A recoil-proton spectrometer based on a p-i-n diode implementing pulse-shape discrimination.

A recoil-proton spectrometer was created by coupling a p-i-n diode with a polyethylene converter. The maximum detectable energy, imposed by the thickness of the totally depleted layer, is approximately 6 MeV. The minimum detectable energy is limited by the contribution of secondary electrons generated by photons in the detector assembly. This limit is approximately 1.5 MeV at full-depletion voltage and was decreased using pulse-shape discrimination. The diode was set up in the 'reverse-injection' configuration (i.e. with the N+ layer adjacent to the converter). This configuration provides longer collection times for the electron-hole pairs generated by the recoil-protons. The pulse-shape discrimination was based on the zero-crossing time of bipolar signals from a (CR)2-(RC)2 filter. The detector was characterised using monoenergetic neutrons generated in the Van De Graaff CN accelerator at the INFN-Laboratori Nazionali di Legnaro. The energy limit for discrimination proved to be approximately 900 keV.

Neutron measurements in the stray field produced by 158 GeV c(-1) per nucleon lead ion beams.

This paper discusses measurements carried out at CERN in the stray radiation field produced by 158 GeV c(-1) per nucleon 208Pb82+ ions. The purpose was to test and intercompare the response of several detectors, mainly neutron measuring devices, and to determine the neutron spectral fluence as well as the microdosimetric (absorbed dose and dose equivalent) distributions in different locations around the shielding. Both active instruments and passive dosimeters were employed, including different types of Andersson-Braun rem counters, a tissue equivalent proportional counter, a set of superheated drop detectors, a Bonner sphere system, and different types of ion chambers. Activation measurements with 12C plastic scintillators and with 32S pellets were also performed to assess the neutron yield of high energy lead ions interacting with a thin gold target. The results are compared with previous measurements and with measurements made during proton runs.

Radiation transport in a radiotherapy room.

The photoneutron dose equivalent in a linac radiotherapy room and its entrance maze was investigated by means of Monte Carlo simulations under different conditions. Particularly, the effect of neutron absorbers and moderator layers placed on the maze walls was considered. The contribution of prompt gamma rays emitted in absorption reactions of thermal neutrons was also taken into account. The simulation results are compared with some experimental measurements in the therapy room and in the maze.

Neutron fluxes in radiotherapy rooms.

The spatial distribution of the neutron flux, originated in an electron accelerator therapy room when energies above the threshold of (y,n) and (e,e'n) reactions are employed, is physically due to a direct flux, coming from the accelerator head, and to a flux diffused from the walls. In this work, the flux is described to a high degree of approximation by a set of functions whose spatial behavior is univocally determined by the angular distributions of the neutrons emitted from the shield of the accelerator head and diffused from the walls. The analytical results are verified with an extended series of Monte Carlo simulations obtained with the MCNP code.

Total tritium measurement in atmosphere.

Measurement of tritium in the atmosphere is of strong interest wherever this radionuclide is used. Therefore, a method is proposed for the joint measurement of burnable tritium, independently from its physico-chemical form, and of tritiated water. The method consists of transforming the tritiated molecules of the gases present in the air volume into tritiated water by burning them together with a known quantity of hydrogen. The water vapor is condensed and added to a liquid scintillator. The scintillator is also able to dissolve conventional filters so that the tritium attached to particulate and concentrated on these filters can be jointly measured, as will be discussed in a future report. The overall detection limit of the method is approximately 64 Bq m-3 for a combustion period of 10 min (which corresponds to sampling an air volume of 15 L) and a counting period of 10 min. This limit, much lower than the derived air concentrations in the most unfavorable cases, allows the application of the method for safety purposes. Moreover, the method can be integrated into a general procedure for the measurement of tritium in different chemical forms, to be applied in case of necessity.

Neutron measurements around medical electron accelerators by active and passive detection techniques.

Passive and active detection techniques have been employed in order to measure neutron fluence rates and corresponding exposure rates around medical electron accelerators operating at energies well above the neutron binding energies of the structural materials. In these conditions from the treatment head, in the direct photon flux and from the shielded region, a fast neutron flux emerges which is partly absorbed and partly scattered by the walls, eventually establishing a nearly uniform thermal and epithermal flux in the room. Both direct and scattered flux contribute to the dose to the patient. A smaller neutron dose rate can also be found outside the treatment room, where the therapy staff works. Passive detectors, of moderation type, have been employed in the treatment room and 3He active detectors in the external zones. For the treatment room the activation data were compared with results of Monte Carlo simulation of the neutron transport in the room. Technical features of the two measures are briefly presented and results obtained around three different types of accelerators are reported. At the higher beam energies, i.e., 25 MV, a neutron dose of 0.36 Sv was estimated in the treatment field in addition to a therapeutic x-ray dose of 50 Gy. At lower energies or out of the treatment field the neutron dose drops significantly. In the external zones the dose rates everywhere are below the recommended limits and normally very low, the highest values being recorded in positions very close to the access door of the treatment room.