle region of fig. 3) can be
385
said ;c. be consistent with the relatively small abundance of members of the actinium series in terrestrial materials (- 1%) as compared to the uranium and thorium series, which are on average approximately equal to one another in their =='Rn and 224 Rn activities [18]. 6. Detection sensitivities The results and analyses so far presented establish the parameters and properties and demonstrate the quantitative aspects attainable with this method . As regards the radionuclidic distribution within the solid material, the associated decay ions that are precipitated out of the helium carrier gas and onto the detector in principle may originate from the surface itself, from a surface layer that is located a distance that is within the range of the recoil associated with its birth, or from deeper-lying sites from which radon diffusion to the surface is able to occur in a timely fashion (see section 5 for possible ===Rn and ""Rn half-life effects). All that can be said here is that the measured activities are clearly the sum of these three components, whose individual contributions or strengths are completely unknown. However, by its very nature, the method identifies the migratory portion of the radionuclidic content of solid matter, and it probably is this portion which ultimately introduces the largest uncertainties associated with searches for naturally occurring SHE via alpha-particle detection in gases and for DBD in 1 "Xe [5] . It is important to emphasize here that the currently-described method generally probes the heavy radionuclidic concentration of an ill-defined surface whose area is very poorly known, and so calculations of average radionuclidic content typically given in Bq/kg (1 Bq= 1 decay/s) cannot be made in any useful fashion nor arc in fact applicable to the "surface" contribution itself . The measurements performed herein employed rudimentary surface preparation which happened to produce activities of the correct order of magnitude for demonstrating the various facets of the method . The history of the chamber itself involved some undetermined additional exposure to =="Ra (via the = "Rn bubbler) prior to cleaning beyond that normally encountered in terrestrial materials [18] . Coupled with the long ==2 Rn half-life (3.82 d), the measurements on the uranium series as compared to the thorium series are much more susceptible to influences involving extraordinary radium contamination ( 2='Rn compared with `=° Ra) prior to and diffusion or leakages involving the chamber's external environment during the present study . It is noted that both the contaminant 21 "Po activity as well as the ` 1 `Po activity associated with the thorium
38 6
A .J. Howard / Radioassay of migratory a-particle emitters
series appear to be time-independent (once transient equilibrium has been reached in the latter case) . It is further noted that the intrinsic overall migration process associated with the thorium series may be hindered by the T, ,,_=55.6 s character of 220 Rn in that emanation and exhalation phenomena [16] are lifetime limited compared to the uranium series . Finally, it is of importance to observe that the counting rate associated with the 8784-keV = ' = po transition observed in preliminary measurements made prior to the present study was (0.19 ± 0 .05) counts/h : Thus the cleaning and component replacements described in section 2 had no discernible effect on the observed activity assigned to the thorium series . The lower limit of sensitivity associated with the current device for alpha-particle transitions in the region of interest (- 6 to 9 MeV) depends on the location and relative strength with respect to higher energy transitions because of the low-energy tail effects described in section 4. It has also been shown in fig. 4 that cosmic-ray interactions with the silicon detector [19] produce aj negligible contribution to the companion spectrum shown in fig. 3 . The presence of uranium, thorium, and actinum series members as depicted in fig. 3 may be readily identified by the respective 7687-, 8784-, and 6623-keV "signature" transitions (see fig. 1) . In this case, the 6623-keV 211 Bi transition is near the sensitivity limit imposed by the tail of the relatively strong 7687-keV 214 po transition. Hence a sensitivity limit of - 1 count per day is indicated in this particular case . Employing e_ = (12 ± 1)% as presented in section 5, this count rate corresponds to - 8 decays per day for atoms which have migrated into the helium gas, or 10 -4 Bq. This rate is independent of the area involved, provided that collection times arc appreciably smaller than the neutralization times for the heavy ions in the current helium environment, which is 7 __ 10 s [10,11]. Assuming that the activities summarized in table 1 exclusively reflect migrations originating from the 0 .7m2 physical surface area of the hollow aluminium shell, the fluxes of the '-== Rn and 220 Rn atoms into the vessel may be estimated by multiplying the 7687- and 8784-keV counting rates tabulated by a factor of 3.3 x 10 - ` to give values in units of atom/m= s. The respective values at t --- 600 h are (6.6 x 10 - -3) and (6.6 x 10 -4 ) atom/m = s . It is of interest to note that the concomitant uranium/thorium ratio associated with these heavy-ion fluxes is - 10i, significantly larger than that routinely found in natural concentrations [18]. Because of the possible "'Rn contamination of the chamber mentioned in section 2, it is assumed that the measured flux associated with the uranium series is in fact largely due to 226 Ra contamination on the surfaces, and hence it is of no further interest here . The thorium case, corresponding, to the (6.6 x 10 -4 ) atom/m = s flux as-
sumed to originate from the aluminum walls, involves -'z4Ra atoms that lie on or within the "surfaces", and it
is here that a large ambiguity arises . This is due to the fact that simple analyses based on recoils originating within a depth equal to the range of the heavy ion in the solid material generally underestimate the flux [17] by up to factors of 10' for macroscopic-sized (>_ 1 wm) minerals [20]. For this reason, it is not possible at the present time to apportion the measured flux associated with 22° Rn migration between the two sites mentioned above . It is for this reason that bulk radionuclidic determinations, such as activities expressed in Bq/kg, arc not appropriate in the present context . A meaningful comparison can, however, be made with the alphaparticle flux measured for the titanium walls of the chamber employed in a recent DBD experiment [5]. Here a total thick-target alpha-particle flux for energies > 2 MeV was measured to be - 1 alpha particle/m= s via a surface-barrier detector. Given that the range of 8-MeV alpha particles in titanium is - 30 Wm [21] and is - 1000 times the range of the 220 Rn recoils [17], the currently measured 220 Rn flux (6.6 x 10 -4 atom/m = s) is consistent with comparable "surface contamination" [5] in the two cases . Clearly, neither measurement is sensitive to the exact locations of the 220 Rn birthplaces, and so "surface contamination" remains an ambiguous term here . 7. Conclusions A sensitive methodology involving electrostatic precipitation and subsequent detection of migratory heavy alpha-particle emitters has been presented . Individual alpha-emission activities characteristic of the individual uranium and thorium series operating in transient equilibrium are shown to be measurable for activities AN_10 -4 Bq, wherein A=0.693/T,,., and N= number of nuclei present . Uncertainties involving the sites of origin with respect to the surface are identified as important open questions . Nonetheless, the method clearly isolates the extremely small fraction of heavy radionuclides present throughout th~ macroscopic volume of materials that transit into regions outside the materials. In consequence, it may be a useful tool in radioassays of particular relevance to SHE and DBD experiments. The simplicity of the physical components involved in the method suggests the possibility of designing them into the chambers used in such experiments with in situ monitoring of radionuclidic growths within the chambers being carried out during intermediate experimental shutdowns for gas analyses, purifications, etc. The performance obtained with the presently employed system merits some final observations . First and foremost, the attainment of the large foreground-to-
A.J. Howard / Radioassay ofmigratory a-particle emitters background ratio exemplified by intercomparison of figs. 3 and 4 is primarily due to the concentrative nature of the electrostatic precipitation process. It is also emphasized that the area of the active area (4 .5 cm` in the current case) is of consequence insofar as it
reduces the perimeter-to-active area ratio, which varies as r- 1 for a circular shape, and hence reduces the relative importance of the low-energy tails discussed in
section 4. As regards the observed background, an undifferentiated combination of mechanical isolation, underbiasing of the detector (45 V instead of 100 V),
physical location (with respect to both the chamber and geographically), and the lack of typical cosmic-ray shielding materials has created a very favorable result.
Therefore, no attempts were made to modify any of these variables except detector orientation from the conditions described in section 2 of this paper. It is noted that while no discernible change in background due to detector orientation was discussed in the present study, the orientation depicted in fig . 2 is inher-
ently less susceptible to microphonics generated by detector vibrations and so is the preferable one. Finally, the large uncertainties that currently sur-
round radon emanation/exhalation phenomena even at room temperatures creates large uncertainties as regards the birthplaces of the migratory radionuclides
studied herein . In consequence, it is not clear to what extent more exotic surface preparation will reduce these migratory fluxes, a factor that ultimately minimizes this source of background in situations such as the SHE and DBD experiments discussed herein .
Acknowledgements It is a pleasure to thank members of the Wright Nuclear Structure Laboratory staff at Yale University for their gracious advice, assistance and support involving this work and Wayne Strange of Trinity College for
assistance in some of the measurements. Enlightment on current problems associated with and the status of DBD experiments were provided by members of the Yale group, namely Craig Levin, Joseph Germani and John Markey.
38 7
References (11 G. Herrmann, Physica Scripta IOA (1974) 71 . [2] G.N. Flerov and G.M. Ter-Akopian, in : Treatise on Heavy-Ion Science, vol . 4, ed. D.A. Bromley (Plenum . New York, 1985) pp. 333-399. [3) A. Forster, H. Kwon, J.K. Markey, F. Boehm and H.E. Henrikson, Phys . Lett . 138E (1984) 301. [41 D.A . Imel and J. Thomas. Nucl . Instr . and Meth. A273 (1988) 291. [51 E. Bellotti et A., Phys. Lett. 221B (1989) 209. [6] J. Germani, C. Levin and J. Markey, Bull . Amer. Phys . Soc. 35 (1990) 1665 . [71 R.D. Evans, The Atomic Nucleus (McGraw-Hill, New York, 1955). [8] C.M . Lederer and V.S. Shirley, Table of Isotopes, 7th ed.
(Wiley, New York, 1978). [9] A.J. Howard, B.K. Johnson and W.P. Strange, Nucl. Instr. and Meth. A293 (1990) 589. [10] A.J. Howard and W.P. Strange, J. Appl . Phys . 69 (1991) 6248. [II] AT Howard and W.P . Strange, Nucl . Instr. and Meth . A311 (1992) 378. [121 E.K. Hyde, 1 . Perlman and G.T. Seaborg, The Nuclear Properties of the Heavy Elements 11 (Prentice-Hall, Englewood, New Jersey, 1964). [13] Ortec Model BA-021-450-100. [14] E.W. McDaniel and E.A . Mason, The Mobility and Dif-
fusion of Ions in Gases (Wiley, New York, 1973) . [15] F.K. Richtmyer, E.H . Kennard and T. Lauritsen, Introduction to Modern Physics (McGraw-Hill, New York, 1955). [161 C. Samuelsson, in: Radon and Its Decay Products, ed. P.K . Hopke (Am. Chem . Soc., Washington, DC, 1987) pp,203-218. [171 T.M. Semkow, Phys . Rev. Lett . 66 (1991) 3012. [18] A.V . Nero, Jr ., in : Radon and Its Decay Products in Indoor Air, eds. W.W . Nazaroff and A.V. Nero, Jr . (Wiley, New York, 1988). [191 J.F . Ziegler, Nucl . Insu . and Meth. 191 (1981) 419. [201 S. Krishnaswami and D.E . Seidemann, Geochim. Cosmochim . Acta 52 (1988) 655. [21] J.F. Ziegler, The Stopping and Ranges of Ions in Matter, vol. 4 (Pergamon, New York, 1977).
Nuclear Instruments and Methods in Physics Research B66 (1992) 379-387 North-Holland
Section B
Method for radioassay of migratory alpha-particle emitters A.J . Howard
Department of Physics, Trinity College, Hartford, CT 06106, USA
Received 5 September 1991 and in revised form 22 November 1991 An electrostatic precipitation method is described for the individual alpha-particle activities associated with radionuclidic decays that trace their origin to sites in the surface layers of solid matter . The energy resolution is sufficient to resolve all of the detected, major alpha-particle groups of the identifying member(s) of the uranium, thorium, and actinium series. The 12% absolute efficiency associated with the device allows determination of individual migration rates from surface layers of solid materials of 10 -4 atom/s, which corresponds to detection of one alpha-particle per day, to be made with negligible background contributions: This efficiency is independent of the material's surface area over a wide range. The background in the detector, presumably produced by cosmic rays, is not found to be measurably dependent upon the orientation of the detector with respect to the cosmic-ray flux . Ambiguities -ssociated with the birthplaces of migratory radon atoms are discussed . 1 . Introduction
studies and are a major limit on the sensitivities associated with current experiments . Focussing on the DBD experiments, a prime nuclear species particularly amenable to experimental scrutiny is 1a6Xe, which chemically is the heaviest stable noble gas . The combined target/detector systems associated with these experiments involve a confinement chamber, and so the (metal) chamber and its detection components are necessary sources of background caused by heavy radionuclides, which are members of the so-called thorium, neptunium, uranium,
There are several experimental areas in fundamental nuclear physics that involve searches for rare nuclear events. Two examples of these involve studies on the possible existence of superheavy elements (SHE) [1,21 and on the strength of the weak interaction as regards double beta decay (DBD) [3-61. In both of these cases, background events caused by naturally occurring radionuclides, particularly those with atomic mass A >_ 208, have proven to be problematic in these zzz
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URANIUM THORIUM ACTINIUM SERIES Fig . 1 . Portions of the uranium, thorium, and actinium series of particular relevance to the present study. The nuclear half-life associated with each species is located at the bottom of its box . The relative alpha (a) and beta (0) branch intensities for 211 13i are given in parentheses as percentages . Alpha-particle transition energies are given in keV. 0168-583X/92/$05 .00 0 1992 - Elsevier Science Publishers B.V . All rights reserved
3811
A .J. li(nvarlI ,l Radioassn3~ of nrigratoKy a-parti,1, emitter.,
and actinium series [7] . These series arc sources of well-identified alpha-, beta-, and gamma-ray transitions [7,8], and members of the three series which are of particular pertinence to the present study arc illustrated in the partial series diagrams shown in fig . 1 . In the construction, execution, and interpretation of these temporally long experiments, it is crucial to understand the nature of this background source in detail, and it is the purpose of this paper to describe a method which should prove of diagnostic utility in current and future SHE and DBD experiments as well as other applications where a detailed quantitative knowledge of heavy radionuclidic content is desired . The method described in this paper evolved from previous measurements involving determinations of air-borne === ltn concentrations [9] and neutralization rates of certain heavy ions in air, argon, and helium [10,11]. Although the system employed is simple, achievable results arc characterized by high sensitivity and high energy resolution . The technique can be applied to identify background contributions from individual components of the detection chamber as well as the assembled device, the latter being the case when the active clement of the current chamber (a surfacebarrier detector) is integrated into the design of future devices for in situ observations . The method involves electrostatic precipitation of radionuclidic ions that arc formed in a hydrostatic carrier gas (helium at atmospheric pressure in the current study) contained in a chamber (a hollow aluminum cylinder with dimensions comparable to those employed in typical DBD experiments [4,5] involving 1 'r'Xe). The precipitation is made onto the surface of a surface-barrier detector, and decay alpha particles are concurrently energy analyzed with an 80-kcV energy resolution that is sufficient to isolate virtually all of the observed alpha-particle transitions associated with those displayed in fig. 1 . Because the nuclear decay schemes associated with members of these radioactive series are very well known [8,12], the observed alphaparticle spectra may also be employed to deduce individual beta- and gamma-ray fluxes within the interior of the chamber as generated by radionuclidic decay originating in the detection volume itself. The time-dependence (field-on-field-off) inherent to the electrostatic precipitation process onto the surface-barrier detector also allows isolation of bacl . ground in the active volume of the detector itself, including that induced by thermal neutrons and/or cosmic rays. The cosmic-ray contribution involving the detector is scrutinized by a variation of the orientation of the 100-Run thickness, 4 .5-cm 2 area active silicon volume with respect to the vertical direction . Measurements of this type (with zero electric field) allow determination of the ultimate sensitivity inherent to the system.
The apparatus employed and the procedural details are presented and explained in section 2. The measurements themselves are performed to ascertain the information as described above and are summarized in section 3 . The experimental findings arc presented in section 4, which is followed by absolute calibration and sensitivity information in sections 5 and 6, respectively. Conclusions drawn from this study are then highlighted in section 7 . 2. Apparatus and procedures The cylindrical chamber employed for the current measurements has been described previously [9] and is shown schematically in fig . 2. Of particular importance to the present considerations are the composition and dimensions of this chamber. The chamber itself is composed of standard grade aluminum and has 0.93-cm wall thickness for the 76.5-cm length cylindrical shell portion and 1 .0-cm thickness for each of the two 32-cm diameter cylindrical end plates. The gas volume enclosed is 38 .2 I, and the total internal area is - 7200 cm` . A silicon surface-barrier detector having a thickness of 100 Wm and an active area of 4 .5 CMZ is employed as the combined collector/detector : It is electrically isolated at the location shown in fig . 2 so that both bias ac :nss the active volume and (negative) precipitation voltage may be concurrently applied. This device was previously calibrated [9,10] by measure-
Fig. 2. Cross-sectional view of the aluminum chamber . During electrostatic precipitation, the combined collector/detector is constant at -1000 V inside the grounded chamber. O-ring assemblies at both ends of the cylindrical shell are not shown.
A.J. Howard / Radioassay ofmigratory a-particle emitters
ments involving 222Rn of known activity contained in various carrier gases, including helium . Such calibrations involved passage of the carrier gas through one of two liquid bubbler sources containing 3 .24- and 3.76nCi 221Ra activities. The emerging gas passed through a filtration tube containing CaS04 crystals (a desiccant) and glass wool into the chamber, which was initially evacuated and also contained -0.1 kg of CaS04 crystals for continuous desiccant action during the calibration measurements. Determinations of equilibrium counting rates for the 6003-keV 2 '"Po and 7687-keV 214Po alpha-particle transitions shown in fig. 1 were made as a function of the applied collection voltage . The respective absolute efficiencies associated with the aforementioned transitions, namely e, and e 2 , represent 100% multiplied by the ratio of the pertinent observed equilibrium counting rate to the known activity of the 222 Rn dispersed throughout the carrier gas in the chamber . Numerical values for e, and e 2 for the current situation as obtained from the results of these previous studies [9,10] are presented in section 5 . It was during these numerous previous calibration procedures that an unknown amount of 226Ra may have inadvertently been transferred from a bubbler into the chamber . Immediately prior to the present study, this chamber was completely disassembled and all internal surfaces were cleaned by sanding with emery paper and wiping with ethanol . This rudimentary procedure was carried out primarily to remove any -'Ra atoms which may have been transferred into the chamber from 222 Rn bubblers employed in the previous investigations [9-11] : The chamber had not been exposed to equivalent 2'9Rn or 22"Rn sources. The gas-handling system and entry ports of the chamber were entirely replaced with larger bore (2 .54-cm diameter) components throughout, which resulted in a radioactively clean system and greatly reduced pumping resistance as compared to the original version [9] : A new molecular drag pump station was employed for the evacuation procedure. Prior to reassembly of the chamber, electrical feedthroughs and connections involving the surface-barrier detector (see fig . 2) were replaced . Thus conventional (not extraordinary) steps were taken to reduce the radionuclidic content associated with all the surfaces involved in the operation of the experiments described herein with the exception of the surface-barrier detector, whose external surfaces were cleaned by a helium jet . This Si(SB) detector used in the current study [13] had been employed in one previous study [11] . It was reemployed (rather than replaced) because its previous exposure to 222Rn decay products resulted in the presence of a 210Pb source (see fig. 1) located within the surface layers that produces 5305-keV alpha-particles with ideal activity for a continuously operative calibration source during the measurements to be described : This alpha-particle en-
381
ergy is just below the major region of interest involved in the present study. Having reassembled the system, the chamber was immediately evacuated to a pressure p - 10 -fi atm and flushed twice with helium of unmeasured radionuclidic content . It was then filled with helium to p = 1 .1 atm . No drying agents (such as CaS0 4 crystals or PROs powder employed in previous measurements [9-11]) were employed because of their potential for radionuclidic contamination of the helium gas. The selection of helium as the carrier gas was primarily based on the relatively high mobility of atomic ions in this particular inert gas [14] and the associated high collection efficiency observed for it [10]. In particular, previous results [9,10] indicate that the ion-collection efficiency is fully saturated in this monopole device when the precipitation voltage has the value (-1000 V) employed in the current study. As discussed in section 6 of this paper, the current results indicate that the helium gas is not the major source of heavy radionuclides detected in the present study. The orientation and location of the detector within the chamber is shown in fig . 2 : The axes of revolution for the detector and chamber are colinear, with the exposed active area of the former facing downward and at a 12 .5-cm distance from the inner surface of the chamber's upper end . The detector is fixed inside the chamber, and so its orientation is determined by the orientation of the entire chamber. Bias for the electrically-isolated detector is applied via an externally located 45-V battery in series with a 22-Mil current-limiting resistor . This underbiasing of the detector (maximum depletion occurs for - 100-V bias) reduces spurious electrical bursts to a negligible level and energy depositions by penetrating high-energy (cosmic-ray) particles . The design of the detector is such that the entire external area (active area and mount) are at the same potential . The precipitating electric field is controlled by a standard nuclear instrumentation module high-voltage power supply whose output is fed through the preamplifier line as indicated in fig. 2. Conventional electronics were employed to carry out the pulse-height analyses described in section 3. Microphonic noise caused by sporadic mechanical vibrations in the building were observed to generate spurious electronic signals during preliminary measurements, and these extraneous pulses were eliminated by mounting the entire system shown in fig . 2 and also the preamplifier on 5-cm thickness foam padding . 3 . Measurements Having filled the chamber with helium gas to p = 1 .1 atm at time t --- 0, this device was located in a quasibasement room (approximately half of which is below
382
A.J. Howard / Radioassay of migratory a-particle emitters
the earth's surface) at 41°N latitude with orientation such that the exposed active area of the detector faced directly towards the center of the earth. Other than the shielding provided by the building and the segment of surrounding earth, no shielding materials were employed in the ensuing measurements. The sequence of measurements; and associated alpha-particle activities are summarized in table 1 . It started with a six-day continuous run commenced at time t( ,=24 h during which the precipitation voltage, HV, was held constant
at -1000 V. Signals from the surface-barrier detector were sent through a simple, standard preamplifier-lincar-amplifier-multichannel-analyzer (MCA) series arrang^ .ment: No pulse-shape discrimination was performed. The MCA was operated in the pulse-height analysis mode, the dispersion being45 keV/channel so that the energy range covered in the 2048-channel memory segment was -0.5 to 23 MeV. Intermediate readouts were performed during this first measurement for the purpose of establishing any time dependences associated either with growth behavior intrinsic to the radioactive series involved or changes in count rates due to (unwanted) sources such as inward leakage of air. A spectrum illustrating the data obtained over the final 70-h segment of this 141-h measurement is shown in fig. 3, where identifications of individual alpha-particle groups is made with the information summarized in fig. 1.
The second measurement in the sequence was identical to that just described except that HV =0 was involved, which results in the condition of no electrostatic precipitation . This measurement was started at t = 166 h and was carried out continuously (except for
100 He HV = -1,000V At=70h
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brief intermediate readouts) for a 93-h interval ; the final 70-h segment, which in this case represents the case where essentially all of the radionuclides precipi-
tated in the first measurement have decayed, is illustrated in fig . 4, where it is clear that the only definitely discernible radionuclidic activity is that associated with 21oPo
the long-lived activity that serves as a continuously operative calibration source that was introduced into the surface layers of the detector during previous
Table I Summary of alpha-particle counting rates observed in the present study ID ` 1 1 1 2 2 2 3 3 3 3 4 4 4 5 5 °) b)
HV (V) -11000 -11000 - IIOw 0 0 0 -11000 -11000 -11000 -11000 0 0 0 -11000 -11000
9
Fig. 3. Semilogarithmic plot of a 70-h spectrum obtained with HV = - 1000 V commencing at t() = 45 h. Energy identifications are in keV and correspond to transitions illustrated in fig. 1. The 5?1,15-keV line results from a previous ion implantation involving the uranium series, and locations of the unob served radon transitions are indicated with arrows. Orientation of the detector is as shown in fig . 2.
t (1 (h)
At (h)
Counts of alpha particles /At (h -1 ) for four transitions 5305( 2 .0 po) h) 6003(2'"Po) 7687(21° Po)
8784( 212 Po)
24 45 116 166 190 213 266 284 308 331 379 427 502 549 597
21 70 50 23 22 48 18 24 23 48 47 49 46 47 52
1 .37±0.26 0.92±0.11 1 .08±0.15 1 .15±0.22 0.85+0.20 1 .02±0.14 0.91 ±0 .23 1 .27+0.26 0.26 1 .25±0.26 1 .24±0.17 1 .20±0.19 1 .65+0.18 1 .67+0.19 1 .43+0.17 1 .47±0.17
0.14±0.08 0.20+0r5 0.l8±0 .05 0 .18±0 .09 0 0 0.28±0.12 0.21±0 .10 0.22±0 .10 0.19±0.06 0.06±0.04 0 0 0.11±0.05 0.21±0.06
0.74±0.19 0.73±0.10 0.98±0.14 0 0 0 1.36±0.35 1.56±0.26 1.55±0.26 1 .39±0.17 0.06±0.04 0 0 1 .24±0.16 1 .47±0.17
Measurement identification number. Alpha-particle energy in keV and between parentheses the associated radionucleus .
0.64±0.18 0.90±0.11 1 .32±0.16 0 0 0 2.05±0 .33 2.15±0 .31 2.20±0 .31 1 .70±0.19 0 .02±0.02 0 0 1 .79±0.19 1 .59±0.17
A.J. Howard / Radioassay of migratory a-particle emitters 100
383
transitions which represent those for the uranium series in transient equilibrium. The fourth measurement was commenced immedi-
r
ately before the measurement just described and was intended to probe the orientational effects associated with background events in the surface-barrier detector .
v
w 10 a
It essentially involved a duplication of the second measurement, for which HV = 0, except that the chamber
z
(and hence the detector fixed within) was rotated 90°
O U
so that the exposed active area faced in a southerly direction. The accumulated spectrum is essentially identical to that shown in fig. 4. 0 2
3
4
5 ENERGY
6 (MeV)
7
8
9
4. Experimental results
Fig. 4. A spectrum similar to that shown in fig. 3, except the illustrated segment was taken with HV = 0 24 h after its establishment at t = 166 h.
The counting rates summarized in table 1 during each of the growth periods (the first and third mea-
measurements [11] : the energy region of interest for the current method (- 6 to 9 MeV) is essentially
clidic series illustrated in fig . I are all consistent with the assignments made in fig. 3, as are observations
devoid of events . The results of the above two measurements as summarized in figs . 3 and 4 as well as table 1 establish the
taken over finer time grids (not recorded in table 1) at the outset of these four individual measurements. It therefore appears that all of these identifications are
that follow migration of members of the associated series from the surface layers of the chamber material
being within ±20 keV of the quoted energies for an assumed linear calibration based on the known [9] 5305-, 6003-, and 7687-keV transition locations in the spectrum displayed in fig . 3. It is noted here that the only unresolved transitions in this spectrum involve the
surements reported in section 3) as well as those for ensuing decay periods (the second and fourth measurements) associated with the portions of the radionu-
complete dominance of identified radionuclidic events
into the helium gas, a situation to be quantitatively considered in section 4. Entries in table 1 give crude,
but nonetheless clearly identifiable, information about the half-life effects associated with the time-depen-
certain, the individual locations of alpha-particle groups
6003-keV
= '"po transition and the 6051 + 6090-keV
dence of the various detected activities. However, the possible presence of time dependences due to any
`1= Bi transitions, the latter of which have 54% of the strength observed for the 8784-keV =' =po case (see fig .
operation of the uranium series involve the 3.82-d of 222 half-life 11n: Any significant residual 222 Rn in the chamber immediately after the evacuation-flush-fill cy-
previously observed [9-11] and are accounted for by
longer-term nonequilibrium situations with respect to the system requires measurements taken over a longer time span. In particular, the activities involved with
cle described in section 2 would manifest itself via a decrease in the observed activities for the 6003- as well as the 7687-keV alpha-particle transitions, whereas air leakage into the chamber would be manifested by an increase in these activities, relative to the anticipated
growth curve [7] associated with 222Rn . In order to further investigate such possible long-term time-dependent effects, the third measurement involved a repeat of the first measurement MV = -1000 V) and was
commenced at t = 266 h. Upon completion of the fourth measurement, which is described in the next paragraph, the fifth (and final) measurement involved HV = -1000 V again: It was commenced at t = 549 h, which corresponds to 6 T, 12 for 222 Rn, and so provides counting rates for the 6003- and 7687-keV
1) . Thus the peak in fig. 3 near 6000 keV is composed primarily (87%) of the lower-energy transition, the weaker higher-energy peak indeed appearing as a shoulder in that spectrum . The low-energy tails observed for all of the alpha-particle groups have been decays which occur along the epoxy seal that forms the perimeter of the exposed active area . Intercomparison of figs. 3 and 4, which involve
identical detection intervals, indicates the vast predominance of radionuclidic-decay transitions in the spectrum associated with transient equilibrium. A similar
intercomparison involving the two zero-field spectra establishes no measured difference in the nonradionuclidic background for the two orientations studied.
Because the incident cosmic-ray flux is strongly peaked in the downward direction [15], the passage of high-en-
ergy cosmic rays down through the large area (4 .5 cm2) but small thickness (100 pm) results in very small energy deposition in the detector when oriented in the position employed for the first three measurements
384
A.J. Howard /Radioassay of inigratory a-particle emitters
(i.e., the orientation depicted in fig. 2). In contrast, the orientation employed for the fourth measurement offers a smaller area (- 0.02 cm') but much larger thickness ( - 2 cm) of the active area to a downwardly-directed cosmic ray. It is apparent that the region of interest in the current method involving particle energies between 6003- and 8784-keV (see fig. 3) is therefore devoid of contributions from primary cosmic rays (see fig . 4), independent of detector orientation. In further considerations of the present experimental results, especially those summarized in table 1, it is important to bear in mind the various time depcndences associated with the current study . A major attribute of the method described is the simultaneous nature of the ion-collection and detection processes, which causes the growth (and decay) of activities to be analyzed in straightforward fashion [7]. When this first time dependence is coupled with the natural ones associated with the operative members of the three radionuclidic series as summarized in fig. 1, it is apparent that a situation of tiansicnt equilibrium has been established in most of the later time intervals summarized in table 1 . In consequence, even the slight ( 20%) differences in efficiencies for some members of the same radionuclidic series as discussed in section 5 below are both understood and quantifiable . The third time dependence involves various diffusion processes, and these manifest themselves collectively as time variations in counting rates that are not manifestations of the two natural time dependences just identified . The three counting rates of major interest with respect to this third time dependence arc displayed in fig. 5 : They arc the summed values for the 6003-keV 21sPo and 7687-keV 21 ; Po members of the uranium series, the constant-activity 5305-keV 2111Po transition caused by previous [11] implantation, and the individual counting rate for the 8784-keV member of the thorium series (see ûg . 1) . In the following discussions, it is crucial to bear in mind the unique (inert-gas) character of radon that is being exploited in the experimental method, the very different lifetimes for the three pertinent radon isotopes (T1i2 = 3.82 d, 55.6 s, and 3 .96 s for 222Rn, 22° Rn and 21"kn, respectively), and also the very different lifetimes for their radium parents (T 1 2 = 1600 y, 3 .67 d, and 111 .4 d for 226 Ra, 22° Ra, and --- Ra, respecti"ely) . An extremely significant consequence of these time-dcpendenccs is associated with the observation, within statistical uncertainties, of a constant counting rate for the 8784-keV 212Po transition in the thorium series illustrated in fig. 5 . This finding implies that no significant amount of 224Ra was transferred into the chamber via the helium gas during its introduction (from the helium storage tank) at time t = 0: Any such a 224 Ra transfer would have manifested itself as a decrease in 2112 Po activity characterized by the 3.67-d
10 -6003 , 7687 o5305 .8784
rr 0
x
~-HV=0-1
[Y w o-
s~
FZ 0 U
01
0
100
¢a¢
4f
i~j ~
TIME
200 300 (HOURS)
400
Fig. 5. Semilogarithmic plot of observed alpha-particle counting rates versus time for the 400-h period commencing at time t = 0. Numeric labels in the legend correspond to alpha-particle transition energies in keV that are fully identified in table I . The delimited segment labelled HV=0 involved the non-precipitative mode of operation, and the counting rate was zero for each of the five datum points within that segment which are absent. The dashed curve represents the growth rate I'llRn predicted for 222 Rn in an isolated system: A similar plot for would indicate a constant counting rate beyond t1) =0.
half-life of the former radionuclide . Coupled with the short 220 Rn half-life and also the previous history of the chamber regarding radon exposures as given in section 2, this result gives strong evidence that the present observations involving the thorium series are associated with emanations from radionuclides that were present inside the chamber prior to t() = 0 . The time dependence observed in fig. 5 as regards members of the uranium series is consistent with the growth curve [7] shown which is characteristic of the comparatively long 222 Rn half-life (91 .8 h). The evacuation-flush-fill cycle that occurred immediately prior to time t,) = 0 of course removes all radon from the hollow region of the chamber at t, ) __ 0, and so this counting-rate behavior is expected, provided longer-term exhalation phenomena are relatively small . The results indicate an equilibrium 222Rn concentration corresponding to - 2 counts/h for the 7687-keV 214Po transition (see the appropriate entries for t >_ 266 h in table 1). Because of the long half-life for 226 Ra (1600 y), it is not possible to rule out any extraneous radon sources (such as might be present from the history of the chamber as described in section 2) by the present consideration . In consequence, the observed counting rate in this case is interpreted as representing an upper limit on the activity of the uranium series that is independent of this previous potential exposure to 22t, Ra .
A.J. Howard / Radioassay of migratory a-partie%emitters
5 . Absolute detection efficiencies The lack of alpha-particles corresponding to decay of 222 Rn in fig . 3 follows from its inert gas nature: Any such ions which are precipitated by the electric field onto the exposed active area of the detector are immediately neutralized there and return into the helium gas . Thus the activities observed in the current method are in fact only those associated with the major alphaparticle transitions as depicted after the radon transitions summarized in fig . 1 . In consequence, previous measurements [10] concerning the migration of ions formed by the alpha-particle decay of 222 Rn in helium allow a determination of the combined collection and detection efficiencies associated with the present device . The calibration procedures have been outlined in section 2 and are fully described elsewhere [9,10] . Using results [f0] for dry helium at p = 1 atm and with radon concentration [ 222 Rn] = 0, it is found that the electric fields generated in the chamber for HV = - 1000 V lead to the saturated value for heavy-ion collection and that the absolute efficiency for collection and subsequent detection of the 21 "Po ions formed by 222 Rn decay in the helium as defined in section 2 is e, --(10± 1)% . This result is about one-fifth the maximum value associated with a surface-barrier detector (when a point source is located at distance r = 0 from the surface), or one-third the value for the ideal geometry in which all of the 2"Po ions are collected onto the active area of the detector. The higher efficiency associated with the integrated = ' 4 Po activity, e 2 -- (12 ± 1)%, is consistent with the previous radon-in-air results [9] for the saturated monopole configuration employed in that as well as the present studies: It is attributed to the additional contribution of =1 "Po atoms that were neutral (or neutralized) before precipitation and subsequently reemerge into the helium as ionized decay products. The shorter half-life of =2 "Rn (T,,_ = 55 .6 s) compared to 222 Rn (T, I , = 3.82 d) is important in considerations involving the physical site of origin for the radon . Any exhalation effects [16] involving diffusion to the material's surface that contribute to the == "Rn content of the helium must occur within seconds of birth, which is not the case for 222 Rn. Nevertheless, the growth (or decay) of the radionuclides under surveillance on surfaces of materials, which is the objective of this methodology as regards the cited experiments [3-5] and similar applications, is possible for both the uranium- and thorium-series cases with the values of e, and e, presented for 222 Rn being also assumed re =="Rn. The comparative limitations presented above for the == "Rn case are even more pronounced for the = ""Rn member of the actinium series (T, /_ = 3 .96 s). The weak evidence discerned herein for its detection (the 6623-kcV alpha-particle region of fig. 3) can be
385
said ;c. be consistent with the relatively small abundance of members of the actinium series in terrestrial materials (- 1%) as compared to the uranium and thorium series, which are on average approximately equal to one another in their =='Rn and 224 Rn activities [18]. 6. Detection sensitivities The results and analyses so far presented establish the parameters and properties and demonstrate the quantitative aspects attainable with this method . As regards the radionuclidic distribution within the solid material, the associated decay ions that are precipitated out of the helium carrier gas and onto the detector in principle may originate from the surface itself, from a surface layer that is located a distance that is within the range of the recoil associated with its birth, or from deeper-lying sites from which radon diffusion to the surface is able to occur in a timely fashion (see section 5 for possible ===Rn and ""Rn half-life effects). All that can be said here is that the measured activities are clearly the sum of these three components, whose individual contributions or strengths are completely unknown. However, by its very nature, the method identifies the migratory portion of the radionuclidic content of solid matter, and it probably is this portion which ultimately introduces the largest uncertainties associated with searches for naturally occurring SHE via alpha-particle detection in gases and for DBD in 1 "Xe [5] . It is important to emphasize here that the currently-described method generally probes the heavy radionuclidic concentration of an ill-defined surface whose area is very poorly known, and so calculations of average radionuclidic content typically given in Bq/kg (1 Bq= 1 decay/s) cannot be made in any useful fashion nor arc in fact applicable to the "surface" contribution itself . The measurements performed herein employed rudimentary surface preparation which happened to produce activities of the correct order of magnitude for demonstrating the various facets of the method . The history of the chamber itself involved some undetermined additional exposure to =="Ra (via the = "Rn bubbler) prior to cleaning beyond that normally encountered in terrestrial materials [18] . Coupled with the long ==2 Rn half-life (3.82 d), the measurements on the uranium series as compared to the thorium series are much more susceptible to influences involving extraordinary radium contamination ( 2='Rn compared with `=° Ra) prior to and diffusion or leakages involving the chamber's external environment during the present study . It is noted that both the contaminant 21 "Po activity as well as the ` 1 `Po activity associated with the thorium
38 6
A .J. Howard / Radioassay of migratory a-particle emitters
series appear to be time-independent (once transient equilibrium has been reached in the latter case) . It is further noted that the intrinsic overall migration process associated with the thorium series may be hindered by the T, ,,_=55.6 s character of 220 Rn in that emanation and exhalation phenomena [16] are lifetime limited compared to the uranium series . Finally, it is of importance to observe that the counting rate associated with the 8784-keV = ' = po transition observed in preliminary measurements made prior to the present study was (0.19 ± 0 .05) counts/h : Thus the cleaning and component replacements described in section 2 had no discernible effect on the observed activity assigned to the thorium series . The lower limit of sensitivity associated with the current device for alpha-particle transitions in the region of interest (- 6 to 9 MeV) depends on the location and relative strength with respect to higher energy transitions because of the low-energy tail effects described in section 4. It has also been shown in fig. 4 that cosmic-ray interactions with the silicon detector [19] produce aj negligible contribution to the companion spectrum shown in fig. 3 . The presence of uranium, thorium, and actinum series members as depicted in fig. 3 may be readily identified by the respective 7687-, 8784-, and 6623-keV "signature" transitions (see fig. 1) . In this case, the 6623-keV 211 Bi transition is near the sensitivity limit imposed by the tail of the relatively strong 7687-keV 214 po transition. Hence a sensitivity limit of - 1 count per day is indicated in this particular case . Employing e_ = (12 ± 1)% as presented in section 5, this count rate corresponds to - 8 decays per day for atoms which have migrated into the helium gas, or 10 -4 Bq. This rate is independent of the area involved, provided that collection times arc appreciably smaller than the neutralization times for the heavy ions in the current helium environment, which is 7 __ 10 s [10,11]. Assuming that the activities summarized in table 1 exclusively reflect migrations originating from the 0 .7m2 physical surface area of the hollow aluminium shell, the fluxes of the '-== Rn and 220 Rn atoms into the vessel may be estimated by multiplying the 7687- and 8784-keV counting rates tabulated by a factor of 3.3 x 10 - ` to give values in units of atom/m= s. The respective values at t --- 600 h are (6.6 x 10 - -3) and (6.6 x 10 -4 ) atom/m = s . It is of interest to note that the concomitant uranium/thorium ratio associated with these heavy-ion fluxes is - 10i, significantly larger than that routinely found in natural concentrations [18]. Because of the possible "'Rn contamination of the chamber mentioned in section 2, it is assumed that the measured flux associated with the uranium series is in fact largely due to 226 Ra contamination on the surfaces, and hence it is of no further interest here . The thorium case, corresponding, to the (6.6 x 10 -4 ) atom/m = s flux as-
sumed to originate from the aluminum walls, involves -'z4Ra atoms that lie on or within the "surfaces", and it
is here that a large ambiguity arises . This is due to the fact that simple analyses based on recoils originating within a depth equal to the range of the heavy ion in the solid material generally underestimate the flux [17] by up to factors of 10' for macroscopic-sized (>_ 1 wm) minerals [20]. For this reason, it is not possible at the present time to apportion the measured flux associated with 22° Rn migration between the two sites mentioned above . It is for this reason that bulk radionuclidic determinations, such as activities expressed in Bq/kg, arc not appropriate in the present context . A meaningful comparison can, however, be made with the alphaparticle flux measured for the titanium walls of the chamber employed in a recent DBD experiment [5]. Here a total thick-target alpha-particle flux for energies > 2 MeV was measured to be - 1 alpha particle/m= s via a surface-barrier detector. Given that the range of 8-MeV alpha particles in titanium is - 30 Wm [21] and is - 1000 times the range of the 220 Rn recoils [17], the currently measured 220 Rn flux (6.6 x 10 -4 atom/m = s) is consistent with comparable "surface contamination" [5] in the two cases . Clearly, neither measurement is sensitive to the exact locations of the 220 Rn birthplaces, and so "surface contamination" remains an ambiguous term here . 7. Conclusions A sensitive methodology involving electrostatic precipitation and subsequent detection of migratory heavy alpha-particle emitters has been presented . Individual alpha-emission activities characteristic of the individual uranium and thorium series operating in transient equilibrium are shown to be measurable for activities AN_10 -4 Bq, wherein A=0.693/T,,., and N= number of nuclei present . Uncertainties involving the sites of origin with respect to the surface are identified as important open questions . Nonetheless, the method clearly isolates the extremely small fraction of heavy radionuclides present throughout th~ macroscopic volume of materials that transit into regions outside the materials. In consequence, it may be a useful tool in radioassays of particular relevance to SHE and DBD experiments. The simplicity of the physical components involved in the method suggests the possibility of designing them into the chambers used in such experiments with in situ monitoring of radionuclidic growths within the chambers being carried out during intermediate experimental shutdowns for gas analyses, purifications, etc. The performance obtained with the presently employed system merits some final observations . First and foremost, the attainment of the large foreground-to-
A.J. Howard / Radioassay ofmigratory a-particle emitters background ratio exemplified by intercomparison of figs. 3 and 4 is primarily due to the concentrative nature of the electrostatic precipitation process. It is also emphasized that the area of the active area (4 .5 cm` in the current case) is of consequence insofar as it
reduces the perimeter-to-active area ratio, which varies as r- 1 for a circular shape, and hence reduces the relative importance of the low-energy tails discussed in
section 4. As regards the observed background, an undifferentiated combination of mechanical isolation, underbiasing of the detector (45 V instead of 100 V),
physical location (with respect to both the chamber and geographically), and the lack of typical cosmic-ray shielding materials has created a very favorable result.
Therefore, no attempts were made to modify any of these variables except detector orientation from the conditions described in section 2 of this paper. It is noted that while no discernible change in background due to detector orientation was discussed in the present study, the orientation depicted in fig . 2 is inher-
ently less susceptible to microphonics generated by detector vibrations and so is the preferable one. Finally, the large uncertainties that currently sur-
round radon emanation/exhalation phenomena even at room temperatures creates large uncertainties as regards the birthplaces of the migratory radionuclides
studied herein . In consequence, it is not clear to what extent more exotic surface preparation will reduce these migratory fluxes, a factor that ultimately minimizes this source of background in situations such as the SHE and DBD experiments discussed herein .
Acknowledgements It is a pleasure to thank members of the Wright Nuclear Structure Laboratory staff at Yale University for their gracious advice, assistance and support involving this work and Wayne Strange of Trinity College for
assistance in some of the measurements. Enlightment on current problems associated with and the status of DBD experiments were provided by members of the Yale group, namely Craig Levin, Joseph Germani and John Markey.
38 7
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fusion of Ions in Gases (Wiley, New York, 1973) . [15] F.K. Richtmyer, E.H . Kennard and T. Lauritsen, Introduction to Modern Physics (McGraw-Hill, New York, 1955). [161 C. Samuelsson, in: Radon and Its Decay Products, ed. P.K . Hopke (Am. Chem . Soc., Washington, DC, 1987) pp,203-218. [171 T.M. Semkow, Phys . Rev. Lett . 66 (1991) 3012. [18] A.V . Nero, Jr ., in : Radon and Its Decay Products in Indoor Air, eds. W.W . Nazaroff and A.V. Nero, Jr . (Wiley, New York, 1988). [191 J.F . Ziegler, Nucl . Insu . and Meth. 191 (1981) 419. [201 S. Krishnaswami and D.E . Seidemann, Geochim. Cosmochim . Acta 52 (1988) 655. [21] J.F. Ziegler, The Stopping and Ranges of Ions in Matter, vol. 4 (Pergamon, New York, 1977).