R. Bedogni

Determination of the response to photons and thermal neutrons of new LiF based TL materials for radiation protection purposes

LiF based thermoluminescent materials are the most used passive detectors in the field of radiation protection. For personal and area dosimetry in mixed neutron and photon fields, /sup 6/LiF and /sup 7/LiF TLD pairs are employed for the simultaneous determination of neutron and photon doses. Therefore, an accurate knowledge of their sensitivity to photons and thermal neutrons is essential. Aimed at providing reliable sensitivity data for new LiF based TL materials, an experimental study was performed in the framework of collaboration between INFN Frascati National Laboratories and ENEA, Italy. The /sup 6/LiF(Mg, Cu, P) material from TLD Poland, was specially manufactured in six versions with different sensitive thickness (t=40,60,80,100,150 and 230 mg/spl middot/cm/sup -2/), which implies different neutron and photon sensitivity. Together with the /sup 7/LiF(Mg, Cu, P) material, namely MCP-7, all seven type of TLDs were irradiated in reference beams of photons and thermal neutrons, at the calibration facilities of INFN and ENEA. The study provides a set of sensitivity data and discusses the performance of the investigated materials for the dosimetry of mixed neutron and photon fields. All uncertainties or error bars reported in the paper refer to 95% confidence level (2 sigma).

A NEW ACTIVE THERMAL NEUTRON DETECTOR

This communication presents the main results about the design and in-house fabrication of a new solid-state neutron detector, which produces a DC output signal proportional to the thermal neutron fluence rate. The detector has been developed within the framework of the 3-y project NESCOFI@BTF of INFN (CSN V). Due to its sensitivity, photon rejection, low cost and minimum size, this device is suited to be used in moderator-based spectrometers.

COMPACT THERMAL NEUTRON SENSORS FOR MODERATOR-BASED NEUTRON SPECTROMETERS

In the framework of the NESCOFI@BTF project of the Italian Institute of Nuclear Physics, different types of active thermal neutron sensors were studied by coupling semiconductor devices with a suitable radiator. The objective was to develop a detector of small dimensions with a proper sensitivity to use at different positions in a novel moderating assembly for neutron spectrometry. This work discusses the experimental activity carried out in the framework of the ERINDA program (PAC 3/9 2012) to characterise the performance of a thermal neutron pulse detector based on (6)Li.

A new online detector for estimation of peripheral neutron equivalent dose in organ.

PURPOSE Peripheral dose in radiotherapy treatments represents a potential source of secondary neoplasic processes. As in the last few years, there has been a fast-growing concern on neutron collateral effects, this work focuses on this component. A previous established methodology to estimate peripheral neutron equivalent doses relied on passive (TLD, CR39) neutron detectors exposed in-phantom, in parallel to an active [static random access memory (SRAMnd)] thermal neutron detector exposed ex-phantom. A newly miniaturized, quick, and reliable active thermal neutron detector (TNRD, Thermal Neutron Rate Detector) was validated for both procedures. This first miniaturized active system eliminates the long postprocessing, required for passive detectors, giving thermal neutron fluences in real time. METHODS To validate TNRD for the established methodology, intrinsic characteristics, characterization of 4 facilities [to correlate monitor value (MU) with risk], and a cohort of 200 real patients (for second cancer risk estimates) were evaluated and compared with the well-established SRAMnd device. Finally, TNRD was compared to TLD pairs for 3 generic radiotherapy treatments through 16 strategic points inside an anthropomorphic phantom. RESULTS The performed tests indicate similar linear dependence with dose for both detectors, TNRD and SRAMnd, while a slightly better reproducibility has been obtained for TNRD (1.7% vs 2.2%). Risk estimates when delivering 1000 MU are in good agreement between both detectors (mean deviation of TNRD measurements with respect to the ones of SRAMnd is 0.07 cases per 1000, with differences always smaller than 0.08 cases per 1000). As far as the in-phantom measurements are concerned, a mean deviation smaller than 1.7% was obtained. CONCLUSIONS The results obtained indicate that direct evaluation of equivalent dose estimation in organs, both in phantom and patients, is perfectly feasible with this new detector. This will open the door to an easy implementation of specific peripheral neutron dose models for any type of treatment and facility.

Development of single-exposure, multidetector neutron spectrometers: the NESCOFI@BTF project

NESCOFI@BTF is a 3-y project (2011-13) supported by the Scientific Commission 5 of INFN (Italy). The target is the development of neutron spectrometers similar to the Bonner spheres, in terms of response energy interval and accuracy, but able to determine the neutron spectrum in only one exposure. These devices embed multiple (10 to 30) thermal neutron detectors (TNDs) within a single moderator. Two prototypes, called SPherical SPectrometer (SP(2)) and cylindrical spectrometer (CYSP), have been set up. Whilst SP(2) has spherical geometry and nearly isotropic response, the CYSP has cylindrical geometry and is intended to be used as a directional spectrometer. Suitable active TNDs will be embedded in the final version of the devices. The resulting instruments could be used as real-time neutron spectrometers in neutron-producing facilities. This communication describes the design criteria, numerical analysis, experimental issues, state-of-the-art and future developments connected with the development of these instruments.

NEUTRON SPECTROMETRY WITH BONNER SPHERES FOR AREA MONITORING IN PARTICLE ACCELERATORS

Selecting the instruments to determine the operational quantities in the neutron fields produced by particle accelerators involves a combination of aspects, which is peculiar to these environments: the energy distribution of the neutron field, the continuous or pulsed time structure of the beam, the presence of other radiations to which the neutron instruments could have significant response and the large variability in the dose rate, which can be observed when moving from areas near the beam line to free-access areas. The use of spectrometric techniques in support of traditional instruments is highly recommended to improve the accuracy of dosimetric evaluations. The multi-sphere or Bonner Sphere Spectrometer (BSS) is certainly the most used device, due to characteristics such as the wide energy range, large variety of active and passive detectors suited for different workplaces, good photon discrimination and the simple signal management. Disadvantages are the poor energy resolution, weight and need to sequentially irradiate the spheres, leading to usually long measurement sessions. Moreover, complex unfolding analyses are needed to obtain the neutron spectra. This work is an overview of the BSS for area monitoring in particle accelerators.

ETHERNES: A new design of radionuclide source-based thermal neutron facility with large homogeneity area.

A new thermal neutron irradiation facility based on an (241)Am-Be source embedded in a polyethylene moderator has been designed, and is called ETHERNES (Extended THERmal NEutron Source). The facility shows a large irradiation cavity (45 cm × 45 cm square section, 63 cm in height), which is separated from the source by means of a polyethylene sphere acting as shadowing object. Taking advantage of multiple scattering of neutrons with the walls of this cavity, the moderation process is especially effective and allows obtaining useful thermal fluence rates from 550 to 800 cm(-2) s(-1) with a source having nominal emission rate 5.7×10(6) s(-1). Irradiation planes parallel to the cavity bottom have been identified. The fluence rate across a given plane is as uniform as 3% (or better) in a disk with 30 cm (or higher) diameter. In practice, the value of thermal fluence rate simply depends on the height from the cavity bottom. The thermal neutron spectral fraction ranges from 77% up to 89%, depending on the irradiation plane. The angular distribution of thermal neutrons is roughly isotropic, with a slight prevalence of directions from bottom to top of the cavity. The mentioned characteristics are expected to be attractive for the scientific community involved in neutron metrology, neutron dosimetry and neutron detector testing.

The thermal neutron facility HOTNES: theoretical design

HOTNES (HOmogeneous Thermal NEutron Source) is a thermal neutron irradiation facility with extended and very uniform irradiation area. A 241Am-B radionuclide neutron source with nominal strenght 3.5×106 s-1 is located on bottom of a large cylindrical cavity (30cm diameter, 70cm in height) delimited by polyethylene walls. The upper part of this volume (30cm diameter, 40cm in height) is used to irradiate samples. A polyethylene cylinder, acting as shadowing object, prevents fast neutrons to directly reach the irradiation volume. Indeed neutrons can only reach the irradiation volume after multiple scattering with the cavity walls. The facility was designed trough extensive calculations with MCNPX. Irradiation planes are disks with 30cm diameter, centred on the cavity axis, and parallel to the cavity bottom. The value of thermal fluence in a given irradiation plane is as uniform as 1-2%. The value of thermal fluence rate simply depends on the height from the cavity bottom. Values of thermal fluence rate in the range 700-1000cm-2s-1 are available, depending on the irradiation plane chosen. The fraction of thermal neutrons is in the order of 90%, also depending on the irradiation plane. The angular distribution of thermal neutrons is roughly isotropic. Taking advantage of the HOTNES design, even large devices can be uniformly irradiated. This work presents HOTNESs design and describes the neutron field in the irradiation volume in terms of spatial, energy and direction distributions.

Feasibility study of a neutron source at the daΦne beam test facility, using Monte Carlo codes

In this paper we report the feasibility study of a photo-neutron source at the Daφne Beam Test Facility. In particular we describe the Monte Carlo simulation results and the comparison of these ones with some important semi-empirical correlations. In the preliminary phase of this project only the FLUKA code has been used, but an extensive use of GEANT4 has been also planned together with MCNPX, to investigate the effect coming from different implementation of the photo-nuclear physics. Finally, we report the expected neutron flux, when high energy electron (510 MeV) beam from the Daφne Linac is sent onto a suitable target at the nowadays maximum admissible rate (5 · 1011e−/s). The status of the overall design is described.

Photo-neutron source by high energy electrons on target: Comparison between Monte Carlo predicitons and experimental measurements

A photo-neutron source has been realized at the DaΦne Beam Test Facility: neutrons are produced by sending high energy electrons (510 MeV) to impinge on an optimized target. Neutron flux and spectra have been measured along well designed extraction lines. The experimental data have been compared with Monte Carlo predictions performed with FLUKA and MCNPX, respectively. The comparison between measured neutron rates, energy spectra and the correspective Monte Carlo expectations is presented, as well as the simulation results from the two different Monte Carlo codes are compared and analyzed.