E. Aza

SECONDARY NEUTRON DOSES IN A PROTON THERAPY CENTRE.

The formation of secondary high-energy neutrons in proton therapy can be a concern for radiation protection of staff. In this joint intercomparative study (CERN, SCK•CEN and IBA/IRISIB/ULB), secondary neutron doses were assessed with different detectors in several positions in the Proton Therapy Centre, Essen (Germany). The ambient dose equivalent H(*)(10) was assessed with Berthold LB 6411, WENDI-2, tissue-equivalent proportional counter (TEPC) and Bonner spheres (BS). The personal dose equivalent Hp(10) was measured with two types of active detectors and with bubble detectors. Using spectral and basic angular information, the reference Hp(10) was estimated. Results concerning staff exposure show H(*)(10) doses between 0.5 and 1 nSv/monitoring unit in a technical room. The LB 6411 showed an underestimation of H(*)(10), while WENDI-2 and TEPC showed good agreement with the BS data. A large overestimation for Hp(10) was observed for the active personal dosemeters, while the bubble detectors showed only a slight overestimation.

Comparison of the performance of different instruments in the stray neutron field around the cern proton synchrotron

This paper discusses an intercomparison campaign carried out in several locations around the CERN Proton Synchrotron. The locations were selected in order to perform the measurements in different stray field conditions. Various neutron detectors were employed: ionisation chambers, conventional and extended range rem counters, both commercial and prototype ones, including a novel instrument called LUPIN, specifically conceived to work in pulsed fields. The attention was focused on the potential differences in the instrument readings due to dead-time losses that are expected to affect most commercial units. The results show that the ionisation chambers and LUPIN agree well with the expected H*(10) values, as derived from FLUKA simulations, showing no relevant underestimations even in strongly pulsed fields. On the contrary, the dead-time losses of the other rem counters induced an underestimation in pulsed fields that was more important for instruments characterised by a higher dead time.

The triple GEM detector as stray neutron monitor

A triple GEM detector for low energy neutrons and high rejection of gamma background was tested at the CERF facility at CERN as stray neutron monitor. The detector was exposed to a wide neutron spectrum generated by a 120 GeV/c positively charged hadron beam hitting a copper target. The beam particle rate on target ranged over three orders of magnitude. The neutron count rate measured with the GEM could be linearly correlated with the beam intensity and also compared well with Monte Carlo simulations. The detector performance suggests that it can be used as an independent low energy neutron monitor at high-energy particle accelerator facilities.

A Bonner Sphere Spectrometer for pulsed fields

The use of conventional Bonner Sphere Spectrometers (BSS) in pulsed neutron fields (PNF) is limited by the fact that proportional counters, usually employed as the thermal neutron detectors, suffer from dead time losses and show severe underestimation of the neutron interaction rate, which leads to strong distortion of the calculated spectrum. In order to avoid these limitations, an innovative BSS, called BSS-LUPIN, has been developed for measuring in PNF. This paper describes the physical characteristics of the device and its working principle, together with the results of Monte Carlo simulations of its response matrix. The BSS-LUPIN has been tested in the stray neutron field at the CERN Proton Synchrotron, by comparing the spectra obtained with the new device, the conventional CERN BSS and via Monte Carlo simulations.

The triple GEM detector as beam monitor for relativistic hadron beams

A triple GEM detector was tested at the CERF facility at CERN as an on-line beam imaging monitor and as a counting reference device. It was exposed to a 120 GeV/c positively charged hadron beam (approximately 2/3 pions and 1/3 protons), which hits a copper target generating a wide spectrum of different kinds of particles used for various experiments. The flux of beam particles ranged over three orders of magnitude, from 8·104 s-1 to 8·107 s-1. The profile of the beam acquired with the GEM was compared to the one measured with a MWPC and no saturation was observed. In addition, the count rate measured with the GEM was compared to the one measured with an Ionization Chamber, which is routinely used for monitoring the beam intensity. Another way of monitoring the intensity of the beam was also explored, which is based on the total current driven from the GEM foils. The digital readout allows making a 2D online image of the beam for the alignment with the copper target in the CERF facility. A low residual activation of the detector was observed shortly after irradiation.

Neutron beam profile measurements with a triple GEM for thermal neutrons at the CERN n_TOF facility

A triple GEM detector for thermal neutrons, with a side-on geometry, was tested at the CERN n_TOF facility as beam profile detector. The n_TOF, neutron time-of-flight facility, provides a neutron beam from thermal energy to 1 GeV with a flight path of 185 m. The triple GEM detector used in this measurement has a cathode specifically conceived for thermal neutron conversion, with a series of thin strips of 10B deposited on glass supports. Filled with an Ar-CO2 70-30% mixture, it was installed in the experimental area, using a step motor to change the position and to image the whole beam area. Furthermore, using the n_TOF trigger it was possible to synchronize the GEM data acquisition in order to select a given neutron energy window and measure the detector efficiency as a function of neutron energy between thermal and 2 eV. Working at low gain, it was possible to perform measurements with a low γ-background level. The neutron beam spot and the efficiency of the detector were measured.