Measurement and simulation of lineal energy distribution at the CERN high energy facility with a tissue equivalent proportional counter

The response of a tissue equivalent proportional counter (TEPC) in a mixed radiation field with a neutron energy distribution similar to the radiation field at commercial flight altitudes has been studied. The measurements have been done at the CERN-EU High-Energy Reference Field (CERF) facility where a well-characterised radiation field is available for intercomparison. The TEPC instrument used by the ARC Seibersdorf Research is filled with pure propane gas at low pressure and can be used to determine the lineal energy distribution of the energy deposition in a mass of gas equivalent to a 2 mum diameter volume of unit density tissue, of similar size to the nuclei of biological cells. The linearity of the detector response was checked both in term of dose and dose rate. The effect of dead-time has been corrected. The influence of the detector exposure location and orientation in the radiation field on the dose distribution was also studied as a function of the total dose. The microdosimetric distribution of the absorbed dose as a function of the lineal energy has been obtained and compared with the same distribution simulated with the FLUKA Monte Carlo transport code. The dose equivalent was calculated by folding this distribution with the quality factor as a function of linear energy transfer. The comparison between the measured and simulated distributions show that they are in good agreement. As a result of this study the detector is well characterised, thanks also to the numerical simulations the instrument response is well understood, and it's currently being used onboard the aircrafts to evaluate the dose to aircraft crew caused by cosmic radiation.     

Neutron spectra and angular distributions of concrete-moderated neutron calibration fields at JAERI

The Facility of Radiation Standards of Japan Atomic Energy Research Institute has been equipped with concrete-moderated neutron calibration fields as simulated workplace neutron fields. The fields use an 241Am-Be neutron source placed in the narrow space surrounded by concrete bricks, walls and floor. The neutron spectra and the neutron fluence rates of the fields were measured with the Bonner multi-sphere spectrometer system (BMS), spherical recoil-proton proportional counters, and a liquid scintillation counter (NE-213). The results were compared with each other. The reference values of H*(10) were determined from the results of BMS. The angular distributions of neutron fluence were calculated using MCNP-4B2 to obtain the reference values of Hp(10). The calculated results show that the scattered neutrons have a wide range of incident angles. The reference Hp(10) values considered the angular distribution were found to be 10-18% smaller than those without consideration.     

An experiment to detect energetic neutrons and gamma rays from the sun

A payload for the detection and measurement of high energy neutrons and gamma rays from the sun was flown onboard the first Indian satelliteAryabhata. The payload system for this neutron-gamma experiment was designed for detecting energetic neutrons in the energy range 10–500 MeV and gamma rays in the energy range 0.2–20 MeV. The details of the design of the payload, various tests carried out on it as well as the preliminary in-orbit performance are presented.     

Intercomparison of absorbed dose to water measurements for 60Co gamma rays using Fricke,alanine and radiochromic dye film dosimetry

Measurements of absorbed dose at 5 cm depth in a 30 x 30 x 30 cm3 water phantom have been performed using three independent dosimetric techniques: Fricke, alanine and radiochromic dye film (GafChromic HD-810). The measurements were carried out in the secondary standard dosimetry laboratory at ININ Mexico using a collimated 60Co gamma source with a radiation field of 10 x 10 cm2 at the phantom front surface. The source to phantom distance was set at 100 cm. The reference absorbed dose at 5 cm depth in the water phantom was obtained using a 0.6 cm3 ionisation chamber. The absorbed dose to water for the test dosimetry techniques was around 100 Gy. The deviations of the dose obtained from these dosimetry techniques were within 4%. The reasons for these deviations are discussed.     

Determination of absorbed dose in the vicinity of a neutron source

A computational method is presented that transfers the spectral neutron fluence from the measuring position to a reference point in a biological sample. The method is applied to the determination of the absorbed dose in adult blood irradiated with monoenergetic neutrons of 0.035 and 14.8 MeV.     

Characterization of gamma rays existing in the NMIJ standard neutron field

Our laboratory provides national standards on fast neutron fluence. Neutron fields are always accompanied by gamma rays produced in neutron sources and surroundings. We have characterised these gamma rays in the 5.0 MeV standard neutron field. Gamma ray measurement was performed using an NE213 liquid scintillator. Pulse shape discrimination was incorporated to separate the events induced by gamma rays from those by neutrons. The measured gamma ray spectra were unfolded with the HEPRO program package to obtain the spectral fluences using the response matrix prepared with the EGS4 code. Corrections were made for the gamma rays produced by neutrons in the detector assembly using the MCNP4C code. The effective dose equivalents were estimated to be of the order of 25 microSv at the neutron fluence of 10(7) neutrons cm(-2).     

Procedures for neutron scattering corrections in a calibration facility with a non-symmetric set-up

When calibrating neutron monitors, it is important to correct for scattering effects. In general, the correction factors depend on the type of source and monitor used, and on the configuration of the calibration room. These correction factors have been determined for the specific non-symmetric configuration of the calibration room used at SCK.CEN by means of the different analytical techniques recommended by the ISO. These results are compared with each other and also with the outcome of numerical calculations performed using the Tripoli-3 and MCNP 4B code.     

Investigation of properties of the TIARA neutron beam facility of importance for calibration applications

Evaluation of the properties for quasi-monoenergetic neutron calibration fields of high energies more than 20 MeV at TIARA is proceeding for development of the field. Among the properties needed for the development as the standard calibration field, we report on measurement of the neutron beam profile using an imaging plate with a polyethylene converter and on estimation of the contribution of scattered neutrons into the irradiation field based on pulse height distribution at various off-beam positions measured using an organic liquid scintillation detector.     

Determination of dose-build-up thickness for absorbed dose measurement in high energy electron-photon radiation at electron storage rings

Radiation field during accidental electron beam loss near electron storage rings comprises of high-energy electrons and bremsstrahlung photons. Due to high-energy nature of the radiation, the absorbed dose is expected to build up with depth of the body of an exposed worker and hence absorbed dose measurements in 30 cm water/tissue equivalent phantom is essential. Carrying out the measurements with in such phantoms is not practically feasible for routine applications and hence the use of an equivalent solid material around the dosemeters would be convenient. Monte Carlo calculations have been carried out in order to determine the dose-build-up thickness required in copper and lead, corresponding to the maximum absorbed within 30 cm depth of water in such radiation fields. Equivalent build-up thickness calculated for a case was verified by measurement using an ion chamber detector. The results are found to be in agreement within +/-20%.     

Degradation of cellulose nitrate with fast neutrons and gamma rays and their application in radiation dosimetry

Fast and moderated neutrons emitted from ²⁵²Cf as well as low doses of gamma rays from ⁶⁰Co, produce damaging effects in cellulose nitrate which can be determined viscometrically by calculating the average molecular weight at different doses.Samples were exposed to different doses of gamma rays (1 × 10^?4 to 1 Gy) and fission neutron fluences (10⁵–10¹¹ n/cm²) in free space and on a paraffin phantom. The effect of phantom thickness and phantom-to-detector distance on the detector readout have been investigated.The results revealed that the predominant bulk effects of radiation on CN is accelerated degradation by random chain scission.Empirical formulae have been given to calculate the absorbed doses of gamma rays and fast neutrons from the measured average molecular weight of the irradiated samples.     

Direction distributions of neutrons and reference values of the personal dose equivalent in workplace fields

Within the EC project EVIDOS, double-differential (energy and direction) fluence spectra were determined by means of novel direction spectrometers. By folding the spectra with fluence-to-dose equivalent conversion coefficients, contributions to H(*)(10) for 14 directions, and values of the personal dose equivalent H(p)(10) and the effective dose E for 6 directions of a person's orientation in the field were determined. The results of the measurements and calculations obtained within the EVIDOS project in workplace fields in nuclear installations in Europe, i.e., at Krümmel (boiling water reactor and transport cask), at Mol (Venus research reactor and fuel facility Belgonucléaire) and at Ringhals (pressurised reactor and transport cask) are presented.     

Characteristics of high-energy neutrons estimated using the radioactive spallation products of Au at the 500-MeV neutron irradiation facility of KENS

We carried out a shielding experiment of high-energy neutrons, generated from a tungsten target bombarded with primary 500-MeV protons at KENS, which penetrated through a concrete shield in the zero-degree direction. We propose a new method to evaluate the spectra of high-energy neutrons ranging from 8 to 500 MeV. Au foils were set in a concrete shield, and the reaction rates for 13 radionuclides produced by the spallation reactions on the Au targets were measured by radiochemical techniques. The experimental results were compared with those obtained by the MARS14 Monte-Carlo code. A good agreement (between them) was found for energies beyond 100 MeV. The profile of the neutron spectrum, ranging from 8 to 500 MeV, does not depend on the thickness of the concrete shield.     

Radionuclide neutron sources in calibration laboratory--neutron and gamma doses and their changes in time

The calibration laboratory, having standard neutron fields of radionuclide sources, should perform regular measurements of fields' parameters in order to check their stability and to get knowledge of any changes. Usually, accompanying gamma radiation is not of serious concern, but some personal dosemeters, old neutron dose equivalent meters with scintillation detectors and the dose meters of mixed radiation require the determination of this component. In the Laboratory of Radiation Protection Measurements in the Institute of Atomic Energy, Poland, the fields of radionuclide neutron sources (252)Cf, (241)Am-Be and (239)Pu-Be were examined for nearly 20 y. A number of detectors and methods have been applied for the determination of neutron ambient dose equivalent rate and for the determination of neutron and gamma dose components. This paper presents the recent results of measurements of gamma and neutron dose and dose equivalent, compared with the results accumulated in nearly 20 y.     

The variation in biological effectiveness of X-rays and gamma rays with energy

The ICRP has attributed the same relative risk for all low-LET (linear energy transfer) radiations, including X and gamma radiations of all energies. However, very low energy X-rays are expected to be more biologically effective, per unit absorbed dose, than high energy X-rays or gamma rays due to the production of lower energy secondary electrons, with a correspondingly higher LET. This increase in relative biological effectiveness (RBE) is also seen experimentally for a range of biological end-points, however, a wide range of RBE values have been reported. The assessment of risks is particularly important due to the use of low energy X-rays for mammography screening. A review of the published data on the variation in biological effectiveness with energy is presented here.     

Characterisation of the IRSN CANEL/T400 facility producing realistic neutron fields for calibration and test purposes

The new CANEL/T400 facility has been set-up at the Institute for Radiological Protection and Nuclear Safety (IRSN) to produce a realistic neutron field. The accurate characterisation of this neutron field is mandatory since this facility will be used as a reference neutron source. For this reason an international measuring campaign, involving four laboratories with extensive expertise in neutron metrology and spectrometry, was organised through a concerted EUROMET project. Measurements were performed with Bonner sphere (BS) systems to determine the energy distribution of the emitted neutrons over the whole energy range (from thermal energy up to a few MeV). Additional measurements were performed with proton recoil detectors to provide detailed information in the energy region above 90 keV. The results obtained by the four laboratories are in agreement with each other and are compared with a calculation performed with the MCNP4C Monte-Carlo code. As a conclusion of this exercise, a reliable characterisation of the CANEL/T400 neutron field is obtained.