R.J. Tanner

The importance of the direction distribution of neutron fluence and methods of determination

For the estimation of non-isotropic quantities such as personal dose equivalent and effective dose, and for the interpretation of the readings of personal dosemeters, it is necessary to determine both the energy and direction distributions of the neutron fluence. In fact, for workplace fields, the fluence and dose-equivalent responses of dosemeters and the relationships of operational and protection quantities, are frequently more dependent on the direction than on the energy distribution. In general, the direction distribution will not be independent of the energy distribution, and simultaneous determination of both may be required, which becomes a complex problem. The extent to which detailed information can be obtained depends on the spectrometric properties and on the angle dependence of the response of the detectors used. Methods for the determination of direction distributions of workplace fields are described.

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 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.

NRPB PADC neutron personal dosimetry after IRCP 60

Abstract The NRPB has been operating a routine neutron personal dosimetry service based upon the electrochemical etching of PADC (poly allyl diglycol carbonate) elements since 1986. This service covers approximately 1000 workers in the UK and abroad, requiring the issue of around 7500 dosemeters annually. It has performed relatively well in international intercomparisons with samples taken from routine stock. The recommendations of ICRP 60 (International Commission on Radiological Protection, 1991) may have serious consequences for many operational neutron dosimetry services. The reduction in dose limits from 50 mSv to an average of 20 mSv per year, the introduction of constraints and the requirements for dose estimation in some circumstances down to a few mSv per year, inevitably result in a need to estimate lower doses with greater precision. Additionally, the proposed change in the Q(L) relationship to be applied in respect of the ICRU operational quantities will effectively decrease the (dose equivalent) sensitivity of detectors.

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 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.

A two frequency electrochemical etch for routine processing of neutron dosemeters at the NRPB

Abstract An overnight two frequency (50 Hz and 2 kHz) electrochemical etch (ECE) for PADC dosemeters has been developed, with the intention of it succeeding the single frequency (2 kHz) ECE currently used for the routine processing of neutron dosemeters. This is discussed in terms of sensitivity and background, and their optimisation with respect to the etching parameters. Particular reference is made to the improvement in energy response and the effect of raised backgrounds upon minimum detectable dose equivalent values.

Re-assessment of PADC neutron dosemeter results

Abstract In an operational neutron personal dosimetry service there is often a requirement for re-assessment of measured doses. The requirement may arise for routine confirmation of high doses, for checking a result at the request of a customer, or where there has been an error in primary processing. Three methods of re-assessment are under consideration for use in NRPBs PADC neutron personal dosimetry service: electrochemical etching of the opposite face of the PADC element; secondary chemical etching of the element; and a second electrochemical etch following a cleaning etch. Advantages and disadvantages of each method are discussed.