Critical requirements for a posteriori track recorder neutron dosimetry at Hiroshima and Nagasaki

Radiation Measurements, Vol. 24, No. 1, pp. 31-42, 1995 Copyright © 1994 Elsevier Science Ltd

Pergamon

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CRITICAL REQUIREMENTS FOR A POSTERIORI TRACK RECORDER N E U T R O N DOSIMETRY AT HIROSHIMA A N D NAGASAKI RAYMOND GOLD Metrology Control Corporation (MC2), Richland, WA 99352, U.S.A.

(Received 28 January 1994; in revised form 31 May 1994) Abstraet--lnternational programs have been carried out over the last four decades to quantify the exposure of atom bomb survivors from Hiroshima and Nagasaki. Unfortunately, the quest for accurate gamma-ray and neutron exposure doses of atom bomb survivors has proven illusive. In the most recent of these programs, designated as Dosimetry System 1986 (DS86), a serious and persistent discrepancy has arisen between neutron transport calculations and radiometric (RM) neutron dosimetry for the Hiroshima site, which has been called the DS86 neutron dosimetry enigma. A recently completed in-depth analysis demonstrates that a simple single factor panacea does not exist to explain the DS86 neutron dosimetry enigma. Careful treatment of a number of specific experimental and calculational effects is required before any progress can be achieved. Within this perspective, the applicability of solid state track recorder (SSTR) neutron dosimetry for the Hiroshima and Nagasaki sites is examined as an independent alternative to radiometric (RM) neutron dosimetry. The utility of the SSTR method for the Hiroshima and Nagasaki sites is analyzed in light of: (i) the current status of the DS86 neutron dosimetry enigma; and (ii) SSTR characteristics that are specificallygermane to the Hiroshima and Nagasaki sites. On this basis, critical SSTR requirements are identified, recommended ways of meeting these critical requirements are advanced and the domain of applicability of SSTR neutron dosimetry at the Hiroshima site is estimated.

1. INTRODUCTION

since the vast majority of survivors come from this region. The current impetus for pursuing SSTR neutron dosimetry at the Hiroshima site is the existence of a large background in RM dosimetry that can be created by environmental neutrons in this critical ground range region. The magnitude of this background component can be appreciated by referring to the companion paper (Gold, 1994), where puzzle piece No. 5: environmental neutrons, has been used to estimate the background-to-foreground (B/F) ratio as a function of ground range for the different slow neutron RM monitors used to date at the Hiroshima site. These efforts show that B/F dramatically increases with increasing ground range in the critical ground range region for all slow neutron RM dosimeters. This behavior underscores the need to vigorously pursue alternative methods of neutron dosimetry for the Hiroshima site. Hence, the first critical requirement, CR 1, is to obtain alternative neutron dosimetry in this critical ground range region at the Hiroshima site. In contrast with RM dosimetry, it will be shown below that the background fission track density induced by environmental neutrons in suitable SSTR neutron dosimetry specimens at the Hiroshima site is negligible compared to the background fission track

A recent in-depth analysis of the DS86 neutron dosimetry enigma can be found in the companion paper (Gold, 1994). This analysis demonstrates that for ground ranges less than approximately 700 m, i.e. r ~<700 m, the discrepancy at the Hiroshima site between thermal neutron activation measurements, i.e. RM dosimetry experiments (E) and neutron transport calculations (C) can be accounted for through proper treatment of the following missing puzzle pieces: (i) Puzzle piece No. 1: Little-Boy replica power normalization. (ii) Puzzle piece No. 2: Angular anisotropy. (iii) Puzzle piece No. 3: Global shielding. Nevertheless, the most significant discrepancy between E and C for the Hiroshima site still persists at larger ground ranges. Even after qualitatively accounting for the effects due to these three missing puzzle pieces, E/C still monotonically increases with increasing ground range in the region lkm~ 1.3 km. This deficiency is extremely unfortunate because the region 1 km-%
3I

32

R. G O L D

density created by spontaneous fission decay of 238U. Hence, the B/F fission track density ratio for suitable SSTR specimens is dominated by the spontaneous fission decay of 238U with a negligible contribution from environmental neutrons. For a suitable SSTR specimen, the age and uranium concentration of the specimen can be used together with the well established 238U spontaneous fission half-life to accurately determine background fission track density. As a consequence, SSTR neutron dosimetry affords complimentary measurements to RM dosimetry for the Hiroshima site that is virtually independent of serious uncertainties that can arise in RM dosimetry from environmental neutron background. 1.1. A posteriori dosimetry

Accurate characterization of radiation environments is not always planned, conducted or realized. Often the need for accurate definition of a radiation field is only recognized long after the irradiation has occurred. Fortunately, certain dosimetry methods possess the ability to retain data that can be used to characterize radiation exposures, long after experiments have ended. The ability to quantify a radiation field or exposure in the absence of explicitly planned or conducted dosimetry for the irradiation of interest is now known as a posteriori dosimetry. The first and most notable example of a posteriori neutron dosimetry has been the effort to quantify the radiation exposures of survivors from Hiroshima and Nagasaki with RM neutron dosimetry. The need for a posteriori neutron dosimetry in reactor environments was recognized more than a decade ago. In 1980, this need was stressed at an international symposium, where it was emphasized by Gold et al. (1980a) that, "the history of reactor dosimetry is replete with after the fact requests". In other words, the need for accurate characterization of radiation fields and environments is often recognized, but unfortunately only after many experiments have long been completed. During the ensuing time, the demand for such data invariably intensifies. Formal recognition of a posteriori neutron dosimetry has arisen through American Society for Testing and Materials (ASTM) activities on the continuation, renewal and plant life extension (PLEX) of light water reactors (LWR). An ASTM task group was first organized for reactor component monitoring (Gold, 1984a) and was subsequently broadened to include L W R - P L E X activities (Gold, 1986). This ASTM task group El0.05.11 was established with a charter that explicitly encourages the development of standard methods of a posteriori neutron dosimetry (Gold, 1987a, b). Through these ASTM activities, this class of radiation metrology methods has become formally known in the US as a posteriori dosimetry, whereas colleagues in the UK have called this method retrospective neutron dosimetry (Banham et al., 1992). Different methods have been identified that

afford the possibility of a posteriori neutron dosimetry, namely: (i) RM neutron dosimetry (ASTM,1984). (ii) SSTR neutron dosimetry (ASTM, 1982). (iii) Helium accumulation fluence monitors (HAFM) (ASTM, 1986). (iv) ln-situ gamma-ray spectrometry (Gold and McElroy, 1986). As a natural outgrowth of these considerations for reactor environments, a posteriori SSTR neutron dosimetry was proposed for the Hiroshima and Nagasaki sites (Gold, 1984b; Gold and Roberts, 1984, 1985). Chapter 5 of the DS86 final report (Loewe et al., 1987) summarizes these developments and points out that these proposals brought to light an earlier fission track study at the Nagasaki site. In a Master's thesis, Matsuda (1977) examined zircon and stained glass SSTR specimens with ground ranges of 415 m and 520m, respectively. Complete details of these earlier efforts at the Nagasaki site are also contained in Appendix 5-15 of the DS86 final report (Matsuda and Sakanoue, 1987). At the very outset of his Master's thesis, Matsuda concludes that estimation of neutron flux by using the fission track technique is hopeless. As a result, his thesis concentrates on using the fission track technique for estimation of temperature. Sakanoue has used these efforts to conclude that it is unlikely that suitable SSTR specimens can be found for neutron fluence measurements in the critical ground range region. As noted by Loewe et al. (1987), there has been no resolution of the Matsuda-Sakanoue conclusions with the position advanced in the Gold-Roberts proposals. This situation has subsequently become even more confused by the conflicting positions taken by Fleischer on this issue. At first, Fleischer (1987) described a posteriori SSTR neutron dosimetry at the Hiroshima and Nagasaki sites as an "opportunity lost". Some five years later, Fleischer (1993) has apparently reversed his position and now believes glass is a likely material for a posteriori SSTR neutron dosimetry at the Hiroshima and Nagasaki sites. More specifically, Fleischer (1993) now claims: "etched tracks in glass of conventional uranium concentration content ( ~ 0.3 parts per million) could allow thermal neutron fluences to be measured to ~ 1 km from the epicenters at Hiroshima". Unfortunately, Fleischer has never supplied the quantitative detail to substantiate either of his contradictory positions, let alone the basis for completely reversing his position. In light of the current state of confusion on a posteriori SSTR neutron dosimetry at the Hiroshima and Nagasaki sites, a critical review of these previous considerations is warranted. To this end, Section 2 reanalyzes the Gold-Roberts proposals and Section 3 evaluates the fission track study of Matsuda and Sakanoue at the Nagasaki site. In order to carry out

T R A C K R E C O R D E R N E U T R O N DOSIMETRY this critical review, a discussion of the negative aspects of past work is unavoidable. Nevertheless the avowed purpose of this review is the constructive use of past efforts. To achieve this goal, critical requirements of a posteriori SSTR neutron dosimetry at the Hiroshima and Nagasaki sites are identified and methods of meeting these critical requirements are recommended. The concluding section provides a summary of these critical requirements and the methods recommended to actually meet these critical requirements. On this basis, an approximate domain of applicability for a posteriori SSTR neutron dosimetry at the Hiroshima and Nagasaki sites is presented.

33

~be, is the environmental flux of thermal neutrons (neutrons/cm ~ s); and s~ is the SSTR asymptotic sensitivity (atoms/cm2). The asymptotic sensitivity is the SSTR efficiency expressed in units of atoms per unit area. It is essentially the number of atoms per cm 2 that can give rise to observable tracks at the surface of the SSTR (after suitable etching). An approximate expression for the asymptotic sensitivity is the simple relation s® = qR/2,

(6)

where q is the optical efficiency of the SSTR and R is the range of fission fragments in the SSTR material expressed in units of atoms/cm 2 (ASTM, 1982).

2. ESTIMATED F I S S I O N TRACK DENSITIES The observed fission track density in a Hiroshima site SSTR specimen, 0,0, can be represented as the linear superposition Oap =

Of -~- 0 b ,

(1)

where Of is the foreground fission track density induced by atomic bomb neutrons and 0b is the background fission track density. The background fission track density is, in turn, a linear superposition of two components, namely 0b = 0~ + 0e,,

(2)

where 0s is the fission track density produced by the spontaneous fission of 238U and 0,, is the fission track density induced by environmental neutrons. Estimates for Of and 0,. will be carried out using only thermal neutron induced fission in 23sU, since the contribution from epi-thermal neutron fission in 23sU and fast neutron fission in 238U are both negligible for the Hiroshima site. Just as in the Gold-Roberts proposals, Hiroshima site fission track densities will be estimated on the basis of the SSTR dosimetry method as described in ASTM standard E854-81 (1982). This standard provides the following simple relations: Of = ~h (r )afazs s~ ,

(3)

O~ = 2Taz8s~,

(4)

2. I. Foreground-to-background fission track densities

The relative magnitude of the two background components of fission track density can be evaluated by taking the ratio of equation (5) to equation (4). One finds

Qen/Qs :

0o, = 4~, T~fa25s~,

(5)

where: • h(r) is the Hiroshima site thermal neutron fluence at ground range r (neutrons/cm2); af is the thermal neutron fission cross-section of 23sU I ( a t = (rrs/2) × 580b = 514 × 10-24 cm2); a2s and a28 are the 23sU and 238U atom fractions in the SSTR specimen, respectively (azs/az8 = 0.720%/ 99.275% = 7.25 x 10-3); 2 is the spontaneous fission decay constant of z38U ( 2 = 7 × 10-~7y ,); T is the age of the SSTR specimen in years;

l(a2s/a28 ).

(7)

A representative value of the environmental flux of thermal neutrons, q~, ~ 8 x 10 -3 neutrons/cm 2 s, has already been used in the companion paper (Gold, 1994). Using this value in equation (7) together with the values for the other physical constants given above, one has 0e,/0~ = 1.3 x I0 2.

(8)

Hence, the background induced by environmental neutrons is only of the order of a percent of the spontaneous fission track density. This result is in sharp contrast with the background produced by environmental neutrons in a posteriori RM neutron dosimetry in the critical ground range region at the Hiroshima site (Gold, 1994). Significant impetus, therefore, exists to pursue a posteriori SSTR neutron dosimetry as an alternative to RM neutron dosimetry at the Hiroshima and Nagasaki sites. As a consequence of equation (8), the foregroundto-background (F/B) ratio can be written as F/B = Of/Qb =

and

b e n O'f,~

0f/0s =

dPh(r)trf(azs/a28)(AT) 1. (9)

Introducing values for the physical constants in equation (9), one has F/B = Qf/Qs= 5.3 x 10 8(~h(r)/T ),

(10)

where T is the age of the SSTR specimen in years. In the Gold-Roberts proposals, the Hiroshima site neutron transport calculations of Loewe (1983) were used for F/B estimates. More recent DS86 neutron transport calculations (Kerr et aL, 1987) will be used here. In addition, the thermal neutron fluence, Oh(r), has been scaled to qualitatively account for puzzle pieces No. 1 and No. 3, as discussed in the companion paper (Gold, 1994). For quantitative experiment-to-

34

R. G O L D

Table 1. Foreground-to-background ratio at the Hiroshima site Ground range (km)

SSTR specimen age (years) 50 1.0 × 10 +3 2.0 x 10+t 4.4 x 10-I 1.5 x 10-2

0.5 1.0 1.5 2.0

100

5.2 X 10 +2 9.8 x 10° 2.2 × 10-l 7.4 x 10-3

1000 5.2 x 10+l 1.0 × 10° 2.2 x 10 2 7.4 x 10-4

calculation (E/C) comparisons it has already been emphasized in the companion paper that one must account for a number of these puzzle pieces on a dosimeter-by-dosimeter basis. In particular, in order to carry out transport calculations which account for puzzle piece No. 3, global shielding, one must possess a detailed description of the locale from which the RM dosimetry specimen has been obtained. The same requirement must obviously be met by SSTR neutron dosimetry. As a consequence, the second critical requirement, CR 2, is identified as sufficiently detailed knowledge of the SSTR specimen locale to carry out quantitative global shielding calculations. For the purpose of this review, only qualitatively scaled DS86 calculations need be considered. On this basis, equation (10) has been used to generate the F/B ratio for SSTR specimens of age T = 50, 100 and 1000 years. The dependence of the F/B ratio upon ground range and specimen age is shown in Table 1 and Fig. 1. Based on past experience with SSTR neutron dosimetry, it can be anticipated that a posteriori SSTR neutron dosimetry can be conducted down to a foreground-to-background ratio of about unity, i.e. F/B ~ 1, without compromising the quantitative data base. It can be seen from Fig. 1 that the condition F/B ~> 1 and CR 1 can be simultaneously

1o3~

K\\

<_,ooo,

,o'I- \ \ \

'°7 \ \ \ I

,o-7

0.5

2.2. Foreground fission track densities As will be shown below, there exist two critical requirements that motivate the need for SSTR to possess high foreground track density, Qr. According to equation (3), Qf depends linearly upon uranium concentration, whereas equation (9) reveals that the F/B ratio is independent of uranium concentration. Hence, SSTR specimens of high uranium concentration are desirable for a posteriori neutron dosimetry at the Hiroshima and Nagasaki sites. It is not surprising, therefore, that the Gold-Roberts proposals and the Matsuda--Sakanoue study focus on the same two SSTR materials, namely zircon crystals and antique glassware, which can possess high uranium concentration. In fact, the Gold-Roberts proposals used data from earlier fission track dating work in Japan (Watanabe and Suzuki, 1969; Nishimura, 1971) to show that these SSTR materials in Japan possessed uranium concentrations in the range of 10 to 105 parts per million (ppm) by weight. In track scanning, just as in pulse counting associated with electronic nuclear detectors, the observation of events possesses inherent statistical uncertainty. Indeed, just as in pulse counting, the applicability of Poisson statistics in track counting (ASTM, 1982) implies that the observation of NI tracks possesses a statistical standard deviation of NL For the purpose of this review, it will be assumed that 10 percent is an acceptable level of statistical uncertainty for a posteriori SSTR neutron dosimetry at the Hiroshima and Nagasaki sites. Consequently, achieving a 10 percent statistical precision is identified as the fifth critical requirement, CR 5. To attain this level of statistical precision, one must scan at least 100 tracks. Hence the fifth critical requirement for a posteriori SSTR neutron dosimetry at the Hiroshima and Nagasaki sites, CR 5, is N t> 100. CR 5 can be written as

N=QfA >t 100,

\

l-~ I,

satisfied by SSTR specimens of only a few hundred years of age. For T = 1000y, Fig. 1 shows F/B < 1 for r > 1 km. Consequently to satisfy CR 1, two additional critical requirements must be met, namely SSTR specimen age can be no more than a few hundred years and background fission track density must be accurately determined. Hence, the third critical requirement, CR 3, is that SSTR specimen age can be no more than a few hundred years and the fourth critical requirement, CR 4, is that background fission track density must be accurately determined.

,

1.0 0.5 Ground range (kin)

2.0

Fig. 1. Foreground-to-background (F/B) fission track density ratio at the Hiroshima site for SSTR specimens of 50, 100 and 1000 years of age.

(11)

where A is the surface area of the SSTR that has been scanned. Even though the F/B ratio is independent of uranium concentration, viz. equation (10), a high SSTR uranium concentration is very desirable since it provides higher track density and thereby facilitates collection of adequate track counting statistics. For

T R A C K R E C O R D E R N E U T R O N DOSIMETRY example, given a track density of Qf = 100 tracks/cm 2, a total surface area of approximately 1 cm 2 would have to be scanned to satisfy CR 5. If SSTR specimens are small, say ~0.01 cm 2 each, then 100 or more specimens would have to be scanned. At higher track densities fewer SSTR specimens would be required. For example, at #r ~ 104 tracks/cm2, only one specimen of ~0.01 cm 2 would be needed. The second independent factor underlying the desirability of a high foreground track density is the existence of a background track density component that has not been discussed above. This background contribution arises from imperfections and artifacts. All SSTR possess imperfections and artifacts that can be confused with etched tracks due to charged particles. The ability to resolve etched particle tracks from imperfections and artifacts depends quite sensitively upon SSTR type. Hence, the imperfection track density background varies widely with SSTR type. The unresolved imperfection track density background, Q~, represents a fundamental limitation in the SSTR method. As a limiting background for low track density applications, it can therefore represent a non-negligible component of experimental uncertainty. The overall significance of ¢~ is described in ASTM standard E854-81 (1982), where it is explained that observer objectivity and optical efficiency depend crucially upon the ability to resolve etched fission tracks from imperfections. Although each SSTR material is unique with regard to Q~, experience with natural mineral SSTR, such as mica and quartz crystal, has established that ~j for natural crystals is no more than a few tracks/cm 2. The distinctive structure of etched fission tracks in natural crystals facilitates the resolution of fission tracks from imperfections and artifacts in these SSTR materials. Since a distinctive structure exists for etched fission tracks in zircon crystals, one can expect nearly the same value of Qj for zircon SSTR. This is not the case for glass SSTR, where p~ can be roughly an order of magnitude higher. Because of the significance of ~ , the sixth critical requirement, CR 6, is that one must accurately account for the background component due to Q~. Hence if one can satisfy the condition

35

lO 100,000 10 4

ppm

~00011:ppm

"_~ 10 3

2

~

lO I

o 10 0

10-I 0.5

,oo,,m '0ppm~'~'-N ~ ~ Ground range (kin) 1.0

1.5

2.0

Fig. 2. The foreground track density, Qf, as a function of ground range at the Hiroshima site for uranium concentrations of Um= 10, 100, 1000, 10,000 and 100,000 ppm.

Hence using equation (13) in equation (3), the dependence of Qf upon Um is given by Of= 6.0 x

lO-l°t~h(r)trfUms~.

(14)

To estimate Qc for the Hiroshima site, a range of uranium concentration from 10 to 105 ppm has been used. As already noted, this range is representative of zircon and antique glassware SSTR specimens found in Japan. For the asymptotic sensitivity in equation (14), a value for zircon and antique glassware SSTR of approximately 3.9 × 10 +19 atoms/cm 2 has been obtained from equation (6). Using the above numerical values in equation (14), one finds Qf= 1.2 x 10 IIOh(r)U m.

(15)

Equation (15) has been used to estimate Hiroshima site foreground track densities for Um = 10, 100, 1000, 10,000 and 100,000 ppm and the results are displayed in Fig. 2. One can satisfy CR 5 and CR 6, i.e. equations ( l l ) and (12), simultaneously if Of/> 100 tracks/cm 2. To attain this condition and also satisfy CR l, Fig. 2 reveals that uranium concentrations must be in excess ~f/~l>> 1, (12) of 1000 ppm, i.e. U m/> 1000. There are, of course, other alternatives to satisfying CR 5 and CR 6. If one does not have a high enough Of, then one must scan then CR 6 is automatically satisfied. Otherwise, Q~ a much larger area to achieve adequate track scanmust be accurately determined so that it can be ning statistics. This task will entail scanning either a subtracted from the a posteriori fission track density, much larger SSTR specimen or many SSTR speciQap • mens. At the same time, the imperfection track To express the Hiroshima site foreground track density background, Ql, must be accurately deterdensity, Qr, in terms of the uranium concentration in mined so that it can be subtracted from the a ppm by weight, Urn, one can use the following simple posteriori track density, O,p. It would clearly be relation between a25 and Urn: advantageous to obtain SSTR specimens of high uranium concentration from the Hiroshima site. For Um~> 1000, such SSTR will possess high Qr and a25 = 6.0 x 10 -l° Urn. (13)

36

R. G O L D

thereby afford higher accuracy and at the same time simultaneously satisfy CR 5 and CR 6. Based on the estimates presented here, it is apparent that Fleischer (1993) has overestimated the capabilities of conventional glass for a posteriori SSTR neutron dosimetry at the Hiroshima site. Using the conventional uranium concentration cited by Fleischer for glass in equation (15), namely Um= 0.3 ppm, one finds foreground fission track densities of Qf ~ 3.5 and Qf,~,6.6 x 10 -2 tracks/cm 2 at ground ranges of 0.5 and 1.0 km, respectively. It would be virtually impossible to satisfy CR 1 and CR 5 with such low foreground track densities. In addition, the imperfection track density background in glass SSTR is typically in the range: 10 ~
According to ASTM standard E854-81(1982), it has been possible to achieve an accuracy of 1-2 percent (1 a ) in absolute fission rate measurements for SSTR neutron dosimetry applications by careful attention to many details. To attain such a quantitative level, one must intensively study, determine and/or account for such factors as: (i) (ii) (iii) (iv)

Etching procedures. Objectivity in track scanning. Quantification of fission deposit masses. SSTR optical efficiency and asymptotic sensitivity. (v) Annealing. (vi) Radiation damage.

All of these factors can introduce distinct requirements for a posteriori SSTR neutron dosimetry at the Hiroshima and Nagasaki sites. However factors (iii), (iv) and (v) are singled out for particular emphasis here because of the difficulties encountered in the Matsuda-Sakanoue (M-S) study at the Nagasaki site (Matsuda, 1977; Matsuda and Sakanoue, 1987). As a consequence, additional critical requirements for a posteriori SSTR neutron dosimetry are identified within the perspective of these earlier M - S efforts. As emphasized above, recommendations are advanced to meet the critical requirements that have been identified. 3.1. Determination o f uranium concentration Accurate quantification of fission deposit mass and uniformity is required in conventional SSTR neutron dosimetry. These data are used to determine the effective number of irradiated target nuclei corresponding to the SSTR area that has been scanned. For a posteriori SSTR neutron dosimetry, the determination of uranium concentration is essentially fac-

tor (iii), i.e. the quantification of fission deposit mass in conventional SSTR neutron dosimetry. The method used for the determination of uranium concentration in the M - S study is described first and then a recommended method that can overcome the difficulties encountered in the M - S method is advanced. 3.1.1. M - S method. Actually, in the M - S study, two different methods were used to determine uranium concentration, one for zircon crystal SSTR and one for stained glass SSTR. 3.1.1.1. Zircon SSTR. Zircon crystals were extracted from the top and bottom of a granite fragment by chemical processing. The granite fragment was located at a ground range of 415 m from the Nagasaki site hypocenter. Near the top surface, 16 zircon crystals were extracted that were exposed directly to the atomic burst. Approximately 5.5 cm below the top (exposed) surface, 23 (bottom) zircon specimens were extracted. After scanning to determine the a posteriori track density, Qap, these zircon SSTR specimens were affixed to sheets of mica and then irradiated in a reactor to a neutron fluence in the range 10~4-10j5 neutrons/cm 2. The induced fission track density observed in the mica sheet, Qi, is then used to determine the uranium concentration in the zircon SSTR specimen. This induced track density can be expressed as Qi ='r/~itrrd25,

(16)

where tI)i is the thermal neutron fluence of the reactor irradiation, r/ is the optical efficiency of the mica SSTR and d25 is the 235U atom density (atoms/cm 2) of the zircon crystal. Consequently, d25 can be determined from equation (16) in terms of the measured value of Qi in the mica together with a knowledge of ~7, tI)i and trr. In the M - S study, the track density induced directly from uranium impurities in the mica is assumed to be negligible and there is no mention of an optical efficiency, which is tantamount to the assumption, ~/= 1. Results reported in the M - S study reveal an a posteriori track density in the neighborhood of Qav ~ 1 x 10 +7 tracks/cm 2 for all the zircon specimens, whereas the induced track density varied from about 2 x 10 +6 u p t o 1.4 x 10 +7 tracks/cm 2. The corresponding uranium concentration in the zircon specimens, as inferred in the M-S study, varied from 100 ppm up to 1000 ppm, with a mean concentration of 545 ppm. Based on the analysis in Section 2, this range of uranium concentration is a favorable one for a posteriori SSTR neutron dosimetry. However, the estimated age of their zircon specimens is at least 10 7 y. AS shown in Table 1 and Fig. 1 of Section 2, a posteriori SSTR neutron dosimetry would be precluded because of the extremely poor F/B ratio possessed by SSTR specimens of this age. In their observations of Q~ in mica, Matsuda and Sakanoue report the presence of frequent 'stars'.

T R A C K RECORDER N E U T R O N DOSIMETRY These 'stars' are regions of extremely high track density that are formed by clusters of fission tracks. Two common sources of 'stars' in SSTR work are: (i) Uranium contamination through inadequate laboratory procedures. (ii) Non-uniform distribution of uranium concentration. At the low levels of uranium concentration that can arise in a posteriori SSTR neutron dosimetry, uranium from the conventional laboratory environment can often contaminate SSTR experiments. At these low levels of concentration, tramp uranium is ubiquitous. Consequently, elimination of contamination by tramp uranium is identified as the seventh critical requirement, CR 7. Contamination can arise through laboratory procedures and chemical reagents. Even the laboratory air can be a source of contamination, so that the use of proper hoods or glove boxes may be necessary. Hence, clean laboratory procedures are mandatory! Specifically recommended for CR 7 are reactor neutron irradiations that actually demonstrate that contamination due to laboratory procedures introduces a negligible level of fission track density. To properly treat non-uniform uranium concentration, one must scan SSTR to obtain either integral or average track density data over a given SSTR surface area that corresponds to either the integral or average uranium concentration over that area. Sampling the track density in a small localized area will not meet this requirement. It is preferable to obtain integral track density data over a given scanning area which corresponds to the integral uranium concentration over that area. To accomplish this using the M-S method, one must obtain both Q~pand ¢~ by scanning the same identical SSTR area. If this is not possible, then one must scan large surface areas in order to obtain the average track density representative of the average uranium concentration. Consequently, the eighth critical requirement, CR 8, is to properly account for non-uniform uranium concentration. This can be accomplished using the above recommendation to obtain integral or suitably averaged track densities in a posteriori SSTR neutron dosimetry. It would be virtually impossible to satisfy CR 8 with the zircon SSTR of the M-S study. The track densities obtained in this study were so high that track overlap, i.e. track pile-up, makes objective scanning extremely difficult. Even using oil immersion optical microscopy to obtain 1000 x magnification, track pile-up would still preclude objective fission track scanning in a manner that satisfies CR 8. 3.1.1.2. Stained glass S S T R . Three stained glass SSTR specimens, identified as fragments A, B and C, were obtained from a Nagasaki site location of 520 m ground range. An a posteriori track density, Oap~ 15 tracks/cm :, was determined by scanning only specimen C. To determine uranium concentration, SSTR

37

fragments A and B were irradiated in a reactor to a neutron fluence of 1.4 x 10 +15 and 2.8 x 10 +16 neutrons/cm2, respectively. The induced track density in SSTR fragments A and B was Qi = 3.3 x 10 +3 and 0i = 6.8 × 10 +4 tracks/cm 2, respectively. In contrast with the zircon SSTR, no mica SSTR surface detectors were affixed to the glass SSTR for the determination of uranium concentration. Instead, induced track densities, 0~, were observed directly in the glass SSTR A and B. The approach used with these glass SSTR is more desirable, in that it eliminates the assumptions entailed when mica is used to determine Qi, namely that the mica possesses negligible uranium concentration and that the optical efficiency of mica is unity, i.e. r / = 1. Although the same uranium concentration was obtained from both SSTR specimens A and B, Um= 0.14 ppm, it was necessary in the M-S study to assume that this uranium concentration was applicable to SSTR fragment C. Actually, this assumption could have been easily eliminated by using all three glass SSTR to determine both O,p and 0i. Moreover, since only modest track densities were attained, track pile-up was negligible. Under these circumstances CR 8 can be satisfied, so that representative uranium concentrations could have been determined corresponding to the a posteriori track density, 0,p, in each of the three glass SSTR. The M-S study uses the track density observed in SSTR glass fragment C, Q,p ~ 15 tracks/cm 2, to infer an upper bound of 6.4 x 10 ÷~2 neutrons/cmz for the Nagasaki site fluence at r = 520m. Using the uranium concentration Um=0.14ppm in equation (4), one finds a background track density 0 b ~ 0 ~ = 3 . 2 × 10 ST,

(17)

where T is the specimen age in years. Using equation (17), any reasonable assumption on the age of SSTR stained glass fragment C yields 0~<<1. Hence, one can use 0,p ~ Of in equation (3), which yields a Nagasaki site fluence, ~ , ~ 8.8 × 10 +12 neutrons/cmz. From DS86 calculations (Kerr et al., 1987), one finds a Nagasaki site fluence of ~ n ~ 8 . 2 × 10 +t° neutrons/cm2 at a ground range of r = 500 m. Here DS86 calculations have been scaled to account for global shielding, i.e. puzzle piece No. 3, as discussed in the companion paper (Gold, 1994). Consequently, this scaled DS86 calculation is roughly two orders of magnitude lower than either the M-S study upper bound or the estimate based on the analysis presented in Section 2 above. It is unlikely that Nagasaki site DS86 calculations could be that far off in the ground range neighborhood of 500m. This conclusion is based on the recent resolution of the DS86 neutron dosimetry enigma in the ground range region r ~<700m for the Hiroshima site (Gold, 1994). Hence, one can only conclude that the uranium concentration determined for SSTR glass fragments A and B is not applicable for SSTR glass fragment C or that the a posteriori track density O,p~ 15

38

R. G O L D

tracks/cm' has not been corrected for other possible sources of background. In this regard, there is no mention in the M - S study of any correction to account for the imperfection track density background, Qi. In fact, based on past experience with glass SSTR, one would expect that most, if not all, of the a posteriori track density in SSTR glass fragment C, i.e. flap~ 15 tracks/cm 2, arises from imperfections and artifacts rather than from neutron induced fission tracks! 3.1.2. R e c o m m e n d e d m e t h o d . Since accurate determination of uranium concentration is crucial, it is identified as the ninth critical requirement, CR 9. For reactor irradiations to determine &, it is recommended that the a posteriori SSTR specimens be irradiated in a face-to-face configuration with a conventional SSTR neutron dosimeter. (ASTM, 1982; Gold et aL, 1968). The conventional SSTR neutron dosimeter should be comprised of a mica SSTR in firm contact with a calibrated 235U fission deposit. The mass density of the 235U fission deposit should be less than 100/~g/cm 2, in order to insure the applicability of the optical efficiency established for mica SSTR, i.e. ~/= 0.9875 _+ 0.0085 (ASTM, 1982). The calibrated fission deposit should be of high quality and, in particular, possess uniform mass density. This characteristic will facilitate determination of uranium concentration, should it be necessary to satisfy CR 8. From the analysis given in Section 2 above, for such a face-to-face irradiation, one has for the induced track density, ~ , in the a posteriori SSTR Qi : (I)i O'fS~ a25 ,

(18)

whereas the track density Qc, of the conventional mica SSTR can be expressed as Qc : /']#c(I)io'f,

(19)

where /1c is the mass density (#g/cm 2) of the calibrated 235U fission deposit. Solving equation (19) for the quantity ~iaf, which is simply the number of fissions per 235U atom created in the irradiation, one finds O~af = Qc/(q#~).

(20)

Using this result in equation (18), one can write S:c a25 =

rl#c(o~i/Qc).

well as af, neutron cross-section values. As a consequence, this recommended method possesses considerably higher accuracy and reliability. Since a2s/a2s = 7.25 x 10 -3, one has from equation (21) a2ssoo = 1.38 x 10 +5 q/zc(Qi/0c),

which is the effective 23sU concentration (atoms/cm 2) in the a posteriori SSTR specimen. In order to satisfy CR 4, this value of a2aso, can be used in equation (4) along with the age of the SSTR specimen to determine the background track density, Qs. In this recommended method, one must obviously observe the a posteriori track density, Q~v, prior to any reactor irradiation. After the reactor irradiation, one observes a total track density, Qt, given by Qt = Qi + Qap'

(23)

Rearranging equation (23) one obtains Qi in terms of the pre-irradiation and post-irradiation track densities as Qi = Qt - Qap.

(24)

In the interpretation of Q~, an important distinction exists between the two types of a posteriori SSTR used in the M - S study. The zircon SSTR specimens extracted from the granite fragment possess surfaces that were in contact with the granite medium. In contrast, the surfaces of the stained glass SSTR were free from contact with any exterior medium. Uranium impurities in the granite medium can contribute to the fission track density, Q~p, observed at the surface of the zircon SSTR, whereas no such contribution can exist for the stained glass SSTR. When an exterior medium exists for a given a posteriori SSTR, one must polish the surface of the SSTR before etching to remove a thickness greater than the fission fragment range in order to remove any fission tracks contributed by uranium impurities in the exterior medium. The procedure outlined below can be used to remove fission tracks contributed by uranium impurities in the exterior medium. I. Determination of a posteriori track density, Q~p. Step 1.

(21)

Hence, the effective 235U concentration (atoms/cm 2) in the a posteriori SSTR, s~ a25, is determined from the ratio of the observed track densities, with Qi track density induced in the a posteriori SSTR and Qc the track density in the mica SSTR. The remaining quantities in equation (21) are known, since r/is the established optical efficiency of mica and #c is the measured mass density of the calibrated 23sU fission deposit. This recommended method of quantifying uranium concentration not only eliminates many of the assumptions entailed in the M - S method, but it is independent of ~ , neutron fluence measurements, as

(22)

Step 2. Step 3.

Polish the a posteriori SSTR surface to remove a thickness greater than the fission fragment range. Etch the a posteriori SSTR. Scan the a posteriori SSTR to determine

~ap" II. Determination of uranium concentration. Step 4.

Step 5. Step 6.

Assemble the a posteriori SSTR and conventional SSTR neutron dosimeter in a face-to-face configuration. Irradiate this assembly in a reactor to the appropriate neutron fluence. Etch and scan the mica from the conventional SSTR neutron dosimeter to determine Q¢.

T R A C K R E C O R D E R N E U T R O N DOSIMETRY Step 7.

Polish the a posteriori SSTR surface in a manner identical to Step 1. Step 8. Etch the a posteriori SSTR in a manner identical to Step 2. Step 9. Scan the a posteriori SSTR to measure the total track density, 0t; determine the induced track density Q~= 0t - Qap. Step 10. Use Qi and Qc in equation (21) to determine the effective 235U concentration, a25sac. It is mandatory that clean laboratory procedures be followed throughout this process. In order to obtain a representative uranium concentration, track scanning in Steps 3 and 9 should be carried out to integrate or average over the same identical a posteriori SSTR surface area. For a posteriori SSTR free from contact with an external medium, one has the alternative of using the above procedure with or without Steps 1 and 7. For this case, if Step 1 is eliminated then Step 7 must be eliminated and conversely if Step 1 is used then Step 7 must be used.

3.2. Annealing effects Annealing is a fundamental attribute of charged particle tracks in SSTR. Investigation of annealing effects to date have been primarily concerned with providing corrections for fission track dating (Fieischer et al., 1965a, b; Storzer and Wagner, 1969; Fleischer et al., 1975; Somogyi and Nagy, 1972). However, more recent interest in annealing phenomena in SSTR has been spurred by the use of SSTR neutron dosimetry at elevated temperatures in power reactors (Gold et al., 1980b, c). Annealing represents an inherent limitation for the applicability and accuracy attained by the SSTR method. Hence, one must quantitatively account for annealing effects in analyzing experimental data obtained at elevated temperatures from specific SSTR, such as the a posteriori SSTR specimens from the Hiroshima and Nagasaki sites. Consequently, the tenth critical requirement, CR 10, is the accurate treatment of annealing effects. The method used to treat annealing in the M - S study is described first. However, in this study only the zircon SSTR data, but not the stained glass SSTR data, were analyzed to infer a Nagasaki site temperature. Hence, only the M - S analysis of the zircon SSTR data need be considered. A recommended method is then advanced, which can overcome the difficulties encountered in the M - S method. 3.2.1. M - S method. The primary purpose stated in the M - S fission track study at the Nagasaki site was the determination of temperature. In this study, annealing characteristics of fission tracks in zircon crystal SSTR specimens were analyzed in an attempt to determine exposure temperatures at the Nagasaki site. After the zircon SSTR were scanned to determine the a posteriori track density, Q,~, the uranium concentration was determined as described in Section

39

3.1.1 above. Following the reactor irradiation, the zircon SSTR were scanned to determine the induced track density, O~. Then the ratio of track density, 0ap/0~ was formed for each zircon SSTR specimen. In the M - S study, it was found that the Qap/Qiratio averaged over 16 top surface zircon specimens was 1.41 + 0.03, whereas the average Qap/Qi ratio for 23 bottom zircon specimens was 1.61 + 0.03. From this change in average track density ratio, it was deduced that the top zircon SSTR specimens possessed approximately a 12 percent fission track density decrease due to annealing. An annealing formulation based on the Arrhenius equation of reaction rate theory (Fleischer et al., 1975) was then used to analyze this inferred 12 percent decrease in track density. Unfortunately, this annealing analysis yielded ambiguous temperature results. This analysis of the inferred decrease in fission track density of the top zircon crystals yielded an exposure temperature in excess of 900°C. However, this temperature is not supported by M - S observations in stained glass SSTR, which by their own admission should have melted at 900°C, but did not show any evidence of such deformation or degradation. Qualitative annealing characteristics of SSTR, as summarized by Fleischer et al. (1975), reveal that zircon crystal exhibits complete (100%) fading of fission tracks at temperatures above approximately 800°C, whereas complete (100%) fading occurs in glass SSTR at temperatures above approximately 500°C. Hence if the exposure temperature was in excess of 900°C, as deduced in the M - S study, all fission tracks recorded prior to the atomic bomb detonation at the Nagasaki site should have been completely eradicated. Only fission tracks produced after the detonation should be observed. As has been demonstrated above, the principal source of these remaining fission tracks arises through the spontaneous fission decay of 238U. Since the zircon crystals of the M - S study possess a typical uranium concentration of Um ~ 500 ppm, the equations introduced in Section 2 above can be used to show that Qs ~< 10 tracks/cm 2. However, the reported a posteriori track density, Qap, in the M-S study was in excess of 106 tracks/cm 2 for all zircon SSTR. There are a number of reasons for the deduced temperature ambiguities in the M - S study. In the first place, the 12 percent decrease observed in average value of Qap/Q~ for the top relative to the bottom zircon SSTR may not be due to annealing. The observed 12 percent decrease could simply be a statistical consequence of the track scanning techniques that were employed, since all the zircon SSTR in the M - S study possess highly non-uniform track density. There is another possible reason for the observed 12 percent decrease that may not be due to annealing from elevated temperatures experienced during the atomic burst. As was already concluded in Section

40

R. G O L D

3.3.1, because of the age of these zircon specimens, Or<
(25)

If all zircon specimens are exposed to the same neutron fluence in a single irradiation, then equation (25) can be written in the form

Qap/Qi "~ T/C,

(26)

where the constant C is given by C = (2azss~)/(qtI)icrfd25).

(27)

Equations (26) and (27) show that even if all zircon SSTR specimens were irradiated to the same neutron fluence, the ratio Q~p/Oi depends on the age of the SSTR specimen. In addition, this ratio also depends on any extended annealing that may have occurred over the age, T/> 107 y, of the zircon SSTR specimen. Neither the age nor the extended annealing history of all these zircon SSTR need be identical. Hence, the observed 12 percent decrease could be due to a slightly different age distribution for the top and bottom zircon SSTR specimens or a slightly different extended annealing history experienced by the top and bottom zircon SSTR specimens. Perhaps a more fundamental reason for these temperature ambiguities lies in the annealing formulation adopted in the M - S study. At the time of Matsuda's thesis (1977), it was fashionable to describe the disappearance of tracks due to annealing with a simple Arrhenius equation of reaction rate theory (Fleischer et al., 1975). Subsequent study of annealing phenomena in SSTR has demonstrated that this formulation is incorrect (Gold et al., 1981). Rather than digress any further into theoretical considerations of track annealing in SSTR, it is more instructive to quote a conclusion from the analysis advanced by Gold et al. (1981): "Use of the Arrhenius equation to describe the decrease in track density resulting from annealing has been shown to be incorrect. The Arrhenius equation describes the temperature dependence of rate constants obtained from rate equations which, in turn, depend on the chemical kinetics for a particular chemical reaction process. The observed reduction in track density induced by annealing is in no way related to these Arrhenius rate constants. Although selected fits obtained in this way can possess useful empirical value, the Arrhenius activation energies extracted in this way are physically meaningless." It automatically follows that the absolute temperatures deduced with such a formulation are also physically meaningless. In spite of these temperature ambiguities, it must be emphasized that the actual significance of the M - S study lies in the demonstration that zircon SSTR from the Nagasaki site have retained a substantial fission track density! 3.2.2. Recommended method. Annealing induced track disappearance is not an instantaneous transition. Experimental results reveal that annealing

dramatically alters the structure of individual tracks (Gold et al., 1979; Roberts et al., 1980). Hence, tracks are not immutable, but are composite macroscopic entities which undergo distinct changes in structure during annealing. These composite entities or tracks consist of many radiation induced damage sites or defects. The radiation damage zone constituting the track can therefore be viewed as an ensemble of defects. Thermal annealing causes individual defects to be modified or removed, resulting in reduced track etch rates. These reduced track etch rates produce smaller track size, which makes some tracks more difficult to observe and can reduce the size of some tracks below the optical threshold of observation. Consequently, under annealing, tracks disappear from view because of this essentially continuous fading process, rather than instantaneous transition assumed in earlier formulations based on the Arrhenius equation of reaction rate theory (Fleischer et al., 1975). Thermal annealing of tracks can be increased by either an increase of temperature for a fixed time duration or an increase in time duration for a fixed (elevated) temperature. By continuously increasing the thermal history of the SSTR, tracks can be reduced in size to the point where they are all eradicated. Annealing studies in a wide variety of SSTR, such as quartz crystals, muscovite mica, quartz glass and cellulose nitrate (CN) (Gold et al., 1979; Roberts et al., 1980; Roberts et al., 1982; Gold et al., 1988) have demonstrated that a characteristic dimension of track size can be used to represent the thermal annealing history of the SSTR. The characteristic track dimension can be any geometric feature of the track, such as the diameter, length, main diagonal, area . . . etc. This characteristic dimension is a representation of the thermal annealing state of the SSTR, independent of the time-temperature path used to reach this annealing state. These annealing studies have demonstrated the validity of an accurate empirical method of correction for track density losses due to annealing. In this recommended method, calibration data for the type of SSTR of interest are generated by measuring track density loss and characteristic track dimension for different annealing states. Since the characteristic track dimension represents the annealing state of the SSTR, these experimental data determine the track density loss as a function of the characteristic track dimension, i.e. an empirical annealing calibration curve. Quantitative correction of track density loss induced by annealing can then be determined by applying this empirical calibration curve for a specific SSTR. Observation of the characteristic track dimension defines the annealing state of the specific SSTR and hence the point at which the calibration curve is evaluated. To cite examples that may be relevant to a posteriori SSTR neutron dosimetry at the Hiroshima and Nagasaki sites, this empirical method affords accurate correction of up to 73 percent loss of

TRACK RECORDER NEUTRON DOSIMETRY

41

Table 2. Critical requirements for a posteriori SSTR neutron dosimetry at the Hiroshima site CR No. 1

Critical requirement description

Recommendations

Alternative a posteriori neutron dosimetry in the critical ground range region 1 km ~
A posteriori SSTR neutron dosimetry

3

A posteriori SSTR specimen age less than a few hundred years

Antique glassware, crystalware, pottery, porcelains, tiles, baked relics. . . . etc.

4

Accurate determination of background from spontaneous fission track density, 0f

Determine a posteriori SSTR specimen age T and uranium concentration Um

5

Attain at least 10% statistical precision

6

Accurate determination of background from imperfection fission track density, 0~

Scan at least 100 foreground tracks; satisfy equation (11) Anneal, etch and scan a posteriori SSTR type to measure QI

7

Eliminate contamination from tramp uranium

8

Account for non-uniform uranium concentration

9

Accurate determination of uranium concentration

Use recommended method described in Section 3.1.2

Accurate treatment of annealing effects

Use recommended method described in Section 3.2.2

2

10

track density in quartz glass SSTR and up to 50 percent loss of track density in natural quartz crystal SSTR due to thermal annealing. In order to carry out this recommended correction method for annealing induced track density loss in a given type of a posteriori SSTR, one must carry out an annealing study to obtain an empirical calibration curve for this type of SSTR. A significant aspect of the annealing study is the selection of the optimum characteristic track dimension for the given type of SSTR. To apply this empirical correction method, one must obviously be able to accurately measure the characteristic track dimension in the a posteriori SSTR of interest!

4. C O N C L U S I O N S It is apparent that conducting a posteriori SSTR neutron dosimetry at the Hiroshima and Nagasaki sites is a formidable endeavor. Table 2 provides a summary of the critical requirements that have been identified in such an endeavor. However, it must be emphasized that satisfying these critical requirements does not insure success. Indeed, these critical requirements are necessary, but not sufficient, conditions. In this regard, annealing effects are a particularly relevant example. The M - S study shows that zircon SSTR specimens can retain fission tracks in spite of the elevated temperatures that were experienced at these sites. However, no definitive conclusions about annealing effects in the stained glass SSTR can be drawn from the M - S study. As stressed earlier, the a posteriori track density observed in glass fragment C, Q~p~ 15 tracks/cm 2, can easily be accounted for by the imperfection track density, Qj, commonly observed in glass SSTR.

Historical records, city plans, maps . . . . etc.

Institute clean laboratory procedures; demonstrate these procedures introduce negligible track density Scan SSTR to obtain integral or suitably averaged track densities

Based on these critical requirements together with the numerical estimates already given in Section 2 above, it appears that a posteriori SSTR with Um/> 103 ppm and T ~< 200y could yield reasonably accurate fluence data in the ground range r ~< 1 km at the Hiroshima site. Of course, one must at the same time assume that track density data from these a posteriori SSTR can be corrected to account for any loss due to annealing. The uncertainty in these a posteriori SSTR neutron fluence data would increase rapidly as r increases beyond 1 km. Even under the present assumptions, the region r ~< 1.2 km is the most one can reasonably expect for the domain of applicability of a posteriori SSTR neutron dosimetry at the Hiroshima site. A somewhat larger domain of applicability, say perhaps r ~< 1.5 km, would arise if the underprediction in DS86 calculations should prove valid. For a qualitative evaluation of this underprediction, see the companion paper (Gold, 1994).

REFERENCES ASTM E854-81 (1982) Standard method for application and analysis of solid state recorder (SSTR) monitors for reactor vessel surveillance. 1982 Annual Book of ASTM Standards, American Society for Testing and Materials, Part 45. ASTM, Philadelphia, PA. ASTM E1004-84 (1984) Analysis of RM monitors for reactor vessel surveillance. 1986 Annual Book of ASTM Standards, American Societyfor Testing and Materials, Part 45. ASTM, Philadelphia, PA. ASTM E910-82 (1986) Application and analysis of helium accumulation fluence monitors for reactor vessel surveillance. 1986 Annual Book of ASTM Standards, American Society for Testing and Materials, Part 45. ASTM, Philadelphia, PA. Banham M. F., Fudge A. J. and Tibbles J. A. (1992) Retrospective neutron dosimetry: a review of possible

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applications for some nuclear reactor materials. Report AEA-FS-0800 (H), AEA Technology Fuel Services, Harwell. Fleischer R. L., Price P. B. and Walker R. M. (1965a) Effects of temperature, pressure and ionization on the formation and stability of fission tracks in minerals and glasses. J. Geophys. Res. 70, 1497-1502. Fleischer R. L., Price P. B. and Walker R. M. (1965b) Solid state track detectors: applications to nuclear science and geophysics. Ann. Rev. Nucl. Sci. 15, 1-28. Fleischer R. L., Price P. B. and Walker R. M. (1975) Nuclear Tracks in Solids: Principles and Applications. University of California Press, Berkeley, CA. Fleischer R. L. (1987) Serendipitous dosimetry--an opportunity and an opportunity lost. Health Phys. 52, 219-221. Fleischer R. L. (1993) Needed: Hiroshima/Nagasaki glass for dosimetry. Health Phys. Soc. Newsletter 21, 4. Gold R., Armani R. J. and Roberts J. H. (t968) Absolute fission rate measurements with solid state track recorders. Nucl. Sci. and Engng. 34, 13-32. Gold R., Ruddy F, H. and Roberts J. H. (1979) Advances in SSTR techniques for dosimetry and radiation damage measurements. In Proc. 3rd International A S T M - E U R A T O M Symposium on Reactor Dosimetry, Ispra (Varese), Italy, pp. 1172 1187. Gold R., McElroy W. N., Lippincott E. P., Mann F. M., Oberg D. L., Roberts J. H. and Ruddy F. H. (1980a) Cross sections required for FMIT dosimetry. Presented at Symposium on Neutron Cross Sections from IO-50MeV. Brookhaven National Laboratory, New York. Gold R., Ruddy F. H. and Roberts J. H. (1980b) Applications of solid state track recorders in United States nuclear reactor energy programs. In Proc. lOth Int. Conf. Solid State Nuclear Track Detectors, Lyon, France, pp. 533-547. Gold R., Ruddy F. H. and Roberts J. H. (1980c) Solid state track recorder applications in United States nuclear reactor energy programs. Trans. Am. Nucl. Soc. 34, 146-148. Gold R., Ruddy F. H. and Roberts J. H. (1981) Annealing phenomena in solid state track recorders. Nucl. Tracks 5, 253-264. Gold R. (1984a) Minutes of ASTM Task Group E10.05.11, on reactor component monitoring, Geesthacht, Federal Republic of Germany, September 24-28, 1984. Gold R. (1984b) Preliminary program plan for SSTR neutron dosimetry at the Hiroshima and Nagasaki sites. WHC Proposal (8452692), August 14, 1984. Gold R. and Roberts J. H. (1984) Addendum on the preliminary program plan for SSTR neutron dosimetry at the Hiroshima and Nagasaki sites. WHC Proposal Addendum, December, 1984. Gold R. and Roberts J. H. (1985) Applicability of track recorder methods for neutron dosimetry at Hiroshima and Nagasaki. Bull. Am. Phys. Soc. 30, 20. Gold R. (1986) Minutes of ASTM Task Group El0.05.11 on plant life extension, Seattle, WA, 26 June, 1986. Gold R. and McElroy W. N. (1986) Non-destructive method for determining neutron exposure and con-

stituent concentrations of a body. U.S. Patent No. 4,622,200. Gold R. (1987a) Minutes of the ASTM Task Group El0.05.11 on plant life extension, Tampa, FL, 27 January, 1987. Gold R. (1987b) Minutes of the ASTM Task Group El0.05.11 on plant life extension, Jackson Hole, WY, 29 May, 1987. Gold G. E., Gold R., Roberts J. H. and Preston C. C. (1988) Annealing of alpha-particle tracks in cellulose nitrate. Nucl. Tracks Radiat. Meas. 14, 467-475. Gold R. (1994) The DS86 neutron dosimetry enigma: an analysis of some missing pieces to the puzzle. Radiat Meas. 24, 9-29. Kerr G. D., Pace III J. V., Mendelsohn E., Loewe W. E., Kaul D. C., Dolastshahi F., Egbert S. D., Gritzner M., Scott W. H. Jr., Marcum J., Kosako T. and Kanda K. (1987) Transport of Initial Radiations in Air Over Ground. In US-Japan Joint Reassessment o f Atomic Bomb Radiation Dosimetry in Hiroshima and Nagasaki, Final Report, Vol. 1, Chapter 3. Radiation Effects Research Foundation, Hiroshima, Japan. Loewe W. E. (1983) Initial radiations from tactical nuclear weapons. UCRL-90018. Loewe W. E., Mendelsohn E., Hamada T., Maruyama T., Okajima S., Pace J. V. III, Sakanoue M., Kondo S., Hashizume T., Marcum J. and Woolson W. A. (1987) Measurements of Neutron Fluences. In US-Japan Joint Reassessment o f Atomic Bomb Radiation Dosimetry in Hiroshima and Nagasaki, Final Report, Vol. 1, Chapter 5. Radiation Effects Research Foundation, Hiroshima, Japan. Matsuda H. (1977) Studies on fission tracks and distributions of uranium and rare earths in granite minerals. Master's thesis, Faculty of Science, Kanazawa, Japan. Matsuda H. and Sakanoue M. (1987) Studies on Fission Tracks and Distributions of Uranium and Rare Earths in Granite Materials. In US-Japan Joint Reassessment o f Atomic Bomb Radiation Dosimetry in Hiroshima and Nagasaki, Final Report, 1Iol. 2, Chapter 5. Appendix 15 Radiation Effects Research Foundation, Hiroshima, Japan. Nishimura S. (1971) Fission track dating of archaeological materials from Japan. Nature 230, 242. Roberts J. H., Gold R. and Ruddy F. H. (1980) Thermal annealing studies in muscovite and in quartz. In Proc. lOth Int. Conf. Solid State Nuclear Track Detectors, Lyons, France, pp. 177 189. Roberts J. H., Gold R. and Ruddy F. H. (1982) Selected etching and annealing properties of Brazilian-quartz crystals for solid state track recorder applications. In Proc. l lth Int. Conf. Solid State Nuclear Track Detectors, Bristol, UK, pp. 417-420. Somogyi G. and Nagy N. (1972) Remarks on fission-track dating in dielectric solids. Radiat. Effects 16, 223-231. Storzer D. and Wagner G. A. (1969) Correction of thermally lowered fission track ages of tektites. Earth Planet. Sci. Lett. 5, 463-468. Watanabe N. and Suzuki M. (1969) Fission track dating of archaeological glass materials from Japan. Nature 222, 1057.