Progress in albedo neutron dosimetry

N U C L E A R I N S T R U M E N T S AND METHODS 145 (1977)

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N O R T H - H O L L A N D P U B L I S H I N G CO.

PROGRESS IN ALBEDO NEUTRON DOSIMETRY E. PIESCH

Karlsruhe Nuclear Research Center, Health Physics Division, Karlsruhe, W. Germany Received 7 April 1977 The review of the current state in albedo dosimetry shows that only dosimeters of the discriminating type fulfill the requirements for routine application in personnel monitoring. Above all, the properties of the recently developed Karlsruhe albedo neutron d6simeter will be discussed and compared to other dosimeter designs.

1. Introduction In the last years many attempts were made to develop albedo neutron dosimeters and to introduce this kind of dosimeter in routine monitoring. In contrast to insufficient results of calculations, albedo dosimeters are applied in personnel dosimetry mainly in high energy neutron fields, where the dose equivalent from neutrons above 10 keV is predominant. The reason for such application can be found in the high thermal neutron response of LiF resulting in a nearly equal dosimeter response for fast neutrons and gamma rays. In addition, in most practical cases there is a broad neutron leakage spectrum due to absorption, moderation and backscattering in shielding and wall, resulting in a lower energy dependence of the albedo dosimeter response compared to the response to monoenergetic neutrons. The special design of the aibedo dosimeter may modify or improve the response only to a small extent. Therefore, the application in personnel dosimetry is limited by additional information about the neutron spectrum at the facility where the dosimeter is used. The recently developed Karlsruhe Albedo Neutron Dosimeter shows favourable properties with respect to a low dependence of the dosimeter reading on neutron energy and the direction of the incident radiation. 2. Dosimeter response Albedo neutron dosimetry is thus based on the effect that incident neutrons are moderated and backscattered by the human body creating a neutron flux at the body surface especially in the thermal and intermediate energy range. These backscattered neutrons - albedo neutrons - can be detected by a dosimeter placed on the body which is usually designed to detect thermal neutrons. An

albedo neutron dosimeter makes use of a cadmium or boron capsule facing the source at least on one side to eliminate incident thermal neutrons and to realize a dosimeter response which is caused mainly by thermal neutrons backscattered from the body. The possibility of detecting backscattered neutrons was investigated earlier by Dennis et al.]), Harvey 2,3) and Alsmiller4). The neutron albedo factor, defined as the ratio of neutron fluence scattered from the body to the total incident neutron fluence entering the body, varies between approximately 0.8 for thermal neutrons and 0.1 for neutrons of 1 MeV. Taking into account fluenceto-dose-equivalent conversion factors, the relative response expected for an albedo dosimeter is presented in fig. 1. The response of an albedo dosimeter expected for exposures to monoenergetic neutrons is found to be high and nearly equal in the thermal and intermediate energy range and decreases rapidly above 10 keV resulting in a relative response of approximately 1% for fast neutrons. Therefore albedo dosimeters may be applied preferably in the measurement of the dose equivalent of neutrons below 10 keV. As has been shown by calculations and experimental results, there is no

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possibility to improve the response of an albedo dosimeter in the energy range above 10 keV by using additional shieldings of cadmium or boron or a special dosimeter design, for instance with additional moderators or changes in the distance of the detector to the body.

3. Dosimeter designs Most of the albedo neutron dosimeters and systems make use of special shield of cadmium, boron or graphite mainly to absorb incident thermal neutrons and thus to exclusively measure backscattered neutrons. Pairs of 6LiF and 7LiF detectors are used to measure thermal neutrons with a high sensitivity via the 6Li(n, ~) reaction and to separate the gamma dose fraction by using the difference in the dosimeter readings. Instead of LiF:Mg, Ti, Li2B407:Mn detectors are also in use. Because of the high energy dependence of the neutron dose measurement there are different kinds of albedo dosimeter systems in use which do or do not allow a discrimination and/or separation of incident and backscattered thermal neutrons (see also fig. 2). 1) The non-discriminating type detects both the incident and the backscattered neutrons with a single detector without using a neutron shield on one part of the capsule. 2) The discriminating type exclusively detects backscattered neutrons by using a single detector inside a neutron shield in front of the

dosimeter which is open on the side facing the body. 3) The discriminating analyser type separately detects incident as well as backscattered neutrons by using at least two detectors one of which is positioned inside and the other outside a neutron shield to get additional information about the incident neutron field. The 6LiF detector of the non-discriminating albedo dosimeter type may be used without any neutron shield directly as a part of the dosimeter or positioned inside a polyethylene and/or in a completely enclosed cadmium shielding5,6). In the latter case such albedo dosimeters preferably detect backscattered intermediate neutrons resulting in a neutron response which is lower by a factor of 0.1. The oversensitivity or undersensitivity of these albedo dosimeter types especially to low energy neutron leakage does not allow an estimation of the neutron dose even in the dose range below 10 keV, unless location-dependent correction factors are applied additionally. Discriminating albedo dosimeters of the type (1) exclude the detection of incident thermal neutrons by using a cadmium 5) or a boron shield as described by Harvey 2'3) on one side of the detector. This albedo dosimeter type allows dose equivalent measurements in the energy range below 10 keV. Albedo dosimeters of the discriminating analyser type make use of two or three different 6LiF detectors which are positioned in special shieldings

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in such a way that at least incident thermal neutrons can be indicated separately in order to correct the oversensitivity to thermal and intermediate neutron leakage. In this case also the dose equivalent of fast neutrons may be measured, for instance with the Harvey boron-plastic capsule and an additional detector outside 8) or with a special cadmium-plastic combination as mentioned by

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Hoy]°). Due to the high influence of the primary spectrum on the dosimeter response, correction factors for the type of the neutron source or of the nuclear facility are applied. As these correction factors may vary in addiation with the location in the radiation field of intermediate neutrons, the Karlsruhe albedo dosimeter 9) makes use of three detectors inside a boronloaded plastic encapsulation allowing a separate indication of incident thermal neutrons (detector a) and of intermediate neutrons (detector m) (fig. 3). The detectors applied are TLD 600 and TLD 700 ribbons of size 3 × 3 × 0.8 mm 3 (manufacturer Harshaw). A constant distance between dosimeter and body of the dosimeter's wearer is realized by a dosimeter belt. The favourable properties of the discriminating analyzer type are demonstrated in fig. 4. Here the change of the neutron response is presented as a function of the relative dose fraction of additional thermal neutrons in the radiation field of 2s2Cf fission neutrons. By using the reading ratio i/a, the energy dependence of the discriminating detector (i) may be reduced to a value of approximately 30%. On the basis of three dosimeters, the energy dependence of dose reading can be corrected more effectively with respect to changes of the primary or local neutron spectrum in the intermediate energy range. The energy dependence of the Karlsruhe albedo dosimeter is shown in fig. 5 for monoenergetic neutrons 26) as well as for different neutron facilities and neutron spectra from neutron sources behind various shieldings which represent a high

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5. Accuracy Up to now there is no albedo dosimeter system in use which may serve as an energy-independent dosimeter in the energy range of interest. With re-

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fraction of thermal and intermediate neutrons due to scattering and moderation. For a neutron source of a given mean energy, therefore, the variation in neutron response is indicated by the 'neutron spectra' area. The response curves before or after correction demonstrate the advantage of the albedo dosimeter of the discriminating analyzer type to reduce energy dependence. 4. D o s i m e t r i c properties The reading o f an albedo neutron dosimeter depends on the special dosimeter design and is highly affected by influences o f field parameters, such as shape and size o f the dosimeter encapsulation, varying distances o f the detector from the body, the fraction o f incident thermal neutrons (fig. 4), and the fraction of thermal neutrons backscattered from the wall and the floor. The main disadvantages o f albedo dosimetry are the relatively low response and the energy dependence in the fast neutron energy range, the high influence o f incident thermal neutrons on the dose indication and therefore the need for a relatively sophisticated dosimeter design if higher accuracy is desired. The main properties of LiF albedo dosimeters are the detection o f neutrons without any energy

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diate and thermal neutron energy range, especially if the dose equivalent of fast neutrons should be measured. T h e accuracy of dose m e a s u r e m e n t is poor w h e n only a single detector is used and no additional information about the local radiation field is available. Local correction factors, therefore, must be taken into account for each location at a reactor or nuclear facility which depend on the dosimeter design and the local change of the neutron spectrum. Local correction factors for the dosimeter response are based on field measurements. In albedo dosimetry, different kinds o f errors must be taken into account such as - Errors due to the evaluation of the T L D detectors which may be relatively small, in the order of 5-10%. - Errors of dosimeter calibration may be in the order of a factor 2 because of the energy dependence of the reference instrument or the secondary standard instrument used for field calibration. This uncertainty is the difference between the estimated dose at the location and its true value. - Errors o f m e a s u r e m e n t defined here as the residual difference between the corrected mea-

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sured dose and the estimated dose at the location of interest. This error highly depends on the design of the albedo dosimeter type, the reference instrument used for field calibration, the technique of calibration and the local change of the neutron spectrum as well as of the dosimeter response. The error of measurement should be discussed in the following part not taking into account local correction factors bdsed on survey measurements. For non-discriminating albedo dosimeters, local changes in the neutron spectrum around one facility may result in a change of the dosimeter response of the order of a factor of 10-15, in optimal cases a factor of 4 when a cadmium encapsulation excludes the detection also of thermal albedo neutrons. Instead of a dosimeter calibration, local calibration factors are used here on the basis of survey measurementsS). The neutron response of discriminating albedo dosimetry may vary by a factor of 15 for different facilities and by a factor of 4-6 around one facility. Further reduction of the energy dependence is based on the dosimeter reading of additional detectors. This can be seen in fig. 7 for a two-detector system (type i in fig. 2) in the radiation field of various fast neutron sources and for different directions of the radiation incidenceS). The results demonstrate the reduction in energy and direction dependence of an albedo dosimeter system, which consists of a dosimeter in front and in the rear of the wearer. The Karlsruhe albedo dosimeters of the discriminating analyzer type may reduce the uncertainty of dose measurement due to the local change of the leakage spectrum around one facility from a factor of 4 to approximately _-+-25% (fig. 8). These data include influences from different directions of the incident radiation on the dose reading of a dosimeter belt. For the neutron spectra and facilities investigated so far, the dosimeter response may vary up to a factor of 6. Albedo dosimeters may, therefore, be applied in fast neutron fields - excluding monoenergetic neutrons - with a sufficient accuracy of dose measurement if different calibration factors for each facility or primary neutron spectrum are used and if the albedo dosimeter system allows to correct for local changes of the neutron spectrum.

7. Application Various field experiments with dosimeter encap-

sulations of different shape and size have been performed in the past to find out an albedo dosimeter capsule suitable for routine applicationS'23). Most of the albedo dosimeters presented in fig. 2 are applied at different installations as routine dosimeters or at an experimental stage as parts of the routine dosimeter. The boron-plastic Harvey dosimeter (type f in fig. 2) is routinely used at reactor sites for the exclusive detection of thermal and epithermal neutrons. Local calibration factors may be applied to correct for fast neutrons above 10 keV 7). The commercially available Karlsruhe albedo dosimeter (type m in fig. 2) makes use of three LiF pairs for long-term monitoring and of a dosimeter belt with one dosimeter at the front and rear side. Local correction factors derived from different dosimeter readings may be applied in addition to source-dependent calibration factors for the estimation of the dose equivalent at reactors and acceleratorsg). The albedo dosimeter was calibrated in fast neutron fields [14 MeV, Am-Be, 252Cf-neutrons and monoenergetic neutrons26)], at reactor sites at Oak Ridge29), Vinca2°), Harwell, Karlsruhe FR 2 as well as in the stray radiation field around the CERN 28 GeV Proton Synchrotron2} and at the Heavy-Ion Accelerator at Darmstadt. The source-dependent calibration factor was found to vary between 0.3 and 3 R/Rein for the spectra investigated. In routine monitoring, especially at reactor sites, the albedo dosimeter indicates high neutron exposures due to intermediate neutrons which have not been detected so far by NTA films or fission fragment track etching detectors. A combination of a 237Np track etching detector and a single albedo dosimeter, for instance of the Harvey type, may detect neutrons below 10 keV and above the energy threshold of 0.7 MeV. A similar combination uses 232Th and an albedo dosimeter30). In this case the accuracy of dose measurement may be improved mainly in the dose range above the energy threshold. In the energy range of 200 keV, for instance, such a dosimeter combination underestimates the dose equivalent by a factor of 3 whereas an albedo dosimeter of the discriminating analyzer type overestimates the dose equivalent by a factor of 3.

References 1) I. A. Dennis, I. W. Smith and D. 1. Boot, Proc. IAEA Symp. on Neutron monitoring (1967) p. 547. 2) I. R. Harvey, Report RD 1 B/N 827 (1967).

ALBEDO N E U T R O N DOSIMETRY 3) I. R. Harvey, W. H. R. Hudd and S. Townsend, Report R D / B / N 1547 (1971). 4) R. G. Alsmiller and I. Barish, Health Phys. 26 (1974) 13. 5) D. E. Hankins, Reports LA-5261 (1973) and LA-4832 (1972). 6) D. E. Hankins, in Report BNWL-1934 (1975) p. 43. 7) I. R. Harvey, W. H. R. Hudd, and S. Townsend, Proc. 1AEA Symp. on Neutron monitoring, vol. 2 (1973) p. 199. s) E. Piesch, B. Burgkhardt, ibid., p. 54. 9) E. Piesch, and B. Burgkhardt, Proc. Int. Conf. on Luminescence dosimetry, Krakow (1974) p. 1124. 1(3,) j. E. Hoy, USAEC Report DP-1277 (1972). ll) j. E. Hoy and R. M. Hall, in Report BNWL-1934 (1975) p. 54. 12) R. B. Falk, Dow Chemical Company Report RFP-1581 (1971). 13) R. V. Griffith, Report UCRL-51362 (1973). 14) C. N. Unruh, Reports BNWL-SA-2978 (1969) and BNWL1616 (1971). 15) H. E. Preston, H. E. and C. O. Peabody, IAEA Symp. on Neutron monitoring (1973). 16) C. Distenfeld, in Report BNWL-1934 (1975) p. 20.

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17) D. E. Hankins, Health Phys. 31 (1976) 170. 18) E. Piesch, B. Burgkhardt and A. M. Sayed, Proc. Int. Conf. on Luminescence dosimetry, Krakow (1974) p. 1201. m) B. Burgkhardt, R. Herrera, and E. Piesch, Nucl. Instr. and Meth. 137 (1976) 41. 20) E. Piesch and B. Burgkhardt, Report KFK 1971 (1974). 21) A. Knight, T. O. Marshall, C. L. Harvey and I. R. Harvey, Proc. IAEA Symp. on Neutron monitoring, vol. 2 (1973) p. 172. 22 E. Piesch and B. Burgkhardt, Proc. IRPA Congress Washington (1974). 23 E. Piesch, B. Burgkhardt and J. Vaane, Report KFK 1666 (1972). 24 T. R. Crites, Health Phys. 31 (1976) 154. 25 G. Lautenbach, G.BA.-memo no. 117 (1976). 26 I. A. Douglas et al., Report AERE-R 8436 (1976). 27 M. Ht~fert, J. Dutrannois, E. Piesch and A. Janett, CERN Report HS/HP 1004 (1976). 28 B. Burgkhardt, R. Herrera and E. Piesch, Proc. Int. Conf. on Luminescence dosimetry, Sao Paulo (1977). 29 L. W. Gilley et al., Report ORNL/TM-5672 (1976). 30 C. H. Distenfeld, CONF-760 202 - 25, BNL-20762 (1976).