Development and Characterization of a Neutron Personal Dose Equivalent Monitor N. Tsujimuraa*, T. Yoshidaa, T. Nunomiyab and K. Aoyamab a
Nuclear Fuel Cycle Engineering Laboratories, Japan Atomic Energy Agency, 4-33, Tokai-mura, Ibaraki-ken 319-1194, Japan. b
Fuji Electric Systems Co., Ltd., Fuji-cho 1, Hino, Tokyo 191-8502, Japan.
Abstract. The authors developed a new neutron-measuring instrument that was designed to measure a neutron personal dose equivalent, Hp(10). This instrument is composed of a conventional moderator-based neutron dose equivalent meter and a neutron shield made of borated polyethylene which covers a backward hemisphere to adjust the angular dependence. The whole design was determined on the basis of MCNP calculations so as to have response characteristics which would generally match both the energy and angular dependencies of Hp(10). The verification experiments for energy and angular responses were done using monoenergetic neutrons produced by the accelerator and polyenergetic neutrons from the moderated 252Cf source. Experiments showed that the new instrument has reasonably acceptable responses as a reference neutron dosemeter. The authors conclude that this new instrument will be a great help in assessing the reference values of neutron Hp(10) during field testing of personal neutron dosemeters in the workplace and also in interpreting their readings. KEYWORDS: personal dose equivalent; neutron dosemeter; MCNP; monoenergetic neutrons; 252Cf; moderator; field calibration. 1. Introduction In personal neutron dosimetry, it is important to establish correction factors to compensate for the differences in dosemeter responses between work environments and a calibration laboratory. This has often been accomplished by the field calibration method [1], in which a dosemeter is placed on a phantom in the representative work environment to simulate personnel wearing a dosemeter. The dosemeter readings are then compared with the reference neutron dose-measuring instruments. Moderator-based neutron dose equivalent rate meters (hereafter denoted as DE meters), operated in an integral mode, are often utilized as reference instruments [2-3] despite the fact that they are not entirely energy-independent. As stated in the ANSI/HPS N13.52 [4], the field calibration method is very practical and continues to play an important role in assuring accuracy in neutron dose assessments. However, there remains a fundamental problem [1]: in the actual workplace where neutron fields consist of multidirectional neutron components, the quantities that are to be measured with personal neutron dosemeters are inconsistent with the quantities normally measured with the instruments used for field dosimetry purposes. This inconsistency arises from the differences in definition between the personal dose equivalent (Hp(10)) and the ambient dose equivalent (H*(10)) and therefore results in inevitable quantitative differences. In other words, instruments such as DE meters have a nearly isotropic response, while personal dosemeters mainly measure neutrons incident within the frontal half hemisphere because of the shield effect by a wearer or a phantom. Therefore, these measurements are not comparable to each other unless both types of instruments are used in unidirectional neutron fields. For this reason, Piesch [5] suggested the necessity of a reference neutron dosemeter having an angular response comparable with that of the Hp(10) rather than that of the H*(10). To deal with this problem, studies have recently been initiated to develop new instruments that can allow a full characterization of the neutron fields as a function of energy and direction. The directional spectrometer, developed by Luszik-Bhadra et al. [6] under the framework of the EVIDOS project [7], can measure doubledifferential neutron fluence in any stray neutron fields and then derive reference values of Hp(10) in *
Presenting author, E-mail:
[email protected]
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conjunction with a set of fluence-to-Hp(10) conversion coefficients. However, this instrument appears to be too complicated in construction and associated electronics. Moreover, it is somewhat doubtful that accurate measurement of Hp(10) necessitates the use of such a complex spectrometer. The purpose of this study was to develop a new, user-friendly neutron Hp(10)-measuring instrument (hereafter the Hp(10) monitor) that can be utilized as a reference instrument for the field calibration method. A portion of the study has already been presented [8]. The present report describes the results of subsequent characterization tests. 2. Description of the Neutron Hp(10) Monitor The design goals for the new instrument were: (1) to match the energy and angular fluence responses to the energy and angular dependencies of fluence-to-Hp(10) conversion coefficients as closely as possible; (2) to obtain a counting efficiency comparable to that of a conventional DE meter along with simplicity in operation; and (3) to obtain transportability in field use. The most feasible approach to achieve all of these goals was to realize an instrument artificially provided with a neutron sensitivity that varies with the neutron energy and angle of incidence in accordance with the conversion coefficients curves given in ICRP Publ.74 [9]. According to this basic approach, the new monitor was configured so that the essential part of the DE meter was embedded into a neutron shield with dimensions large enough to cover the backward hemisphere. This asymmetric geometrical configuration allows the angular response of the instrument to be reduced in the larger angles of incidence (> 90º) without spoiling the flatness of the energy dependence in normal angles of incidence. This relatively simple configuration is advantageous for reliable operation. The dimensions of the components (moderator/absorber) and their arrangement were parametrically determined on the basis of the Monte Carlo calculations by modifying Nakamura’s design [10] of a stationary DE meter, and final adjustments were made by trial and error. Figure 1 shows the external view of the prototype Hp(10) monitor. This monitor is composed of a central spherical moderator, a front hemispherical shell moderator and a rear-lateral hemispherical/annular neutron shield. The central and front moderators are of polyethylene with a density of 0.92 g cm-3, and have radii of 4.3 cm and 11 cm, respectively. The rear-lateral shield is constructed of 30-cm-diameter borated polyethylene containing a boron oxide (B2O3) of 21 w%, with a density of 1.057 g cm-3. A fenestrated thermal neutron absorber made of a mixture of 50% boron carbide and 50% silicon, with a density of 1.17 g cm-3, is placed between the central and front polyethylene moderators so as to enable the instrument to have a significant response to thermal neutrons. The thermal neutron sensor at the center of the central moderator is a small cylindrical 3He proportional counter, type 0.5NH1/1K, manufactured by Eurisys Mesures. The counter is 10 mm in outer diameter by 10 mm long and is filled with a mixture of 3He of 0.8 MPa and additive Kr of 0.2 MPa. Figure 1: External view of the neutron Hp(10) monitor. The monitor is assembled by fixing components with aluminum flanges to facilitate deassembling for a future upgrade.
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The counter output is connected to a charge sensitive preamplifier, and then to an amplifier in a remote area away from the counter. The pulse signals are accumulated in a multichannel analyzer (or a scaler). The monitor is set on a turntable, as shown in Fig. 1, for a rotational orientation adjustment. The monitor weighs about 20 kg. 3. Methods of Characterization 3.1 Simulations To determine the final design of the Hp(10) monitor, the energy and angular responses of the monitor were calculated with the MCNP-4C code [11] with the ENDF/B-VI cross section library [12]. The MCNP calculation model is depicted in Fig. 2. The prototype monitor shown in Fig. 1 was built to reproduce the calculation model with the exceptions of flanges and bolts/nuts. The S(α,β) data tables were used to treat thermal neutron scattering from chemically bound hydrogen in the polyethylene. The number of the 3He(n,p)3H reactions in the 3He counter was scored by the track length estimate tally (F4 in MCNP) with the multiplier of the microscopic cross section taken from the LLLDOS library [13]. For the energy-dependence calculations, neutrons with selected energies ranging from thermal Maxwellian to 10 MeV were generated as a parallel beam source. Neutrons from 252Cf spontaneous fission were also used. For the angular-dependence calculations, the geometry model was rotated with the ordinates transformation card in 15º steps. In the calculations for each neutron energy and angle of incidence, neutrons were emitted perpendicularly from a circular surface, efficiently covering what appears from the source as a silhouette of the instrument. Figure 2: MCNP geometrical view of the neutron Hp(10) monitor. The right figure is a cut away view showing the internal structure. An annular guide ring shield is placed to improve the angular resolution. Boron polyethylene
He-3 counter
Polyethylene
3.2 Experiments To verify the responses obtained from the MCNP calculations, neutron irradiation experiments were performed using monoenergetic neutrons produced from an accelerator and polyenergetic neutrons from a 252Cf neutron source plus moderators. 3.2.1 The FRS monoenergetic neutron calibration field Monoenergetic neutrons with energies of 0.144, 0.565 and 5.0 MeV using a Van-de-Graff accelerator were provided at the Facility for Radiation Standards (FRS) of the Nuclear Science Research Institute, the Japan Atomic Energy Agency. The neutron irradiations were performed in a concrete-walled room of 16.5 m × 11.5 m × 12.3 m (H) with an aluminum grating floor (mezzanine) at the midheight of the room. The neutron production target was located 1.5 m above the aluminum floor. The effective center of the Hp(10) monitor was placed on the beam axis at a target-to-detector distance of 150 cm. The monitor was horizontally rotated to examine the angular dependency of the fluence responses. The influence of room-scattered neutrons was compensated for by making shadow-cone measurements. A precision long counter, placed at 220 cm at an angle of 60º to the beam line, was used to monitor the
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neutron fluence during the irradiations. A detailed description of the production of the neutron beams and their characteristics is given by Tanimura et al. [14]. 3.2.2 The ICF moderated neutron calibration field The irradiations of neutrons from a 252Cf neutron source were performed at the Instrument Calibration Facility (ICF) of the Nuclear Fuel Cycle Engineering Laboratories (NCL), the Japan Atomic Energy Agency. The ICF was specifically designed to provide in-house calibration of the neutron-measuring devices used to monitor the MOX fuel fabrication facilities of NCL. While the radio-isotopic neutron sources of 252Cf and 241Am-Be are generally used for routine calibration, a moderated neutron field consisting of a 252Cf source plus moderators was established for determining the calibration factors specific to workplaces and performance testing of neutron dosemeters [15]. Figure 3 is a schematic view of the experimental set up. The 252Cf source of nominal 1 GBq is remotely moved to the irradiation positions of A and B on a guide tube using a pneumatic/mechanical transfer system. The source position of A is at a height of 126 cm from a steel grating floor of a large low-scatter room and produces the neutron fields simulating the spectrum of neutrons transmitted through MOX-handling glove boxes and its shielding window. The position of B is at a height of 126 cm from the concrete floor of a concrete-walled basement, and produces the neutron field simulating the spectrum of neutrons in typical work environments, where large glove boxes are installed in a concrete wall-enclosed room. The moderator arrangements have been described in greater detail elsewhere [16]. The response of the Hp(10) monitor was determined by exposing it at locations where the reference personal dose equivalent had been determined from neutron spectral measurements and Monte Carlo computations. For irradiations at the position of A, the monitor was turned in a horizontal plane through angles from 0º to 180º in steps of 30º. Since a shadow cone or shadow block made of 20-cmthick steel and 30-cm-thick polyethylene containing boron was used to make scatter correction, the neutron field can be interpreted as being in a unidirectional condition. In contrast, for irradiations at the position of B, the monitor was fixed facing the source (0º). The room-scattered neutrons are included in the measurements, and hence the neutron fields are interpreted as being in a multidirectional condition. Figure 3: Experimental arrangement at the ICF. 252
Cf
A Ground level
B
Moderator material and thickness A: Annular Cylinders Steel: 40 mm (max.) PMMA: 100 mm (max.) B: Slabs/Blocks Steel: 100 mm (max.) Graphite: 100 mm (max.) PMMA: 100 mm (max.) (PMMA: Polymethyl methacrylate)
4. Results and Discussion 4.1 Energy dependence Figure 4(a) shows the computer-expected dependence of the fluence response upon incident neutron energy for the irradiation angles of 0º, 45º, 60º, 75º, 90º, 105º, 120º and 180º. The plotted uncertainties
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represent one standard deviation from the statistical component in the Monte Carlo calculations. For comparison, the figure also presents the 0º fluence response of the NDN1 [17], which is a stationary ambient dose equivalent rate meter with a completely spherical symmetrical geometry and hence is less angular-dependent. The data shown in the figure are normalized to unity for neutrons for 252Cf and α=0º. It can be easily seen that the new instrument and the NDN1 are very similar in response at normal incidences and that both present typical energy dependence in the energy region from thermal to 10 MeV. Most importantly, the angular responses of the new instrument are markedly dependent on the angle of incidence. For the irradiation angles up to 75º, the angular variations of fluence response are relatively small in the MeV region, while they are gradually enlarged in the lower energy region with decreasing neutron energy. This characteristic is quite different from general DE meters that have been developed to achieve approximately directionally isotropic responses. Figure 4(b) compares the dependence of the Hp(10) response for 0º, 45º, 60º and 75º irradiations. In this figure, the data for the 90º-180º irradiations are not shown because a comparable conversion-coefficients data set is not available from any International Standards. For irradiation angles from 0º to 60º, the new instrument exhibits an almost ideal response to 0.1 - 5 MeV neutrons and an over-response by a factor of 6-10 in the intermediate energy region around 10 keV. These results predict that the Hp(10) monitor might slightly overestimate Hp(10) in neutron fields with energies spanning from thermal to few MeV. Figure 5 shows the fluence responses of the Hp(10) monitor obtained from the FRS experiments. The experimental results were in reasonable agreement with the calculations. The Hp(10) response under test energy regions (0.144 – 5.0 MeV) in normal incidence is approximately 0.16 s-1 µSv-1 h.
Figure 4: Calculated fluence response (a) and Hp(10) response (b) as a function of neutron energy for irradiation angles of 0º to 180º. All data are normalized to unity for 252Cf neutrons of α=0º. The broken line represents the fluence and H*(10) response of NDN1 [17].
(a)
(b)
Figure 5: Neutron-energy dependency of the fluence response. Filled symbols were determined from monoenergetic neutron irradiation experiments. ‘X’ symbols with solid lines are from the MCNP calculations.
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4.2 Angular dependence Figure 6 shows the angular dependence of fluence responses for monoenergetic neutrons ((a) 5.0 MeV, (b) 0.565 MeV, and (c) 0.144 MeV) and for isotopic neutrons ((d) a bare 252Cf, and (e) a 252Cf plus steel/PMMA combination moderator). For comparison, the ratio of Hp(10,α)/Hp(10,α=0º) is shown with solid lines from 0º to 75º. This ratio is considered as a criterion on an expected angular dependence of a dosemeter reading. Additionally, the ratio at angles > 90º, taken from the calculations by Siebert et al. [19], is also indicated with broken lines as an index of the instrument’s intrinsic performance at higher angles. As is shown, the experimental results agree well with the calculated ones, and also fairly follow the Hp(10,α)/Hp(10,α=0º) curve over all angles from 0º to 180º. Particularly in the angles up to 75º, experimental data demonstrates that this monitor exhibits an excellent angular dependence that matches that of Hp(10) within an accuracy of ±15% in both monoenergetic and polyenergetic neutron fields. Significant discrepancies can be seen around 90º because of the smaller attenuation in the lateral shield. However, this may be preferable in that the monitor’s angular dependency is closer to that of an effective dose (E) rather than Hp(10), thereby allowing a reasonably conservative estimate of E in stray neutron fields. The close agreements between experimental and calculated results in Figs. 5 and 6 lend credibility to the validity of the measurements and the Monte Carlo simulations.
Figure 6: Angular dependence of the fluence response. The ratio of Hp(10,α)/Hp(10,α=0º) is indicated by a bold line (0º to 75º, taken from ISO8529-3[18]) and a broken line (75º to 180º, from the calculations by Siebert et al. [19]).
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3.3 Performance of the Hp(10) monitor The performance of the Hp(10) monitor was evaluated by the neutron irradiation tests under conditions simulating the work environments typically encountered in the MOX fuel facilities. The irradiation tests were performed at the ICF by exposing the monitor to twelve different neutron fields; seven of the twelve were performed at position A in Fig. 3, and the other five were performed at position of B. The parameters of the test neutron fields, which showed neutron spectral and dosimetric features, were as follows: Position A: unidirectional, Eφ= 0.85 to 2.0 MeV, h*= 196 to 380 pSv cm2, Position B: multidirectional, Eφ= 0.43 to 0.80 MeV, h*= 107 to 171 pSv cm2, where Eφ is the fluence-average energy and h* is the spectrum-average conversion coefficient. Each neutron spectrum is composed of three neutron components (fission, intermediate 1/E and thermal) with different proportions of spectral fluences. Figure 7 presents the variation of dose equivalent responses of the monitor as a function of Eφ. To clarify the uniqueness and characteristics of the Hp(10) monitor, both the H*(10) response (reading per H*(10)) and the Hp(10) response (reading per Hp(10)) are shown in Figs. (a) and (b), respectively. Both responses are normalized to unity at 252Cf. Note that the H*(10) response in Fig. (a) is for purpose of comparison only. As for the unidirectional neutrons irradiated at position A, the Hp(10) response and H*(10) response give similar results. However, apparent discrepancies in energy dependence can be seen in the lower energy region that consists of multidirectional neutron components. This can be explained by the angular-dependent response of the Hp(10) monitor. Since the monitor has a small response at large angles of neutron incidence, the monitor misses a portion of the neutrons and thus tends to give a significant under-response with respect to H*(10) in the multidirectional neutron field. In contrast, the monitor gives a slight over-response (up to ~8%) to Hp(10) in the lower energy region (i.e., for the highly-moderated neutron spectra) as a result of its energy dependence (shown in Fig. 4(b)). This result is thought to be sufficiently acceptable for most neutron spectra likely to be encountered in representative workplaces of the MOX fuel facilities, even without a correction factor.
Figure 7: Comparison of the H*(10) response and Hp(10) response of the Hp(10) monitor as a function of the neutron energy averaged over the spectrum. Upper: H*(10) response; Lower: Hp(10) response. The open and closed symbols are experimentally determined responses in the unidirectional and multidirectional neutron fields, respectively.
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In the laboratory irradiation tests, the monitor demonstrates a variation of ±10% for energy dependency and ±15% for angular dependency up to 75º. One should bear in mind that where a large isotropic or rotational component is present, there is a potential for the overestimation of Hp(10). Nevertheless, the new instrument seems to have an acceptable response function in terms of both energy and angle of incidence as long as it is used with its orientation facing the direction from which neutrons are dominantly coming. 4. Conclusion The authors report progress on the development and performance testing of a new type of neutron Hp(10)-measuring instrument, allowing its use as a reference dosemeter for field calibration of personal neutron dosemeters. This instrument is designed to have response characteristics which generally match both the energy and angular characteristics of Hp(10). The expected performance of the instrument was verified from experiments at the accelerator-produced monoenergetic neutron fields and moderated 252Cf neutron fields. It is expected that this new instrument will be of great help in assessing the reference values of neutron Hp(10) during field testing of personal neutron dosemeters in the workplace and also in interpreting their readings. Acknowledgements The authors wish to thank Dr. Y. Tanimura for his assistance in arranging the experiments and operating the accelerator at the FRS. REFERENCES [1] [2] [3]
[4] [5] [6] [7] [8] [9] [10]
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PIESCH, E., Calibration techniques for personnel dosemeters in stray neutron fields, Radiat. Prot. Dosim. 10, 159-173 (1985). HARVEY, W. F., et al., Personnel neutron dosimetry improvements at Los Alamos National Laboratory, Radiat. Prot. Dosim. 47, 391-395 (1993). INTERNATIONAL COMMISSION ON RADIATION UNITS AND MEASUREMENTS, Determination of operational dose equivalent quantities for neutrons, ICRU Report 66, J. ICRU 1, Nuclear Technology Publishing (2001). AMERICAN NATIONAL STANDARDS, Personnel neutron dosimeters (neutron energies less than 20 MeV), ANSI/HPS N13.52-1999 (1999). PIESCH, E., et al., Properties of personnel neutron dosemeters on the basis of intercomparison results, Radiat. Prot. Dosim. 44, 267-270 (1992). LUSZIK-BHADRA, M., et al., A wide-range direction neutron spectrometer, Nucl. Instrum. Meth. Res. A 476, 291-297 (2002). BOLOGNESE-MILSZTAJN, T., et al., Individual neutron monitoring in workplaces with mixed neutron/photon radiation, Radiat. Prot. Dosim. 110, 753-758 (2004). TSUJIMURA, N., et al., Development of a neutron personal dose equivalent detector, Radiat. Prot. Dosim. 126, 261-264 (2007). NAKAMURA, T., et al., Realization of a high sensitivity neutron rem counter, Nucl. Instrum. Meth. Res. A 241, 554-560 (1985). INTERNATIONAL COMMISION ON RADIOLOGICAL PROTECTION, Conversion Coefficients for use in radiological protection against external radiation, ICRP Publication 74, Ann. ICRP 26(3/4), Pergamon Press, Oxford (1996). BRIESMEISTER, J. F. (Ed.), MCNP – a general Monte Carlo N-particle transport code, version 4C, Report LA-13709-M, Los Alamos National Laboratory (2000). HENDRICKS, J.S., et al., ENDF/B-VI data for MCNP, Report LA-12891, Los Alamos National Laboratory (1994). LITTLE, R.C., et al., Dosimetry/activation cross section for MCNP, Los Alamos National Laboratory memorandum, March 13, 1984. TANIMURA, Y., et al., Construction of 144, 565 keV and 5.0 MeV monoenergetic neutron calibration fields at JAERI, Radiat. Prot. Dosim. 110, 85-89 (2004).
[15] TSUJIMURA, N., et al., Development of moderated neutron calibration fields simulating workplaces of MOX fuel facilities, HOKEN BUTSURI (Jpn. J. Health Phys.) 40, 354-359 (2005), (in Japanese). [16] TSUJIMURA, N., et al., “Performance test of the electronic personal neutron dosemeter in neutron fields simulating workplaces of MOX fuel fabrication facilities”, Proceedings of 12th Int. Congress of Int. Radiat. Prot. Asso. (IRPA12), Buenos Aires (2008) (CD-ROM). [17] SAEGUSA, J., et al., Evaluation of energy responses for neutron dose-equivalent meters made in Japan, Nucl. Instrum. Meth. Res. A 516, 193-202 (2004). [18] INTERNATIONAL ORGANIZATION FOR STANDARDIZATION, Reference Neutron Radiations – Part 3: calibration of area and personal dosimeters and determination of their response as a function of neutron energy and angle of incidence, ISO 8529-3, Geneva (1998). [19] SIEBERT, B. R. L., et al., Calculated fluence-to-directional and personal dose equivalent conversion coefficients for neutrons, Radiat. Prot. Dosim. 54, 231-238 (1994). [20] BARTLETT, D. T., et al., The importance of the direction distribution of neutron fluence, and methods of determination, Nucl. Instrum. Meth. Res. A 476, 386-394 (2002).
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