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Practical Implications of Neutron Survey Instrument Performance

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Practical Implications of Neutron Survey Instrument Performance HPA-RPD-016 Practical Implications of Neutron Survey Instrument Performance R J Tanner1, C Molinos2, N J Roberts3, D T Bartlett1, L G Hager1, L N Jones3, G C Taylor3 and D J Thomas3 1 RADIATION PROTECTION DIVISION, HEALTH PROTECTION AGENCY, CHILTON, DIDCOT, OXON OX11 0RQ 2 FORMERLY NRPB. 3 NEUTRON METROLOGY GROUP, DQL, NATIONAL PHYSICAL LABORATORY, TEDDINGTON, MIDDLESEX, TW11 0LW ABSTRACT Neutron area survey instruments are used to detect neutrons with a wide range of energies and directions. They are designed to have a response that is as independent of neutron energy and angle of incidence as possible, but given the difficulty of the problem it is unsurprising that they are all deficient in terms of both energy and angle dependence of response to some extent. Simple inspection of the maximum systematic errors that could occur would lead to a very pessimistic view of their performance in the workplace because the energy and direction distributions of the neutrons will tend to reduce the maximum bias that can occur. To estimate the magnitudes of these biases improved energy and angle dependence of response characteristics for the three most commonly used designs in the UK have been calculated using MCNP. These calculations have been augmented by measurements. The new response data have then been used to calculate the response in workplaces and assess the implications of the deficiencies of the response characteristics. Data have also been obtained to enable a less thorough assessment to be made for other instruments. The performances of the instruments are also assessed in terms of effective dose and for situations where the user perturbs the response. This study was funded by the National Measurement System Policy Unit of the Department of Trade and Industry as Project 3.5.5 in the Ionising Radiation Metrology Programme IR(01-04) © Health Protection Agency Approval: August 2006 Centre for Radiation, Chemical and Environmental Hazards Publication: November 2006 Radiation Protection Division £35.00 Chilton, Didcot, Oxfordshire OX11 0RQ ISBN 0 85951 580 X This report from HPA Radiation Protection Division reflects understanding and evaluation of the current scientific evidence as presented and referenced in this document. EXECUTIVE SUMMARY Detailed Monte Carlo modelling has been performed for three models of neutron area survey instrument, namely the Leake 0949, the NM2B and the Studsvik 2202D. The geometric specifications of the instruments have been improved compared to earlier modelling of the same devices, and the energy and angle dependence of response has been modelled in smaller increments. The resultant response characteristics have been applied to understanding the behaviour of the instruments in workplaces, given assumptions about the direction distributions of the fields. Such assumptions are required to take account of the angle dependence of response of the instruments. They are also needed to assess whether the H*(10) assessment provides a conservative or reasonable estimate of effective dose. Published response data for the Berthold LB6411 and the Thermo Electron SWENDI-II have also been included in the study. Unfortunately, the variation of the response characteristics with angle of incidence are not available for either of these instruments, so analysis of their response in terms of effective dose has had to assume perfect isotropic response. All of these single detector designs are found to make generally conservative estimates of H*(10). However, the Leake 0949 tends to underestimate H*(10) in hard neutron fields to avoid excessive overestimates in soft fields. The significantly heavier NM2B and Studsvik 2202D have better H*(10) response in workplace fields, although their direction dependence is less satisfactory. The spherical symmetry of the Leake is clearly preferable in this respect. The newer designs of instrument, the LB6411 and SWENDI-II do not have significantly better H*(10) response, although this assessment depends on the workplace application. The LB6411 has a reduced overestimate in the 1-10 keV energy region, but this leads to a significant under-response to thermal neutrons. In workplaces, this is seen to give very good integral H*(10) response, unless the field is very soft. This effect is most significant for a sub-set of the reactor fields. The SWENDI-II, requires a slightly more complex analysis because its calibration is appropriate for high-energy neutron fields. It hence has a large overestimate in terms of H*(10) for energies between thermal and 1 MeV, and would overestimate in all of the fields in this study. To avoid subjective judgements about the correct calibration response, the data have also been presented for a bare 252Cf calibration. Generally, the instruments avoid underestimates of effective dose when calibrated in terms of H*(10) using bare radionuclide sources. There are exceptions, such as the LB6411 for unidirectional, very soft fields. However, fields that are so soft are unlikely to be unidirectional. In terms of the isotropic geometries considered, these estimates of effective dose are still conservative. One field that causes problems for the instruments in this respect is the bare 241Am-Be calibration field. EAP/Φ is higher than H*(10)/Φ for this field. Hence, if the instrument is calibrated using this field it will underestimate effective dose by 5%. This makes the field particularly problematic for the lightest instrument, the Leake, for which there is a 28% underestimate of effective dose. The unscattered field from a point source will be iii PRACTICAL IMPLICATIONS OF NEUTRON SURVEY INSTRUMENT PERFORMANCE approximately unidirectional so the assumption of some degree of isotropy is not realistic for that field. Because EAP/Φ represents the maximum for effective dose in the energy range studied, when rotational or spherical isotropy is assumed large overestimates of effective dose occur. All of the instruments suffer from this to differing degrees, but when the H*(10) from thermal neutrons is overestimated, the problem is worse. It is particularly true for soft fields for the Leake, for which the instrument overestimates H*(10). Consequently, for spherically isotropic fields the instrument can overestimate effective dose by up to a factor of six. Unfortunately, there are few simultaneous direction and energy distribution determinations that have been made in workplaces. The only ones available to this study are from calculations that were performed during the design process of a facility that is now being commissioned. The direction distributions are interesting, since there are many relatively unshielded sources in the room, and operator positions that have sources located on all sides. Because the direction distributions are known, the reading of the instrument can be more accurately determined, but more importantly, the effective dose can be calculated more correctly. Hence, it is possible to show that whilst the assessments of ambient dose equivalent are relatively good, the instruments overestimate effective dose by factors of up to 2.5. This conclusion can only be justified for these specific calculations, which may not represent the plant as it is to be operated when it is fully operational. Two other, more sophisticated instruments have been studied. Both designs have smaller deviations in their H*(10) response to monoenergetic neutrons. In workplace fields, the improvement in their response characteristics is not so significant. For the HPA/BNFL Novel Survey Instrument, this is caused by its underestimate of H*(10) at 100 keV. However, despite underestimating H*(10) in some of these fields by up to 30%, it does not underestimate effective dose in any field by more than 4%. The avoidance of any significant overestimates of H*(10) assists the effective dose performance: the overestimates of isotropic fields reach a factor of 3.5, which is slightly more than the maximum for the LB6411, equivalent to the NM2B and better than the other single detector instruments. The Hybrid Survey Instrument developed by the University of Lancaster, despite being lighter than the original HPA/BNFL design, performs better. Its estimates of H*(10) are within the range from –26% to +31% of the true value, for the fields included in this study when incident from the reference direction. The corresponding values for the HPA/BNFL version are –30% to +35% and those for the best single detector design in this respect, the NM2B, –25% to +31%. Clearly, although these designs have improved energy dependence of response in monoenergetic fields they do not offer a major advance over the single detector systems in workplace fields. However, they do have the potential for providing warnings or even corrections to the reading based on the direction distribution of the field. Published response data show that the behaviour of a particular instrument type varies from instrument to instrument. Perturbation calculations have been used to understand iv whether the intrinsic variability of the response between instruments is caused by manufacturing uncertainties. Several parameters have been tested: a Polyethylene density b Accuracy of construction of the attenuating layer construction, specifically the holes diameters c Composition of the attenuating layer d Central detector gas pressure e Accuracy of the cross-section data for hydrogen The polyethylene density is found most likely to cause variation between instruments. Commercial products have a range of densities and the density has an energy- dependent influence on the response. This is found to have a potentially significant impact on the response in workplace fields: the H*(10) response for some fields falls by up to 10% relative to the calibration response for plausible changes to the polyethylene density. The response is even more sensitive to the sizes of the holes in the thermal neutron attenuation layer. The likely variation in this parameter is, however, not so easy to assess but there are good grounds for believing that it may be systematically rather than randomly perturbed. The effect on the response is energy dependent and greatest for low energies.
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