Angular distribution of neutron spectral fluence around phantom irradiated with high energy protons

Extended Bonner Spheres spectrometer was used to measure the angular distribution of neutron spectral fluence around NYLON6 phantom irradiated with pencil beam of 100, 150 and 200 MeV protons at the Proton Therapy Center Praha. Measurements were supplemented by a calculation of neutron spectral fluences at different depths of the phantom. The calculation of neutron spectral fluence at different depth of the phantom demonstrated that the majority of high energy neutrons was generated at the beginning of the proton trajectory in the phantom and the neutron yield decreased with increasing depth, with a minimum at the depth corresponding to the Bragg peak. Therefore, attention should be paid not only to the tissue behind the irradiated volume, but also to the preceding tissue. However, the neutron spectral fluence in the vicinity of the treated tissue can only be determined by calculation, mainly due to the dimensions of the neutron spectroscopic instrumentation. This paper presents a suitable technique and experimental conditions to acquire reliable data necessary for the proper determination of neutron spectral fluence. From the measured spectral fluences, the neutron fluence in whole-range and partial energy intervals were determined together with the corresponding ambient dose equivalents at measurement positions. The obtained results indicate that high energy neutrons predominate at the direction of the proton beam and more neutrons are generated by higher energy protons.     

Electron volt neutron spectrometers

The advent of pulsed neutron sources has made available intense fluxes of epithermal neutrons (500 meV ≤E≤100 eV ). The possibility to open new investigations on condensed matter with eV neutron scattering techniques, is related to the development of methods, concepts and devices that drive, or are inspired by, emerging studies at this energy scale. Electron volt spectrometers have undergone continuous improvements since the construction of the first prototype instruments, but in the last decade major breakthroughs have been accomplished in terms of resolution and counting statistics, leading, for example, to the direct measurement of the proton 3-D Born–Oppenheimer potential in any material, or to quantitatively probe nuclear quantum effects in hydrogen bonded systems. This paper reports on the most effective methods and concepts for energy analysis and detection, as well as devices for the optimization of electron volt spectrometers for different applications. This is set in the context of the progress made up to date in instrument development. Starting from early stages of development of the technique, particular emphasis will be given to the Vesuvio eV spectrometer at the ISIS neutron source, the first spectrometer where extensive scientific, as well as research and development programmes have been carried out. The potential offered by this type of instrumentation, from single particle excitations to momentum distribution studies, is then put in perspective into the emerging fields of eV spectroscopy applied to cultural heritages and neutron irradiation effects in electronics.     

The influence of field size and off-axis distance on photoneutron spectra of the 18 MV Siemens Oncor linear accelerator beam

At present, high energy electron linear accelerators (LINACs) producing photons with energies higher than 10 MeV have a wide use in radiotherapy (RT). However, in these beams fast neutrons could be generated, which results in undesired contamination of the therapeutic beams. These neutrons affect the shielding requirements in RT rooms and also increase the out-of-field radiation dose to patients. The neutron flux becomes even more important when high numbers of monitor units are used, as in the intensity modulated radiotherapy. Herein, to evaluate the exposure of patients and medical personnel, it is important to determine the full radiation field correctly. A model of the dual photon beam medical LINAC, Siemens ONCOR, used at the University Hospital Centre of Osijek was built using the MCNP611 code. We tuned the model according to measured photon percentage depth dose curves and profiles. Only 18 MV photon beams were modeled. The dependence of neutron dose equivalent and energy spectrum on field size and off-axis distance in the patient plane was analyzed. The neutron source strength (Q) defined as a number of neutrons coming from the head of the treatment unit per x-ray dose (Gy) delivered at the isocenter was calculated and found to be 1.12 × 10¹² neutrons per photon Gy at isocenter. The simulation showed that the neutron flux increases with increasing field size but field size has almost no effect on the shape of neutron dose profiles. The calculated neutron dose equivalent of different field sizes was between 1 and 3 mSv per photon Gy at isocenter. The mean energy changed from 0.21 MeV to 0.63 MeV with collimator opening from 0 × 0 cm² to 40 × 40 cm². At the 50 cm off-axis the change was less pronounced. According to the results, it is reasonable to conclude that the neutron dose equivalent to the patient is proportional to the photon beam-on time as suggested before. Since the beam-on time is much higher when advanced radiotherapy techniques are used to fulfill high conformity demands, this makes the neutron flux determination even more important. We also showed that the neutron energy in the patient plane significantly changes with field size. This can introduce significant uncertainty in dosimetry of neutrons due to strong dependence of the neutron detector response on the neutron energy in the interval 0.1–5 MeV.     

Influence of Bonner sphere response functions above 20 MeV on unfolded neutron spectra and doses

Monte Carlo (MC) codes for neutron transport calculations such as MCNP, MCNPX, FLUKA, PHITS, and GEANT4, crucially rely on cross sections that describe the interaction of neutrons with nuclei. For neutron energies below 20 MeV, evaluated cross sections are available that are validated against experimental data. In contrast, for high energies (above 20 MeV) experimental data are scarce and, for this reason, every neutron transport code is based on theoretical nuclear models to describe interactions of neutrons with nuclei in matter. Here we report on the calculation of a complete set of response functions for a Bonner spheres spectrometer (BSS), by means of GEANT4 using the Bertini and Binary Intranuclear Cascade (INC) models for energies above 20 MeV. The recent results were compared with those calculated by MCNP/LAHET and MCNP/HADRON MC codes. It turns out that, whatever code used, the response functions were rather similar for neutron energies below 20 MeV, for all 16 detector/moderator combinations of the considered BSS system. For higher energies, however, differences of more than a factor of 2 were observed, depending on neutron energy, detector/moderator combination, MC code, and nuclear model used. These differences are discussed in terms of neutron fluence rates measured at the environmental research station (UFS), “Schneefernerhaus”, (Zugspitze mountain, Germany, 2650 m a.s.l.) for energies below 0.4 eV (thermal neutrons), between 0.4 eV and 100 keV (epithermal neutrons), between 100 keV and 20 MeV (evaporation neutrons), and above 20 MeV (cascade neutrons). In terms of total neutron fluence rates, relative differences of up to 4% were obtained when compared to the standard MCNP/LAHET results, while in terms of total ambient dose equivalent, relative differences of up to 8% were obtained. Both the GEANT4 Binary INC and Bertini INC gave somewhat larger fluence and dose rates in the epithermal region. Most relevant for dose, however, those response functions calculated with the GEANT4 Bertini INC model provided about 18% less neutrons in the cascade region, which led to a roughly 13% smaller contribution of these neutrons to ambient dose equivalent. It is concluded that doses from secondary neutrons from cosmic radiation as deduced from BSS measurements are uncertain by about 10%, simply because of some differences in nuclear models used by various neutron transport codes.     

Passive neutron area monitor with pairs of TLDs as neutron detector

A passive neutron area monitor has been designed using Monte Carlo methods; the monitor is a polyethylene cylinder with pairs of thermoluminescent dosimeters (TLD600 and TLD700) as thermal neutron detector. The monitor was calibrated with a bare and a thermalzed ²⁴¹AmBe neutron sources and its performance was evaluated measuring the ambient dose equivalent due to photoneutrons produced by a 15 MV linear accelerator for radiotherapy and the neutrons in the output of a TRIGA Mark III radial beam port.     

Evaluation of target photon dose mixed in mono-energetic neutron fields using 7Li(p,n)7Be reaction

Target photons mixed in the 144, 250 and 565 keV mono-energetic neutron calibration fields were measured using a cylindrical NaI(Tl) detector with 7.62 cm both in diameter and in length. The ambient dose equivalent H*(10) of the photons was evaluated by applying the “G(E) function” to the measured pulse height spectrum. Neutrons induce photons by nuclear reactions in the NaI(Tl) detector and affect the pulse height spectrum. In order to eliminate the influence of these neutron events, the time-of-flight technique was applied with operating the accelerator in the pulse mode. The ratios by the ambient dose equivalent H*(10) of the photons to the 144, 250 and 565 keV neutrons were evaluated to be 3.3%, 4.7% and 0.9%, respectively. Although high energy photons ranging from 6 to 7 MeV are emitted by the ¹⁹F(p,αγ)¹⁶O reactions, the dose of the target photons is low enough to calibrate neutron dosemeters except for ones with high sensitivity to the photons.     

In-phantom neutron dose distribution for bladder cancer cases treated with high-energy photons

This work presents an estimation of the neutron dose distribution for common bladder cancer cases treated with high-energy photons of 15 MV therapy accelerators. Neutron doses were measured in an Alderson phantom, using TLD 700 and 600 thermoluminescence dosimeters, resembling bladder cancer cases treated with high-energy photons from 15 MV LINAC and having a treatment plan using the four-field pelvic box technique. Thermal neutron dose distribution in the target area and the surrounding tissue was estimated. The sensitivity of all detectors for both gamma and neutrons was estimated and used for correction of the TL reading. TLD detectors were irradiated with a Co⁶⁰ gamma standard source and thermal neutrons at the irradiation facility of the National Institute for Standards (NIS). The TL to dose conversion factor was estimated in terms of both Co⁶⁰ neutron equivalent dose and thermal neutron dose. The dose distribution of photo-neutrons throughout each target was estimated and presented in three-dimensional charts and isodose curves. The distribution was found to be non-isotropic through the target. It varied from a minimum of 0.23 mSv/h to a maximum of 2.07 mSv/h at 6 cm off-axis. The mean neutron dose equivalent was found to be 0.63 mSv/h, which agrees with other published literature. The estimated average neutron equivalent to the bladder per administered therapeutic dose was found to be 0.39 mSv Gy^?1, which is also in good agreement with published literature. As a consequence of a complete therapeutic treatment of 50 Gy high-energy photons at 15 MV, the total thermal neutron equivalent dose to the abdomen was found to be about 0.012 Sv.     

CYSP: A new cylindrical directional neutron spectrometer. Conceptual design

This communication describes in detail the design of a new cylindrical neutron spectrometer (CYSP) embedding 7 active thermal neutron detectors in a moderating structure made of polyethylene, borated plastic and lead. The device provides a strong directional response within the energy interval from thermal to hundreds of MeV, being nearly insensitive to neutrons coming from directions other than the cylinder axis with energies up to about 10 MeV. Therefore it will be especially suitable for applications where the neutron spectrum as a function of the emission angle needs to be measured. The Monte Carlo transport code MCNPX has been used to reach the final configuration for the spectrometer in terms of size, collimator, and arrangement of borated plastic and lead layers, number and position of the detectors. Moreover, MCNPX has been also used to calculate the response matrix of the instrument.     

Peripheral doses in radiotherapy: A comparison between IMRT,VMAT and Tomotherapy

The goal of this intercomparison is to determine the peripheral doses during treatment of prostate and head and neck (H&N) cancers. In the case of prostate cancer, two different treatment techniques are compared: intensity-modulated radiation therapy (IMRT – 10 MV and 18 MV), on a Varian Clinac 2100 C/D and Tomotherapy. VMAT (also on a Varian Clinac 2100 C/D) was compared to Tomotherapy, for H&N cancer. The treatment devices are located at the university hospitals of Leuven and Brussels, respectively. A common treatment protocol was agreed between the two clinical centers and this same protocol was used by each partner. For the higher energy modalities (10 MV and 18 MV) we also assessed the neutron contribution to the total dose, by using bubble detectors. In this way, the performance (in terms of peripheral doses) of the different treatment techniques, when faced with the same dose distribution constraints, was evaluated. The doses were evaluated with an anthropomorphic phantom loaded with TLD detectors. Summarizing our results, we can conclude that low energy radiation techniques, namely VMAT and Tomotherapy, have more interesting performances when compared to IMRT at energies of 10 MV and 18 MV, with respect to peripheral dose. On the one hand the former are associated with lower photon doses and, on the other hand, there is no contribution from neutrons to the total dose.     

Measurement of peak fluence of neutron beams using Bi-fission detectors

Fission fragments and other charged particles leave tracks of permanent damage in most of the insulating solids. Damage track detectors are useful for personal dosimeters and for flux/dose determination of high-energy particles from accelerators or cosmic rays. A detector that has its principal response at nucleon energy above 50 MeV is provided by the fission of Bi-209. Neutrons produce the largest percentage of hadron dose in most high-energy radiation fields. In these fields, the neutron spectrum is typically formed by low-energy neutrons (evaporation spectrum) and high-energy neutrons (knock-on spectrum). We used Bi-fission detectors to measure neutron peak fluence and compared the result with the calculated value of neutron peak fluence. For the exposure to 100 MeV we have used the iThemba Facility in South Africa.     

Number of elastic scatterings on free stationary nuclei to slow down a neutron

We report the formulation of the number of elastic scatterings required to slow down a neutron. By establishing its analytical expression, we show that this number displays a discontinuity and an oscillatory transient that progressively dampens when the neutron energy decreases. This result does not apply to neutrons with energies lower than a few eV, as we restrict our study to scatterings on free stationary nuclei.     

Use of CR-39 solid state nuclear track detector in neutron personnel monitoring

Neutron sources like ²⁴¹Am–Be, ²³⁹Pu–Be, ²⁵²Cf and 14.6 MeV neutron generator are being used in oil exploration industries as well as in research institutions. While handling these neutron sources, personnel may be exposed to neutrons. Also personnel working in reactors, accelerators may receive dose from neutrons. These exposed individuals need to be monitored regularly for measurement of neutron doses. The individual neutron doses can be estimated by using Kodak NTA films and CR-39 Solid State Nuclear Track Detector with a polyethylene radiator to increase sensitivity in front in holder. Nearly 1450 personnel are being monitored regularly throughout the country on a quarterly basis. In India, the monitoring system adopted for individual neutron dose estimation having energy from 100 keV and above is described in this paper. Background counts of 0.20 mSv could be measured with CR-39 SSNTD foil system and it has been successfully introduced for Fast Neutron Personnel Monitoring for the country.     

Microdosimetry for the characterization of the THOR epithermal neutron beam

The epithermal neutron beam of the Tsing Hua Open-pool Reactor (THOR) was constructed for the study of boron neutron capture therapy (BNCT). The THOR epithermal neutron beam was mainly composed of thermal neutrons, fast neutrons, and photons. For fast neutrons and photons, the absorbed dose and the relative biological effectiveness (RBE) were used to characterize radiation dose and radiation quality. The short-ranged alpha particles and lithium ions produced from ¹⁰B(n,α)⁷Li reactions in the BNCT required cellular- and micro-dosimetry characterizations. Due to the non-uniform microdistribution of boron in cells, these characterizations should depend on the source–target geometry. In this case, the geometry-dependent specific cellular dose and lineal energy could be used to describe radiation dose and radiation quality. In the present work, cellular- and micro-dosimetry were studied for the THOR epithermal neutron beam. The specific cellular dose and lineal energy were calculated for thermal neutron-induced α-particles and ⁷Li-ions with different source–target geometry and various cell sizes. Applying the linear energy dependent-biological weighting function, the geometry-dependent RBE of thermal neutron-induced heavy particles was determined. Finally, the effective RBE of the THOR epithermal neutron beam was estimated for tumors and normal tissues of specified ¹⁰B concentrations. This effective RBE should be multiplied by the total absorbed dose to determine the corresponding biological dose required in the treatment planning.     

Measurements of neutron radiation and induced radioactivity for the new medical linear accelerator,the Varian TrueBeam

Contemporary linear accelerators applied in radiotherapy generate X-ray and electron beams with energies up to 20 MeV. Such high-energy therapeutic beams induce undesirable photonuclear (γ,n) and electronuclear (e,e'n) reactions in which neutrons and radioisotopes are produced. The originated neutron can also induce reactions such as simple capture, (n,γ), reactions that produce radioisotopes. In this work measurements of the non-therapeutic neutrons and the induced gamma radiation were carried out in the vicinity of a new medical accelerator, namely the Varian TrueBeam. The TrueBeam is a new generation Varian medical linac making it possible to generate the X-ray beams with a dose rate higher than in the case of the previous models by Varian. This work was performed for the X-ray beams with nominal potentials of 10 MV (flattening filter free), 15 MV and 20 MV, and for a 22 MeV electron beam. The neutron measurements were performed by means of a helium chamber and the induced activity method. The identification of radioisotopes produced during emission of the therapeutic beams was based on measurements of the energy spectra of gammas emitted in decays of the produced nuclei. The gamma energy spectra were measured with the use of the high-purity germanium detector. The correlation between the neutron field and the mode and nominal potential was observed. The strongest neutron fluence of 3.1 × 10⁶ cm⁻² Gy⁻¹ and 2.0 × 10⁶ cm⁻² Gy⁻¹ for the thermal and resonance energies, respectively, was measured during emission of the 20 MV X-ray beam. The thermal and resonance neutron fluence measured for the 15 MV X-rays was somewhat less, at 1.1 × 10⁶ cm⁻² Gy⁻¹ for thermal neutrons and 6.7 × 10⁵ cm⁻² Gy⁻¹ for resonance neutrons. The thermal and resonance neutron fluences were smallest for the 10 MV FFF beam and the 22 MeV electron beam and were around two orders of magnitude smaller than those of the 20 MV X-ray beam. This work has shown that the neutron reactions are dominant because of relatively high cross sections for many elements used in the accelerator construction. The detailed analysis of the measured spectra made it possible to identify 11 radioisotopes induced during TrueBeam delivery. In this work the following radioisotopes were identified: ⁵⁶Mn, ¹²²Sb, ¹²⁴Sb, ¹³¹Ba, ⁸²Br, ⁵⁷Ni, ⁵⁷Co, ⁵¹Cr, ¹⁸⁷W, ²⁴Na and ³⁸Cl.     

Influence of neutron irradiation and temperature on the electric conductivity of SiO2 nanoparticles

At different frequency range it has been studied the influence of temperature and amount of falling neutrons on nano SiO₂ irradiated with neutrons. It has been revealed that it is generated additional electroactive radiation defects under the influence of rays omitted from activation products or direct neutron. Thus, the change of neutron flux at 6.7 × 10¹⁷ ∼ 2.7 × 10¹⁸ cm⁻² s⁻¹ range increases the electric conductivity of nano SiO₂ for approximately 30 times. It has been revealed two temperature ranges at temperature dependence of non-irradiated sample and three temperature ranges at a neutron irradiated-sample. It has been put forward the mechanism that explains the obtained results.     

Franco-Russian comparison of mixed neutron and gamma radiation field dosimeters at the Silène reactor

This paper gives the results of dosimetry measurements carried out in the Silène reactor at Valduc (France) with neutron and photon dosimeters in mixed neutron and gamma radiation fields, in the frame of a Franco-Russian comparison of dosimeters. Neutron dosimetry was supplied by passive semiconductors, activation detectors and nuclear track detectors. For photon dosimetry, thermoluminescent and passive semiconductor detectors were used. The experiments were located at 3 m from the reactor core, in free air and also at the front and back of a tissue-equivalent phantom. The pulse operating mode of the reactor was used to simulate a criticality accident with solid fissile material, while the free evolution mode simulated a criticality accident in a fissile solution. The photon absorbed dose showed a slight increase on entering the phantom compared to measurements in free air, probably due to backscattering by the phantom. At the rear of the phantom, the neutron kerma was four times lower than on the front, whereas the photon dose was only two times lower. The heterogeneity of dose inside the phantom was far greater for neutrons than for photons.     

A telescope detection system for direct and high resolution spectrometry of intense neutron fields

A high energy- and spatial-resolution telescope detector was designed and constructed for neutron spectrometry of intense neutron fields. The detector is constituted by a plastic scintillator coupled to a monolithic silicon telescope (MST), in turn consisting of a ΔE and an E stage. The scintillator behaves as an “active” recoil-proton converter, since it measures the deposited energy of the recoil-protons generated across. The MST measures the residual energy of recoil-protons downstream of the converter and also discriminates recoil-protons from photons associated to the neutron field. The lay-out of the scintillator/MST system was optimized through an analytical model for selecting the angular range of the scattered protons. The use of unfolding techniques for reconstructing the neutron energy distribution was thus avoided with reasonable uncertainty (about 1.6% in neutron energy) and efficiency (of the order of 10⁻⁶ counts per unit neutron fluence). A semi-empirical procedure was also developed for correcting the non-linearity in light emission from the organic scintillator. The spectrometer was characterized with quasi-monoenergetic and continuous fields of neutrons generated at the CN Van De Graaff accelerator of the INFN-Legnaro National Laboratory, Italy, showing satisfactory agreement with literature data.     

Neutron spectrometry and determination of neutron ambient dose equivalents in different LINAC radiotherapy rooms

A project has been set up to study the effect on a radiotherapy patient of the neutrons produced around the LINAC accelerator head by photonuclear reactions induced by photons above ～8 MeV. These neutrons may reach directly the patient, or they may interact with the surrounding materials until they become thermalised, scattering all over the treatment room and affecting the patient as well, contributing to peripheral dose. Spectrometry was performed with a calibrated and validated set of Bonner spheres at a point located at 50 cm from the isocenter, as well as at the place where a digital device for measuring neutrons, based on the upset of SRAM memories induced by thermal neutrons, is located inside the treatment room. Exposures have taken place in six LINAC accelerators with different energies (from 15 to 23 MV) with the aim of relating the spectrometer measurements with the readings of the digital device under various exposure and room geometry conditions. The final purpose of the project is to be able to relate, under any given treatment condition and room geometry, the readings of this digital device to patient neutron effective dose and peripheral dose in organs of interest. This would allow inferring the probability of developing second malignancies as a consequence of the treatment. Results indicate that unit neutron fluence spectra at 50 cm from the isocenter do not depend on accelerator characteristics, while spectra at the place of the digital device are strongly influenced by the treatment room geometry.     

Evaluation of neutron production in new accelerators for radiotherapy

Ambient dose equivalent, H*(10), and personal dose equivalent, Hp(10), were calculated in different points located inside two different treatment rooms. 15-MV Varian and 15-MV Elekta accelerators were used in these studies. The geometry of both accelerators heads and treatment rooms were built up to perform the Monte Carlo simulations. The patient was also simulated using an ICRU phantom. Calculations were done using the MCNPX code. Ambient dose equivalents rates from neutrons range between 1.2 and 419 mSv/h in the Elekta treatment room and between 0.96 and 1140 mSv/h in the Varian treatment room, depending on the location. These values suggest a larger neutron production in the Varian than in the Elekta accelerator.     

The relative biological effectiveness of a high energy neutron beam for micronuclei induction in T-lymphocytes of different individuals

In assessing the radiation risk of personnel exposed to cosmic radiation fields as it pertains to radiological damage during travel in civilian aircrafts, it is particularly important to know the relative biological effectiveness (RBE) for high energy neutrons. It has been the subject of numerous investigations in recent years using different neutron energies and cytogenetic examinations. Variations in the radiosensitivity of white blood cells for different individuals are likely to influence the estimate of the relative biological effectiveness for high energy neutrons. This as such observations have been noted in the response of different cancer cell lines with varying inherent sensitivities. In this work the radiosensitivities of T-lymphocytes of different individuals to the p(66)/Be neutron beam at iThemba LABS were measured using micronuclei formations and compared to that noted following exposure to ⁶⁰Co γ-rays. The principle objective of this investigation was to establish if a relationship between neutron RBE and variation in biological response to ⁶⁰Co γ-rays for lymphocytes from different individuals could be determined. Peripheral blood samples were collected from four healthy donors and isolated lymphocytes were exposed to different doses of ⁶⁰Co γ-rays (1–5 Gy) and p(66)/Be neutrons (0.5–2.5 Gy). One sample per donor was not exposed to radiation and served as a control. Lymphocytes were stimulated using PHA and cultured to induce micronuclei in cytokinesis-blocked cells. Micronuclei yields were numerated using fluorescent microscopy. Radiosensitivities and RBE values were calculated from the fitted parameters describing the micronuclei frequency dose response data. Dissimilar dose response curves for different donors were observed reflecting varying inherent sensitivities to both neutron and gamma radiation. A clear reduction in the dose limiting RBE_M is noted for donors with lymphocytes more sensitive to γ-rays (p = 0.032, R² = 0.94). Unlike observations made with different cancer cell lines exposed to the same clinical neutron beam, the variations in neutron RBE observed in T-lymphocytes of different individuals is related to the cellular radioresistance to photons.