L.G. Hager

Recent enhancements to the understanding of the response of the NRPB neutron personal dosemeter

Abstract The response of the NRPB neutron personal dosemeter has been determined for intermediate energy neutrons using the code MCNP. These calculations fill the energy gap for which it is difficult to obtain calibration fields, and thereby enable the response function to cover the energy range from thermal to 15 MeV in an effectively continuous fashion. Recent free-in-air irradiations of the dosemeter around its fast neutron threshold have enabled the energy dependence of response in the 100 keV – 1.2 MeV energy range to be determined with high resolution. The effect of performing these irradiations free-in-air has been investigated using monoenergetic irradiations and MCNP calculations.

Measurements of the high energy neutron component of cosmic radiation fields in aircraft using etched track dosemeters

Measurements of the complex cosmic radiation field in aircraft at altitude are made with a passive survey meter comprising routine-use thermoluminescent detectors and etched track detectors. The energy dependence of response of the etched track detectors used to determine the neutron component has been characterized, partly, up to a neutron energy of 180 MeV. The neutron detectors are routinely calibrated in the CERN/EC Reference Field. The 15% determination level for total dose equivalent is 100 microSv. The evidence is that the passive survey meter provides a reliable determination of route dose.

The measurement using passive dosemeters of the neutron component of aircraft crew dose

The cosmic radiation field at aviation altitudes can be measured with simple passive detectors. The non-neutron component may be measured by means of thermoluminescence dosimetry or other techniques, and the neutron component may be measured using poly allyl diglycol carbonate (PADC) dosemeters as described in this paper. Effective dose from neutron radiation becomes the larger component for altitudes above about 10 km, in general. The dominance is more pronounced for higher latitudes. The neutron energies range up to the maximum of the incident protons, that is many GeV. However the majority of the dose is contributed by neutrons of a few hundred MeV and less, with two maxima in the fluence spectrum, one between 1 and 10 MeV and the other between 50 and 150 MeV. We have used PADC dosemeters, electrochemically etched, to estimate the neutron component of effective dose. Up to 50 dosemeters are used in a single measurement to obtain an estimate of sufficient precision for total neutron effective doses of 50 microSv and less. The neutron fluence response characteristics of the dosemeter have been measured up to 70 MeV. These are extrapolated up to 180 MeV. This extrapolation is validated, partially, by a comparison of measured and predicted readings in the CERN reference field. From the dosemeter readings for exposure on board aircraft, neutron fluence may be estimated assuming an isotropic radiation field and the estimated neutron fluence spectrum. The neutron fluence may then be converted to effective dose using published values of conversion coefficients with the same assumptions of isotropy and known fluence spectrum. For the measurement results reported here, the calculated spectrum for the CERN concrete shielded field is used.

The determination of the neutron component of cosmic radiation fields in spacecraft

Abstract Poly allyl diglycol carbonate (PADC or CR-39®) etched track detectors may be used to estimate the neutron component of the cosmic radiation in spacecraft using simple techniques developed for neutron personal dosimetry. Electrochemically etched pits are identified and counted using fully automated read-out procedures. The neutron component of the radiation field at the location of the dosimeter will produce electrochemically etchable tracks, as will the proton and energetic heavy charged particle components, depending on particle type, energy and angle of incidence. The response to incident charged particles which produce tracks and are counted as if produced by a neutron, will lead to an over-estimate of the neutron component. A correction can be applied to take account of this, or an additional chemical etch carried out which allows discrimination. Recent results for exposures in low-Earth orbit are reported.

The energy and angle dependence of neutron response for PADC manufactured using two different cure cycles

The energy and angle dependence of the neutron responses for PADC manufactured using two different cure cycles have been determined: they are compared in terms of their suitability for neutron personal dosimetry.

Comparison of neutron dose quantities and instrument and dosemeter readings at representative locations in an MOX fuel fabrication plant

Abstract The relationships between operational and protection quantities, and values of personal dosemeter and instrument readings have been determined for a recently designed MOX fuel fabrication plant. The relationships between the quantities, and the readings of personal dosemeters are sensitive to both the energy and direction distribution of neutron fluence. The energy distributions were calculated using the Monte Carlo code MCBEND. The direction distribution was addressed by calculating independently, spectral components for which the direction distribution could be reasonably assumed. At representative locations, and for assumed worker orientations, the radiation field is analysed as having, in general, three components—a direct, unidirectional component from the nearest identified discrete source, which is considered incident A-P, several unidirectional components from other such sources which are treated as a rotational component and a scattered isotropic component. The calculated spectra were folded with conversion coefficients for personal dose equivalent, Hp(10)slab (A-P, ROT and ISO), effective dose, E, (A-P, ROT and ISO), ambient dose equivalent, H*(10), personal dosemeter (AP, ROT and ISO) and survey instrument response characteristics.

The energy dependence of response of the NRPB neutron personal dosemeter and its response to workplace neutron spectra

The energy dependence of response of the NRPB PADC neutron personal dosemeter has been measured using accelerator generated neutrons over the energy range from 100 keV to 15 MeV. This is the most comprehensive recent determination of the response, which is here compared to that measured previously. The response to workplace spectra is then analysed to ascertain the accuracy with which the dosemeter responds in the working environment.

The influence of the energy distribution of workplace fields on neutron personal dosemeter reading

Variations in the energy dependence of response of neutron personal dosemeters cause systematic errors in the readings obtained in workplace fields. The magnitude of these errors has been determined theoretically by folding measured and calculated workplace energy distributions with dosemeter response functions, to determine the response of a given personal dosemeter in that field. These results have been analysed with consideration of the dosemeter response to various calibration spectra, and with reference to different workplaces. The dosemeters in the study are discussed in terms of the workplaces for which they can be suitably calibrated. Deficiencies in the published neutron energy distributions are identified.

The time dependence of neutron response for PADC manufactured using two different cure cycles

The time dependence of the neutron response of PADC cured using one of the cycles proposed by Ahmad and Stejny has been measured and compared with that of PADC cured using the cycle of Adams.