Neutron dosimetry in the particle accelerator environment

Radiological safety aspects in general and neutron dosimetry in particular, around medium and high-energy particle accelerators pose some unique challenges to the practitioners of radiation protection. This is mainly because the source of radiations are directional, dynamic, pulsed and a mixture of different types. In conventional dosimetry, measurements are done in the units of the quantities in which the radiological protection limits are expressed. In the accelerator environment, measurement of energy and angular distribution of radiations is preferred instead. Research activities being carried out (particularly in India) in the field of neutron dosimetry are discussed. Measurements of neutron ambient dose equivalent directly using conventional rem-meters as well as neutron energy distributions using the time-of-flight technique employing proton recoil scintillators have been done at different directions with respect to light and heavy ion projectiles incident on various thick elemental targets. The observations and conclusions are summarized. Finally, a discussion on the concept of dose and radiological protection and operational quantities is done along with the recommendation of using Evidence theory instead of Bayesian probability in assessing radiological risk.     

Response of a Si-diode-based device to fast neutrons

Semiconductor devices based on a Si-detector are frequently used for charged particle's detection; one application being in the investigation of cosmic radiation fields. From the spectra of energy deposition events in such devices, the total energy deposited by the radiation in silicon can be derived. This contribution presents the results of studies concerning the response of this type of detector to fast neutrons. First, the spectrum of energy deposition was established in fast neutron radiation fields with average energies from 0.5 to 50 MeV. It was found that these spectra vary significantly with the neutron energy. The comparison with the spectra registered in photon beams permitted an estimation of the part of energy deposited that could be attributed to neutrons. It was found that this part increases rapidly with neutron energy. The possibilities to use this type of detector for neutron detection and dosimetry for radiation protection are analysed and discussed.     

Response functions of Cs2LiYCl6: Ce scintillator to neutron and gamma radiation

Research into advanced screening technologies has become high priority in all aspects of occupational nuclear safety and environmental radiation protection. Neutrons are a fundamental part of radiation encountered in various fields of nuclear science and technology and their detection is still employing detectors with a high thermal neutron response embedded in a thermalizing medium where helium based devices have been a dominant choice in many applications. Recently, there have been newly developed sensors based on multi-elements that include ⁶Li and ³⁵Cl isotopes to detect neutrons and gamma radiation. Among these new sensors one can cite the elpasolite scintillator, known as CLYC. This sensor contains two neutron sensitive isotopes and may serve as a dual detector for gamma as well as for neutron radiation. In this paper, the response functions of this sensor have been investigated in different fields of neutron and gamma-radiation. The sensor responses have been simulated using Monte Carlo N-Particle MCNPX code and a series of experiments have been carried out to validate the simulated data. Both sets of data are presented and discussed.     

Monte Carlo simulation of mini TEPC microdosimetric spectra: Influence of low energy electrons

In the past few years, miniaturized tissue-equivalent gas detectors (mini TEPCs) have been developed for application of microdosimetry in radiotherapy. These mini-TEPCs are characterised by millimetric dimensions. They are equipped neither with an internal calibration source nor with electric field tubes, which would properly define the sensitive volume hence the simulated site size. In spite of these lacks, mini TEPCs working in gas flow conditions have proven to be precise and reliable detectors. However, for future therapeutic plans including microdosimetric data, consistency between experimental and calculated data is important. Existing general-purpose Monte Carlo codes have proven to be very useful to calculate the energy deposition due to ionization in macroscopic targets, even in various complex radiation fields. However, theoretical models implemented in these codes for simulating electron transport and straggling are valid only for energies above a few keV. This restricts their applicability for simulating radiation transport at a micrometric level, where low-energy electrons play a dominant role. In this work, we calculate frequency distributions of deposited energy in a mini TEPC (with sizes equivalent to 1 and 2 μm) due to photons using the Monte Carlo code FLUKA. Comparisons between simulated and experimental data show a rather good agreement. Differences due to different FLUKA settings are discussed.     

均匀磁场下碳离子束剂量平均LET及纳剂量学量的分布

随着磁共振成像(MRI)技术的发展,图像引导放射治疗在放射肿瘤学中的作用和重要性正在迅速增加,本研究分析了外加均匀磁场对碳离子束的剂量平均LET以及纳剂量学量的影响。通过基于GEANT4内核的GATE蒙特卡罗(Monte Carlo, MC)模拟平台,模拟计算了不同磁场环境下,不同能量碳离子束剂量平均LET和纳剂量学量的分布。结果发现,平行磁场对碳离子束的剂量平均LET和纳剂量学量均无显著影响,垂直磁场对碳离子束的剂量平均LET及纳剂量学量的影响主要集中在布拉格峰区域,其影响主要是碳离子束在磁场中受到洛伦兹力作用而发生横向偏转,进而使得碳离子束布拉格峰位置发生横向侧移导致的。这些结果为进一步研究磁场对碳离子束治疗性能的影响打下了坚实的基础。     

Novel applications of particle accelerators to radiotherapy

Charged hadrons (protons and heavier ions) have very definite advantages over photons as far as radiotherapy applications are concerned. They allow for much better spatial dose localization due to their charge, relatively high mass and nature of the energy deposition process. In the frame of an attempt to promote the introduction of hadrontherapy in Argentina we have installed and started using an external beam facility at our tandem accelerator TANDAR. The advantages of heavy ions can only be fully exploited for tumors of well defined localization. In certain types of malignancies, however, the region infiltrated by tumor cells is diffuse, with no sharp boundaries and with microscopic ramifications. In such cases (particularly in certain brain cancers) a more sophisticated scheme has been suggested called boron neutron capture therapy (BNCT). In this work, the use of the Tandar accelerator to produce neutrons for feasibility studies for BNCT through low-energy proton beams on a thick LiF target is being briefly described. Studies on the ¹³C(d,n) reaction and a comparison with other neutron-producing reactions are also mentioned. Simulation work to optimize an accelerator-based neutron production target is discussed. A project is being prepared to develop a small proton accelerator in Argentina. Technical specifications of this machine will be briefly discussed.     

Estimation of neutron and other radiation exposure components in low earth orbit

The interaction of high-energy space radiation with spacecraft materials generates a host of secondary particles, some, such as neutrons, are more biologically damaging and penetrating than the original primary particles. Before committing astronauts to long term exposure in such high radiation environments, a quantitative understanding of the exposure and estimates of the associated risks are required. Energetic neutrons are traditionally difficult to measure due to their neutral charge. Measurement methods have been limited by mass and weight requirements in space to nuclear emulsion, activation foils, a limited number of Bonner spheres, and TEPCs. Such measurements have had limited success in quantifying the neutron component relative to the charged components. We will show that a combination of computational models and experimental measurements can be used as a quantitative tool to evaluate the radiation environment within the Shuttle, including neutrons. Comparisons with space measurements are made with special emphasis on neutron sensitive and insensitive devices.     

Predictions of secondary neutrons and their importance to radiation effects inside the International Space Station

As part of a study funded by NASA MSFC to assess thecontribution of secondary particles in producing radiation damage to optoelectronics devices located on the International Space Station (IS), Monte Carlo calculations have been made to predict secondary spectra vs. shielding inside ISS modules and in electronics boxes attached on the truss (Armstrong and Colborn, 1998). The calculations take into account secondary neutron, proton, and charged pion production from the ambient galactic cosmic-ray (GCR) proton, trapped proton, and neutron albedo environments. Comparisons of the predicted neutron spectra with measurments made on the Mir space station and other spacecraft have also been made (Armstrong and Colborn, 1998). In this paper, some initial results from folding the predicted neutron spectrum inside ISS modules from Armstrong and Colborn (1998) with several types of radiation effects response functions related to electronics damage and astronaut-dose are given. These results provide an estimate of the practical importance of neutrons compared to protons in assessing radiation effects for the ISS. Also, the important neutron energy ranges for producing these effects have been estimated, which provides guidance for onboard neutron measurement requirements.     

Indirect method to monitor the site size of sealed TEPCs

Tissue equivalent proportional counters (TEPCs) can be used as radiation monitors in different radiation environments, including on board of a spacecraft or aircraft. Usually, the gas density of the counter is set to simulate site sizes of 1 μm or 2 μm. However, especially for sealed counters which should be operative for long time without being refilled, the gas density inside the counter could change significantly. The immediate consequence of density variations is a gas gain shift. This in turn is not a serious problem, because the gas gain can be measured and corrected by increasing the bias voltage. A more critical consequence of an increase of the density is that the shape of the microdosimetric spectrum changes, affecting the response of the detector. Consequently, this limits the reliability of sealed TEPC over time, especially because the gas pressure is not directly monitored. Therefore, the aim of this study is to propose an experimental procedure to estimate the TEPC sensitive-site size by performing two independent measurements, one in a gamma radiation field and the other one in a neutron field. The results show that it is possible to assess the site size of sealed TEPCs with an accuracy of 5%.     

Regularities of nuclear physics experiment aboard spacecrafts with long-term active functioning

Although many efforts have been made to ensure the radiation resistance of spacecraft equipment, the active life of space instruments and onboard astrophysical equipment is limited to a large extent by the action of ionizing cosmic rays. Hence, it is necessary to refine the techniques used to predict the radiation resistance, improve the methods for monitoring radiation fields aboard spacecrafts, and find more effective ways to provide radiation protection of the electronic components.     

重离子加速器示范装置被动式束流配送系统次级中子特征的蒙特卡罗研究

在碳离子放射治疗中,碳离子束在剂量配送过程中会与束流输运线相互作用,形成以中子辐射为主的外辐射场.由于中子是高LET射线,具有较高的相对生物学效应,减少碳离子放疗中产生的次级中子有助于降低放疗后正常组织并发症几率及二次肿瘤风险.利用蒙特卡罗方法对保守情况(能量为400 MeV/u,多叶光栅完全闭合)下碳离子治疗被动式束...     

Neutron reflectometry with vector polarization analysis: First steps

The most detailed and reliable information about the magnetic state (magnetization depth profiles) of layers can be obtained by neutron reflectometry with vector polarization analysis. Two schemes of realization of this technique are considered. Precession coils designed to manipulate the polarization vector of monochromatic beams are used in scheme I. This scheme was tested at the neutron reflectometer NR-4M (PNPI, Gatchina). The earliest experimental data on the polarization vector rotation are reported, giving direct evidence of the wave function phase shift of a massive particle, the neutron, under total reflection. The basic elements for scheme II are remanent supermirrors. This scheme is designed for use with a white beam and is advantageous for pulse neutron sources. The effect of stray fields produced by remanent supermirrors on the neutron polarization has been theoretically and experimentally evaluated; efficient ways of compensating the stray fields are proposed.     

Intercomparison of the response of different photon and neutron detectors around a spent fuel cask

An intercomparison of the response of different photon and neutron detectors was performed in several measurement positions around a spent fuel cask (type TN 12/2B) filled with 4 MOX and 8 UO₂ 15 × 15 PWR fuel assemblies at the nuclear power plant Gösgen (KKG) in Switzerland. The instruments used in the study were both active and passive, photon and neutron detectors calibrated either for ambient or personal dose equivalent.The aim of the measurement campaign was to compare the responses of the radiation instruments to routinely used detectors.It has been shown that especially the indications of the neutron detectors are strongly dependent on the neutron spectra around the cask due to their different energy responses. However, routinely used active photon and neutron detectors were shown to be reliable instruments.     

Test measurements with a perfect crystal neutron interferometer

Two widely separated coherent neutron beams from dynamical diffraction in a perfect Si-crystal are used for neutron interferometric measurements. Al and Bi samples cause phase shifts within the individual beams which result in a marked intensity oscillation of the interfering beams behind theE-shaped interferometer crystal. With this interferometer interesting features of various physical quantities can be investigated in a new way, e.g. refracting index, scattering amplitudes, magnetic domain structures, density inhomogeneities, and coherence properties of the neutron beams.     

A preliminary study of the performance of a novel design of multi-element tissue equivalent proportional counter for neutron monitoring

In this study we investigate the dependence of the sensitivity of a TEPC upon its surface area and demonstrate that a compact multi-element tissue equivalent proportional counter (METEPC) has a counting sensitivity comparable to a commercially available 12.7 cm (5 inch) diameter spherical TEPC. The METEPC incorporates 61 cylindrical counting volumes of internal diameter of 0.5 cm and height 5 cm, machined in a single block of tissue equivalent plastic. It is the simplest design available in the multi-element geometrical configuration and is approximately nine times smaller in volume than that of a conventional 12.7 cm spherical TEPC. The neutron sensitivity of commercially available TEPCs and the METEPC simulating a 2 μm tissue site size was examined experimentally using the McMaster University 1.25 MV double stage Tandetron accelerator, which produces low energy neutrons via the ⁷Li(p, n)⁷Be reaction. The mean energy of the neutron beams produced ranged from 34 keV to 354 keV. The results presented in this study suggests that the compact METEPC is able to produce measurements in low dose rate radiation environments with the same precision in a given length of time as could be obtained with a 12.7 cm diameter spherical TEPC.     

VITESS polarized neutron suite: allows for the simulation of performance of any existing polarized neutron scattering instrument

As neutron simulations packages are used for analysis of the expected performance for practically all newly built neutron instruments, possibilities for simulations with polarized neutrons have been relatively underdeveloped.During the last years we developed a new approach for the representation of time-dependent magnetic fields (both in magnitude and direction) for the VITESS simulation package. This allowed us to simulate the neutron spin dynamics in practically all polarized neutron devices (RF neutron flipper, adiabatic gradient RF flipper, the Drabkin resonator, etc.). In this article the above-mentioned VITESS instrument components (modules) will be presented and the simulated performance of a number of polarized neutron scattering instruments (NRSE, MIEZE, SESANS, etc.) will be demonstrated.Thus, we practically complete the polarized neutron suite of the VITESS, which seems sufficient for the simulation of performance of any existing polarized neutron scattering instrument. Future work will be concentrated on developments of dedicated sample modules (kernels) to allow for virtual experiments with VITESS.     

241Am-Be中子参考辐射的蒙特卡罗模拟研究

研究用于校准场所中子剂量监测仪表的241Am-Be中子参考辐射场计量特性。采用蒙特卡罗方法模拟了空气自由中子参考辐射(FRNR),GB/T 14055规定的最小尺寸中子参考辐射(SRNR)和实际中子参考辐射(ARNR)中不同检验点处中子周围剂量当量率、散射中子占比和能谱分布特征。研究结果表明,空气对FRNR中的剂量率和能谱分布影响小,近似为理想中子参考辐射;采用5%含硼聚乙烯作屏蔽的最小尺寸SRNR可减少热中子,降低散射中子占比,影锥法不适用于小尺寸中子参考辐射中对散射中子的修正;ARNR中的散射中子更少、占比更低,影锥法所得散射中子占比与理论值基本一致。     

Energy deposition distributions caused by fast neutrons in converter type silicon detectors

Polyethylene converter silicon detectors have been developed as part of a dosimetry system for the application in neutron fields. Signals have been investigated which are mainly caused by reactions occurring when neutrons hit the PE converter or the silicon diode directly. In this work, neutron interactions in the components of a PE converter silicon sensor have been calculated to determine energy deposition distributions for neutron energies from 1.2 v MeV to 14.8 v MeV. Experiments in quasi-monoenergetic neutron fields have been performed. For the simulation of the neutron interactions with the detector layers, the GSF neutron interaction code NISD has been applied. The transport of ions has been calculated separately by means of the program TRIM. It has been shown that by the use of these concepts and models, a rather good agreement of measured and computed pulse height distributions can be obtained and, consequently, the response of converter type detectors to neutron radiation can be determined.     

The simulated workplace field with a high-energy neutron component produced by irradiating a Fe-target with 200 MeV/u 12C-ions

Recent developments in accelerator physics have led to new challenges for radiation protection dosimetry. Doses have to be determined for workplace fields which are characterized by high-energy radiation, a dominant contribution from neutrons, high intensities and pulsed time structure This may present problems for active measuring devices. As is well known, the ambient dose equivalent is often underestimated by area monitors operating in high-energy neutron fields behind shielding. Therefore, it is desirable to calibrate survey monitors in a characterized neutron field with the type of spectral fluence distribution that is expected behind shielding, i.e. where the main dose from neutrons arises from two peaks with mean energies of about 1 MeV and 100 MeV, respectively. Such a neutron fluence distribution is produced by the irradiation of a Fe-target with 200 MeV/u ¹²C-ions. Measurements with the extended range Bonner sphere spectrometer NEMUS of PTB were performed at two positions inside the experimental area Cave A of the heavy-ion synchrotron SIS at GSI. The measured neutron spectra show different fluence contributions for the two peaks at the two positions. The results were compared to Monte Carlo Simulations with MCNPX and FLUKA.     

Measurement of photoneutron doses in and out of high-energy X-ray beam of a SATURNE-20 medical linear accelerator by ECE polycarbonate detectors

Photoneutron contaminations in and out of high energy X-ray beams of the medical linear accelerator SATURNE 20 (CGR) of the Radiotherapy Department of Omeed Hospital in Isfahan, Iran, have been determined using 250 μm polycarbonate (PC) dosimeters, in strips or in sheets, processed by electrochemical etching (ECE) using specially designed ECE chambers to etch larger sheets. A two dimensional or topographical distribution of neutron contamination was also determined in a full size beam. The neutron dose equivalents (Hn) in the beam of 18 MV X-rays at 80 cm FSD were determined to be linear functions of X-ray dose equivalents (Hx) up to 1400 cSv. The distribution of the Hn at different X-ray doses showed bell-shape profiles with maxima at the isocenter. The ratios of dose equivalents of neutrons to those of X-rays increased as the field size increased having values of 0.22%, 0.28%, 0.31% and 0.37% for field sizes of 10×10, 20×20, 30×30, and 40×40 cm² respectively. Although such neutron dose equivalents can be corrected for patient treatment, it can cause radiation protection problems for workers where the design of the facility is not well planned.