Neutron Ambient Dose Equivalent Research Articles

Generation of neutron spectra and emission yields from proton-induced reactions on natC and natLi targets using CR-39 detectors

Fast neutron spectrometry and dosimetry at a specified location in a compact radiation environment is difficult due to large size of the conventional neutron spectrometers. In such situations, a small-size bare CR-39 detector can be used as a neutron detector to quantify the fast neutron fraction. In the present study, CR-39 detectors were placed very close to natural lithium and carbon targets during the proton irradiations to acquire the emission neutron spectra. The neutron spectra and the ambient dose equivalents in the forward and lateral directions with respect to the incident proton beam were estimated for both targets at incident proton energies between 8 to 20 MeV. The measured neutron spectra using CR-39 detectors could identify the quasi mono-energetic neutron features from the Li(p,n) and 13C(p,n) reactions effectively. An important observation of the present study is the identification of the fast neutron signature from the 13C(p,n) system from the discrete state de-excitations of excited 14N composite nuclei. The theoretical evaluation of the neutron spectral features and relative neutron energy distributions were performed using the FLUKA: FLUktuierende KAskade, a Monte Carlo simulation package, and the estimates agreed with the experimental results for both systems. The neutron ambient dose equivalents were also estimated from the measured spectra. These neutron fluence and dose estimates at close vicinity to the target can serve as an essential basis for shielding calculations and planning the pertinent radiation protection strategies.

Measurement of energy and directional distribution of neutron ambient dose equivalent for the [formula omitted]Li(p,n)[formula omitted]Be reaction

Double differential neutron fluence distributions were measured in the 7Li(p,n)7Be reaction for proton beam energies 7, 9 and 12 MeV. Seven liquid scintillator based detectors were employed to measure neutron fluence distributions using the Time of Flight technique. Neutron ambient dose equivalents were determined from the measured fluence distribution using ICRP (International Commission on Radiological Protection) recommended fluence to dose equivalent conversion coefficients. Neutron dose equivalents were also measured using a conventional BF3 detector based REM counter. Ambient dose equivalent measured by the REM counter is found to be in agreement with that determined from the neutron fluence spectra within their uncertainties. Angular distributions of the ambient dose equivalents were also determined from the measured fluence distributions at different angles.

LET spectrometry for p+nat(Li+C) system at different proton energies using CR-39: Development of autoLET code for automated generation of LET spectra and dose values

The present work describes measurement of LET (linear energy transfer) spectra due to neutrons generated from the p+nat(Li+C) system at 4 different proton energies (8, 12, 16 and 20 MeV) using CR-39 detector. A computer program (autoLET) was developed to process the track and the etching parameters for automated generation of the LET spectra and the corresponding dosimetric quantities. After chemical etching of irradiated CR-39 detectors, the track parameters were fed as inputs to the autoLET code to generate the LET spectra as well as to determine the track density and dosimetric quantities. FLUKA Monte Carlo simulations were performed to estimate the neutron fluence, energy spectrum and ambient dose equivalent for the same p+nat(Li+C) system. The measured track density and the dose equivalent were found to decrease initially from 8 to 12 MeV and further increased with increase in proton energy. Similar estimations for neutron fluence and ambient dose equivalent were also observed from the FLUKA simulations. The relative response of CR-39 in terms of track and dose equivalent determined for this radiation environment was found to be 1.60 × 10−4±3.85×10−5 track/n and 0.394 ± 0.054 respectively. The dose equivalent response (HLET/H*(10)) obtained in this study was compared with the data available in literature. The present study would be useful in estimating the neutron field around particle accelerator environment and for the radiation protection purposes in neutron generating facilities.

Dosimetry and radioprotection evaluations of very high energy electron beams

Very high energy electrons (VHEEs) represent a promising alternative for the treatment of deep-seated tumors over conventional radiotherapy (RT), owing to their favourable dosimetric characteristics. Given the high energy of the electrons, one of the concerns has been the production of photoneutrons. In this article we explore the consequence, in terms of neutron yield in a water phantom, of using a typical electron applicator in conjunction with a 2 GeV and 200 MeV VHEE beam. Additionally, we evaluate the resulting ambient neutron dose equivalent at various locations between the phantom and a concrete wall. Through Monte Carlo (MC) simulations it was found that an applicator acts to reduce the depth of the dose build-up region, giving rise to lower exit doses but higher entrance doses. Furthermore, neutrons are injected into the entrance region of the phantom. The highest dose equivalent found was approximately 1.7 mSv/Gy in the vicinity of the concrete wall. Nevertheless, we concluded that configurations of VHEEs studied in this article are similar to conventional proton therapy treatments in terms of their neutron yield and ambient dose equivalent. Therefore, a clinical implementation of VHEEs would likely not warrant additional radioprotection safeguards compared to conventional RT treatments.

Secondary Neutron Dose in Carbon-ion Radiotherapy: Investigations in QST–NIRS

Background: The National Institutes for Quantum and Radiological Science and Technology–National Institute of Radiological Sciences (QST–NIRS) has continuously investigated the undesired radiation exposure in ion beam radiotherapy mainly in carbon-ion radiotherapy (CIRT). This review introduces our investigations on the secondary neutron dose in CIRT with the broad and scanning beam methods.Materials and Methods: The neutron ambient dose equivalents in CIRT are evaluated based on rem meter (WENDI-II) measurements. The out-of-field organ doses assuming prostate cancer and pediatric brain tumor treatments are also evaluated through the Monte Carlo simulation. This evaluation of the out-of-field dose includes contributions from secondary neutrons and secondary charged particles.Results and Discussion: The measurements of the neutron ambient dose equivalents at a 90° angle to the beam axis in CIRT with the broad beam method show that the neutron dose per treatment dose in CIRT is lower than that in proton radiotherapy (PRT). For the scanning beam with the energy scanning technique, the neutron dose per treatment dose in CIRT is lower than that in PRT. Moreover, the out-of-field organ doses in CIRT decreased with distance to the target and are less than the lower bound in intensity-modulated radiotherapy (IMRT) shown in AAPM TG-158 (American Association of Physicists in Medicine Task Group).Conclusion: The evaluation of the out-of-field doses is important from the viewpoint of secondary cancer risk after radiotherapy. Secondary neutrons are the major source in CIRT, especially in the distant area from the target volume. However, the dose level in CIRT is similar or lower than that in PRT and IMRT, even if the contributions from all radiation species are included in the evaluation.

Cosmic ray neutron spectrometry and dosimetry at High Altitude Research Laboratory, Gulmarg, Kashmir, India

Cosmic ray neutrons are produced by the interaction of primary cosmic rays with the air molecules present in the atmosphere. At the ground level, these secondary neutrons are one of the important contributors of natural cosmic ray radiation background dose. With two major peaks around 1 MeV and 100 MeV, the neutron energies can extend up to 10 GeV. At the ground level, considerable presence of thermal neutrons can also be observed due to collisions and moderation of fast neutrons with the soil and other surrounding materials. A study on the evaluation of cosmic ray neutron spectra, and the resulting ambient dose equivalents was carried out at High Altitude Research Laboratory, Gulmarg, India (altitude 2650 m, N34.0484°, E74.3805°) considering the enhanced contribution of cosmic ray background in the region. Extended energy Bonner Sphere Spectrometer (BSS) was used for the continuous measurement of cosmic ray neutrons during the period of October 2018 to June 2019. Neutron spectra and ambient dose equivalents were unfolded from the BSS counts with the Monte Carlo generated response matrix and the FRUIT unfolding code. Along with Bonner sphere spectrometer, Tissue Equivalent Proportional Counter (TEPC) was also used for a limited period for independent measurement of cosmic ray dose equivalents and Linear Energy Transfer (LET) distribution. For comparison, neutron spectra and dose equivalent rates were also calculated using Monte Carlo code FLUKA and EXPACS software program. The ambient dose equivalent rates were found to vary in the range of 24–34 nSv. h−1 during the entire observation period of 9 months. No considerable seasonal variations were observed during the course of the study. Dose equivalent rates, estimated by the TEPC were slightly higher than the neutron dose equivalent rates measured by BSS. Experimentally measured values match reasonably well with theoretically simulated results.

EVALUATION OF NEUTRON AMBIENT DOSE EQUIVALENT IN INTENSITY-MODULATED COMPOSITE PARTICLE THERAPY.

Several studies have reported benefits derived from cancer treatment using various heavy-ion beams. Based on these reports, the National Institutes for Quantum and Radiological Science and Technology started developing intensity-modulated composite particle therapy (IMPACT) using He-, C-, O-, and Ne-ions. In ion beam therapy, nuclear interactions in the beamline devices or patient produce secondary neutrons. This study evaluated the characteristics of secondary neutrons in IMPACT. Neutron ambient dose equivalents were measured using WENDI-II. Measurements were performed under realistic case scenarios using He-, C-, O- and Ne-ion beams. Moreover, neutron ambient dose equivalents generated by He-, C-, O- and Ne-ion beams were compared with neutron ambient dose equivalents in proton therapy. No differences exist in the distance-dependence even when the primary ions are different. Neutrons generated by primary ion beams of high atomic numbers tend to emit forward. Moreover, in contrast with proton therapy, IMPACT can reduce neutron doses.

DEVELOPMENT OF A NEUTRON DOSIMETRY SYSTEM BASED ON DOUBLE SELF-ACTIVATED CSI DETECTORS FOR MEDICAL LINAC ENVIRONMENTS.

In the present study, by using double self-activated CsI detectors, the development of a neutron dosemeter system whose response indicates better agreement with the International Commission on Radiological Protection-74rem-response was carried out to simply evaluate the neutron dose with high accuracy. The present double neutron dosemeter system, using a slow-neutron dosemeter (thermal to 10keV) and a fast-neutron dosemeter (above 10keV), consists of CsI scintillators wrapped with two types of neutron energy filtering materials: polyethylene and B4C silicon rubber. After optimization of each filter thickness, to confirm the validity of our method, the neutron ambient dose equivalents under several operating conditions of medical linear accelerators (Linacs) were evaluated using a Monte Carlo simulation and an experiment with the present dosemeter. From these results, the present dosimetry system has enabled a more accurate neutron dose evaluation than our conventional dosemeter, and the present dosemeter was suitable for the neutron dosimetry for 10 MV Linac environments.

Characterization of the Secondary Neutron Field Produced in a Thick Aluminum Shield by 1 GeV/u 56Fe Ions Using TLD-Based Ambient Dosimeters

The neutron ambient dose equivalent induced by galactic cosmic ray-like (1 GeV/u 56Fe) radiation stopped in a thick aluminum shield was measured at different angles with GSI neutron ball, the standard TLD (thermoluminescent dosimeters) based neutron dosimeter for area monitoring at the GSI facility. In order to measure reliably at large angles, a modified version of the GSI ball, including a set of three more sensitive TLD600H/700H cards, instead of one standard TLD600/700, cards was used. The modified GSI balls were calibrated in neutron reference fields of 241Am-Be(alpha,n) available at the Physikalisch-Technische Bundesanstalt (PTB). The neutron ambient dose equivalent was measured at 5 different angles (15, 40, 90, 115 and 130 degree) with respect to the beam direction and compared to the calculated detector response and ambient neutron dose equivalent results from FLUKA simulations. The dosimeter readings were corrected for signal contributions coming from secondary charged particles. An agreement within 15 % was found between the measured and calculated GSI ball response and an agreement within 30 % was found between experiments and calculated neutron dose equivalents.

EVALUATION OF NEUTRON AMBIENT DOSE EQUIVALENT IN CARBON-ION RADIOTHERAPY WITH ENERGY SCANNING.

In carbon-ion radiotherapy (CIRT), secondary neutrons are produced by nuclear interactions in the beamline devices or patient. Herein, the characteristics of secondary neutrons in CIRT with energy scanning (ES) were evaluated. Neutron ambient dose equivalents (H*(10)) were measured using WENDI-II. The neutron energy spectrum was calculated using the Monte Carlo simulation. Measurement and calculation were performed under realistic case scenarios using maximum beam energies (Emax) of 290, 350 and 400MeVu -1. Moreover, H*(10) in ES was compared with H*(10) in range-shifter scanning (RS) and hybrid scanning (HS). H*(10) in Emax=290MeVu-1 was 65% less than that in Emax=400MeVu-1. At Emax=350MeVu-1, H*(10) in ES at θ=120 was 42% of that at θ=60. The neutron dose in ES CIRT decreased to approximately 60 and 70% of that in RS and HS CIRT, respectively, at 50-cm distance from the beam axis.

A New Application of Sohrabi Albedo Neutron Dosimeters around a Plasma Focus Device.

A new application of the Sohrabi albedo neutron dosimeters is reported for the first time for determination of very low-level neutron ambient dose equivalents on and around a 3.5 kJ plasma focus device (PFD). The Sohrabi dosimeters basically use a polycarbonate track detector as bare and/or in contact with B convertor(s) under special cadmium cover arrangements. Its sensitivity was improved by using enriched B under new cadmium arrangements in order to detect epithermal neutrons in addition to fast and thermal neutrons. Results of 12 dosimeters installed externally around the PFD at different azimuthal (φ) and polar (θ) angles showed that azimuthal (φ) fast, epithermal, thermal, and total neutron ambient dose equivalents were symmetric and isotropic, respectively, with values 55.15 ± 8.36, 1.36 ± 02, 0.53 ± 03, and 57.04 ± 8.62 μSv/shot at ~25 cm from anode top. Polar (θ) neutron ambient dose equivalent values on z-axis relative to 90 angle were relatively higher. Results of 38 dosimeters placed on PFD facility walls for workplace monitoring and on a BOMAB phantom at operator's location for personal dose equivalent determination showed values below minimum detection limits after exposure to 130 PFD shots. However, an operator's personal dose equivalents at ~1.0 and ~3.0 m from the anode top were estimated to be, respectively, ~13.7 and ~1.52 mSv y using azimuthal angle (φ) values if the PFD operates, for example, up to 20 shots per day for 200 d y. Even under such an extreme assumption, annual personal dose equivalent is still much below 20 mSv, the annual ICRP dose limit for workers.

Monte Carlo Simulations of Neutron Ambient Dose Equivalent in a Novel Proton Therapy Facility Design.

Purpose:The neutron shielding properties of the concrete structures of a proposed proton therapy facility were evaluated with help of the Monte Carlo technique. The planned facility's design omits the typical maze-structured entrances to the treatment rooms to facilitate more efficient access and, instead, proposes the use of massive concrete/steel doors. Furthermore, straight conduits in the treatment room walls were used in the design of the facility, necessitating a detailed investigation of the neutron radiation outside the rooms to determine if the design can be applied without violating existing radiation protection regulations. This study was performed to investigate whether the operation of a proton therapy unit using such a facility design will be in compliance with radiation protection requirements.Methods:A detailed model of the planned proton therapy expansion project of the University of Texas, M. D. Anderson Cancer Center in Houston, Texas, was produced to simulate secondary neutron production from clinical proton beams using the MCNPX Monte Carlo radiation transport code. Neutron spectral fluences were collected at locations of interest and converted to ambient dose equivalents using an in-house code based on fluence to dose-conversion factors provided by the International Commission on Radiological Protection.Results and Conclusions:At all investigated locations of interest, the ambient dose equivalent values were below the occupational dose limits and the dose limits for individual members of the public. The impact of straight conduits was negligible because their location and orientation were such that no line of sight to the neutron sources (ie, the isocenter locations) was established. Finally, the treatment room doors were specially designed to provide spatial efficiency and, compared with traditional maze designs, showed that while it would be possible to achieve a lower neutron ambient dose equivalent with a maze, the increased spatial (and financial) requirements may offset this advantage.

Neutron measurements from the interaction of a thick Ta target with protons at different energies

Neutron dosimetry and spectrometry with a Ta target is of concern in an accelerator environment considering its enormous use in a proton accelerator. The Ta target has a unique advantage considering the generation of short half lived radionuclides upon interaction with protons. This advantage extends its use as the beam stopping material at various locations of an accelerator. However, this target has a varied range of reactions to produce neutrons and prompt gammas depending on proton energies. So in the present work, the neutron spectra were generated at various incident proton energies and neutron ambient dose equivalents were estimated close to the target projectile interaction at 90°with respect to the incident beam direction. A comparison of the spectral nature and doses were also done with measurements made by our earlier studies carried out at 0°. The comparison clearly showed that, apart from an increase in the low energy neutron yields at 90°, the overall yield distribution remains similar at both 0°and 90°angles with respect to the incoming beam direction. The ratio of the ambient neutron dose equivalent is found to reduce by a factor of 3–4 at 90°compared to 0°over the entire incident proton energy range. So, the dose equivalent ratio at two angles can be considered as independent of the incident projectile energy.

Analysis of neutron streaming through penetrations in the JET biological shielding

In the present work neutron streaming through large ducts and labyrinths of the Joint European Torus (JET) biological shielding was evaluated. Neutron fluence and ambient dose equivalent were calculated along the total length of the ducts. Monte Carlo calculations using the MCNP code were performed for both Deuterium-Deuterium (D-D) and Deuterium-Tritium (D-T) toroidal plasma discharge sources. The results of the calculations were compared against measurements performed using thermoluminescence detectors. This work contributes to the operational radiation protection effort to minimize collective radiation dose to personnel at JET and, moreover, provides important information from JET experience that may assist in the optimization and validation of the radiation shielding design methodology used in future fusion plants, such as ITER and DEMO.

OPTIMIZATION OF AN ADDITIONAL COLLIMATOR IN A BEAM DELIVERY SYSTEM FOR REDUCTION OF THE SECONDARY NEUTRON EXPOSURE IN PASSIVE CARBON-ION THERAPY.

The aim of this work is to optimize an additional collimator in a beam delivery system to reduce neutron exposure to patients in passive carbon-ion therapy. All studies were performed by Monte Carlo simulation assuming the beam delivery system at Heavy-Ion Medical Accelerator in Chiba. We calculated the neutron ambient dose equivalent at patient positions with an additional collimator, and optimized the position, aperture size and material of the collimator to reduce the neutron ambient dose equivalent. The collimator located 125 and 470 cm upstream from the isocenter could reduce the dose equivalent near the isocenter by 35%, while the collimator located 813 cm upstream from the isocenter was ineffective. As for the material of the collimator, iron and nickel could conduct reduction slightly better than aluminum and polymethyl methacrylate. The additional collimator is an effective method for the reduction of the neutron ambient dose equivalent near the isocenter.

The effect of the flattening filter on photoneutron production at 10MV in the Varian TrueBeam linear accelerator.

Neutrons are an unavoidable by-product of high-energy radiation therapy treatments that deliver unwanted nontarget dose to patients. Use of flattening-filter-free (FFF) photon beams has been shown to significantly reduce photoneutron production per monitor unit (MU) of dose delivered. The purpose of this investigation was to characterize the photoneutron production of the 10MV and 10MV FFF beams of the Varian TrueBeamTM linear accelerator. Neutron fluence spectra were measured using a Nested Neutron SpectrometerTM (NNS, Detec Inc., Gatineau, Canada). The ratios of neutron fluence and ambient dose equivalent for the 10MV FFF beam relative to the 10MV beam, dubbed FF-ratios (FFF/FF), were used to characterize the difference between the two beams. FF-ratios were compared under the following three conditions (a) per MU, at various locations in the treatment room, (b) per MU, with the linac jaws opened and closed, and (c) per electron striking the bremsstrahlung target, as opposed to per MU, at one location with the jaws closed. On average, the neutron fluence for the 10MV FFF beam was 37% lower per MU than the 10MV beam (FF-ratio=0.63). The FF-ratio in neutron fluence and ambient dose equivalent did not vary by much between different locations within the treatment room. However, the FF-ratio in neutron ambient dose equivalent was reduced significantly when the linac jaws were opened compared to closed, which implies that the jaws contribute more to the photoneutron spectrum of the 10MV FFF beam than to the 10MV beam. Finally, it was found that the 10MV FFF beam produces more photoneutrons per electron striking the bremsstrahlung target than the 10MV beam (FF-ratio=2.56). The photoneutron fluence per MU produced by the 10MV FFF beam is 37% lower than the 10MV beam of a Varian TrueBeam linac. Accordingly, a reduction in neutron dose received by patients is achieved through use of the unflattened beam, provided that treatment plans for each beam require approximately the same number of MU. It was found to be instructive to compare the photoneutron yield per source electron between the two beams as it helped provide an understanding of the physics underlying photoneutron production in both beams.

Abstract ID: 98 Measurement of the induced neutron ambient dose equivalent during proton therapy in scanning mode

The purpose of this study is to evaluate secondary neutron ambient dose equivalent, which simulated and measured four positions around a plastic water phantom on a couch. We compared and analyzed the secondary neutron dose during proton treatment with scanning mode of each gantry. [1] , [2] . The volume of the plastic water phantom was 30 × 30 × 60 cm 3 and was located at the isocenter. The phantom was irradiated by a proton beam at 190 MeV. Proton beams were used to irradiate a volume of 10 × 10 × 10 cm 3 centered at the isocenter in each nozzle. In addition, WENDI 2 detector was used as a neutron measuring device to measure the neutron ambient dose equivalent. We performed experiments under the same conditions as those of FLUKA simulation. The ambient dose equivalent value was normalized to the maximum value at each nozzle of scanning mode. Each normalized neutron flux in the simulation and experiment showed similar tendencies. We successfully simulated and measured the neutron ambient dose equivalents at four positions generated by the scanning mode. These results prove that the simulation data provides reliable data of the induced neutron dose. Our results provide sufficient reference for the occurrence of secondary neutron during proton therapy.

Improvement of the response of a rem-meter based on prompt gamma ray detection to high-energy neutrons up to 1 GeV

Prompt gamma emissions from a system containing Borated High Density Polyethylene (BHDPE) covered by lead (Pb) have been estimated for incident neutron energies from 0.01 eV up to 1 GeV using the FLUKA Monte Carlo simulation code. The areas under the photo peaks of the emitted prompt gammas are linearly combined to approximate the ICRP recommended energy dependent neutron dose conversion coefficients to estimate the neutron ambient dose equivalent. An enhancement in the prompt gamma emission is observed in the system above 14 MeV of incident neutron energy because of neutron multiplication due to (n, xn) reactions in Pb. Monte Carlo simulation results are used to optimize the thickness of the Pb-cover and the boron content in BHDPE. Different workplace neutron spectra are used to computationally validate the present method. The photo peak areas from different prompt gammas emitted from H, B, C and Pb in the proposed system, when multiplied with appropriate coefficients and summed, can predict neutron ambient dose equivalent with good accuracy for both high energy (up to 1 GeV) and low energy (down to 0.01 eV) neutrons.

Study of neutron dose equivalent at the HIRFL deep tumor therapy terminal

The secondary neutron fields at the deep tumor therapy terminal at HIRFL (Heavy Ion Research Facility in Lanzhou) were investigated. The distributions of neutron ambient dose equivalent were measured with a FHT762 Wendi-II neutron ambient dose equivalent meter as 12C ions with energies of 165, 207, 270, and 350 MeV/u were bombarded on thick tissue-like targets. The thickness of targets used in the experiments was larger than the range of the carbon ions. The neutron spectra and dose equivalent were simulated by using FLUKA code, and the results agree well with the experimental data. The experiment results showed that the neutron dose produced by fragmentation reactions in tissue can be neglected in carbon-ion therapy, even considering their enhanced biological effectiveness. These results are also valuable for radiation protection, especially in the shielding design of high energy heavy ion medical machines.

MEASUREMENT OF NEUTRON AMBIENT DOSE EQUIVALENT IN PROTON RADIOTHERAPY WITH LINE-SCANNING AND WOBBLING MODE TREATMENT SYSTEM.

The primary objective of this study was to measure secondary neutron dose during proton therapy using a detector that covers the entire neutron energy range produced in proton therapy. We analyzed and compared the neutron dose during proton treatment with passive scattering and line scanning. The neutron ambient dose equivalents were measured with a 190 MeV wobbling and line-scanning proton beam. The center of a plastic water phantom (30 × 30 × 60 cm3) was placed at the isocenter. A Wide-Energy Neutron Detection Instrument (WENDI-2) was located 1m from the isocenter at four different angles (0°, 45°, 90° and 135°). Both wobbling and line-scanning modes of a multipurpose and pencil beam scanning dedicated nozzles were used to obtain a spread-out Bragg peak with 10-cm-width for the measurements. The ambient dose equivalent H*(10) value was normalized by the proton therapeutic dose at the isocenter. For wobbling mode and line-scanning mode, the highest H*(10) values were 1.972 and 0.099 mSv/Gy, respectively. We successfully measured the neutron ambient dose equivalents at six positions generated by a 190 MeV proton beam using wobbling and line-scanning mode with the WENDI-2. These reference data could be used for neutron dose reduction methods and other analysis for advanced proton treatment in the near future.