Neutron Dose Measurements Research Articles

Design, fabrication and modeling of an AmBe neutron irradiator for TLD screening for neutron dose measurement in mixed radiation fields

TLDs dosimeters are frequently presented as a viable choice for dosimetric studies when dealing with mixed neutron-gamma radiation fields. However, this choice is not without some drawbacks, because not only TLD response is highly dependent on particle type but also on neutron energy spectrum. Therefore, a correct screening and calibration of the dosimeter are required, and a simple shift from gamma screening methodology for mixed field is not suitable. This paper presents the design, fabrication and tests of an irradiator for TLD screening for neutron dose measurement using an AmBe source and polyethylene as moderator material. The design of the irradiator was conducted through Monte Carlo simulations using the MCNP5 code. The experimental validation and tests were performed using Indium activation foils and TLD 600 dosimeters. The manufactured irradiator demonstrated to be suitable for TLD screening under neutron source radiation field, offering very good homogeneity conditions in the radiation field so to guarantee same radiation dose delivered to the TLDs.

Structure and luminescence properties of $$\hbox {K}_{{2}}\hbox {GdF}_{{5}}$$ K 2 GdF 5 : $$\hbox {Tb}^{3+}$$ Tb 3 + synthesized by solid-state reaction method

\(\hbox {K}_{{2}}\hbox {GdF}_{{5}}\):\(\hbox {Tb}^{3+}\) material used in dosimetry field was synthesized by solid-state reaction method. The scanning electron microscopy image showed that the material exhibits porous surface with a very large surface area. The photoluminescence spectrum confirmed energy transfer process from \(\hbox {Gd}^{3+}\) ions to \(\hbox {Tb}^{3+}\) ions, and the luminescence of this material was entirely due to Tb ions. The high luminescent intensity at 542 nm was perfectly consistent with the sensitive wavelength range of photomultiplier tubes in the thermoluminescence (TL) reader. The TL-glow curve was very simple with the temperatures of the main peak in the range from 200 to \(230^{\circ }\hbox {C}\), and the glow-curve shape was suitable for dosimetry purposes. The TL intensities of \(\hbox {K}_{{2}}\hbox {GdF}_{{5}}\):10 mol% \(\hbox {Tb}^{3+}\) was higher than that of common \(\hbox {CaSO}_{{4}}\):\(\hbox {Dy}^{3+}\) dosimeters. The estimation on available application of the prepared material in neutron dose measurement was carried out.

In-phantom neutron dose measurement using Gafchromic film dosimeter for QA of BNCT beams

In order to improve the quality assurance (QA) procedure for beams of boron neutron capture therapy (BNCT), this study introduced using the Gafchromic film dosimeter for neutron dose measurement of BNCT beams. The crucial part of this study was investigating an approach to employ the Gafchromic film dosimeter placed inside a PMMA phantom irradiated by a BNCT beam. The spatial distribution of neutron dose of the film was determined using measurements and Monte Carlo calculations. By employing the present approach, the two-dimensional distributions of the neutron dose component of the film at specific depths in the phantom were successfully obtained. The determined neutron dose profiles were in good agreement with the calculated ones. This study also confirmed the finding that the film dosimeter is sensitive to thermal neutrons by comparing the depth-capture-reaction-rate and depth-dose distributions. Results of this work not only proved the feasibility of using the proposed method for the QA measurement of beam delivery but also revealed the advantages of easy-handling and remarkable spatial resolution of the film dosimeter when applied to BNCT fields. The present work can help to verify the dose uniformity and output stability of BNCT beams prior to clinical irradiation.

Cancer Risk Assessment due to Accidental Exposure inside Neutron Laboratories using BEIR VII Model

Introduction: Environmental and occupational human exposure from neutron source can lead to the serious biologic effects. The aim of this study is to evaluate the cancer incidence risk for various human organs at different neutron dose levels due to exposure from an Americium-241/Beryllium (Am-241/Be), a standard neutron source for calibration purposes. Material and Methods: We measured ambient dose equivalent H*(10) at different distances from Am-241/Be mixed neutron source by Berthold LB 6411 detector and determined cancer incidence risk for different organs of both male and female subjects at different neutron exposure levels by BEIR VII model. Results: Exposure age had a reverse impact on cancer incidence risk of different organs. We found that as H*(10) increases, cancer incidence risk increments as well. Colon (for men) and bladder (for women) had the highest sensitivity to neutron exposure, while prostate and uterus showed the lowest risk of cancer incidence among male and female subjects, respectively. Conclusion: Older exposed persons are at a lower risk of cancer incidence. The risk of cancer incidence for various organs is considerably associated with gender, such that radiation sensitivity of female organs was higher at all the measured neutron dose levels.

A prototype neutron dose measuring instrument based on prompt gamma detection

Neutron detection and the measurement of ambient dose equivalent are essential to ensure radiation safety in the workplace neutron environment existing in neutron-generating facilities, like particle accelerators, nuclear reactors and other neutron-generating facilities. Based on the measurement of the prompt gammas emitted from elements like hydrogen, boron and carbon, consequent to neutron interactions, a prototype neutron dose measuring instrument has been fabricated using high-density polyethylene and borated high-density polyethylene. This instrument provides an optimized response that closely approximates the fluence to dose conversion coefficients recommended by the International Commission on Radiological Protection (ICRP) to estimate the ambient neutron dose equivalent. Experimental evaluation of the performance of the instrument has been done using neutrons emitted from a 241 Am-Be source, kept inside a concrete enclosure with a square opening on one side. The present system can measure neutron dose with a match of about 5% with theoretical estimates and an existing rem-meter. The measured minimum detection level (MDL) of the instrument is 8.35 μSvh-1 in the presence of significant gamma background and can be improved using a larger NaI(Tl) detector.

Effects of 10 MV and Flattening-Filter-Free Beams on Peripheral Dose in a Cohort of Pediatric Patients

To assess the effect of flattening-filter-free (FFF) and 10MV radiation therapy beams on the peripheral dose received by a population of pediatric patients undergoing volumetric modulated arc therapy (VMAT). Twenty-six previously delivered 6MV flattened VMAT pediatric radiation therapy treatments plans were replanned with 6MV flattened, 6MV FFF, and 10MV FFF VMAT. Monte Carlo simulation code EGSnrc was used in conjunction with a measurement-based model to obtain 3-dimensional dose distributions. Peripheral dose delivered by FFF beams was compared with that delivered by 6MV flattened beams. A statistical analysis was performed to determine whether certain clinical factors (eg, target volume, location) were associated with a change in integral relative radiation dose. Neutron dose measurements assessed the neutron contribution from the 6MV flattened and 10MV FFF x-ray beams. Both the 6MV FFF and 10MV FFF beams delivered significantly lowerperipheral radiation doses than 6MV flattened (P<.01). The dose reduction was of 3.9% (95% confidence interval [CI] 2.1-5.7) and 9.8% (95% CI, 8.0-11.6) at 5cm from the PTV and 21.9% (95% CI, 13.7-30.1) and 25.6% (95% CI, 17.6-33.6) at 30cm for 6MV FFF and 10MV FFF beams, respectively. The clinical factorsexamined did not have a significant effect on the relative magnitude of the peripheral dose reduction. The upper limit on the neutron dose was determinedto be 203μSv for the 6MV flattened and 522μSv for the 10MV FFF beam. Both FFF beams significantly (P<.01) reduced the peripheral dose. 10MV FFF was more effective at reducing peripheral dose at distances <5cm from the PTV edge. The neutron doses delivered by all beams were <1% compared with the photon doses. 10MV FFF should be used to minimize peripheral dose.

Detection of neutrons with LiF fluorescent nuclear track detectors

Fluorescent nuclear track detectors (FNTD) based on natLiF crystals were successfully applied for measurements of neutron doses. FNTDs were exposed on a phantom to Hp(10) doses ranging from 1 mSv to 20 mSv. The number of observed tracks was found to be linear with the dose. The actual limit of detection is certainly much lower than 1 mSv and it depends on the scanned volume of a crystal. The very advantageous feature of LiF FNTDs is that the unirradiated detectors show no tracks similar to that produced by thermal neutrons, which means that background (zero-dose signal) is basically equal to zero. These results were obtained with detectors based on natural lithium and might be substantially improved if 6Li enriched lithium is used for crystal growth.It was also found that LiF FNTDs can be used for direct detection (without any converter) not only of thermal, but also of fast neutrons by using recoil of lithium nuclei.

Electronic neutron dosimeter in high-energy neutron fields

In neutron fields including neutron energies above 20 MeV a conventional neutron dosimeter is not suitable for measurements of neutron personal dose equivalent, Hp(10), over the whole energy range. Therefore, for such fields an electronic neutron dosimeter has been developed recently at Helmholtz Zentrum München (HMGU). In general, neutron dose measurements performed with this dosimeter at neutron energies below 2 MeV show an accuracy of about 30% [1]. Here we report the use of this dosimeter at the CERN-EU high-energy Reference Field (CERF) facility in Geneva, Switzerland. At this facility the available neutron fields include neutrons with energies below, but also above 20 MeV.In the present paper, personal dose equivalent (Hp(10)) values obtained with the ELectronic neutron DOsimeter (ELDO) are compared to neutron personal dose equivalent (Hp(10)) values obtained with the HMGU extended-range Bonner Sphere Spectrometer, and to reference values from FLUKA Monte Carlo simulations provided by CERF. It is shown that for continuous neutron spectra as those at CERF behind concrete shielding or secondary neutrons from cosmic rays, the dosimeter results are satisfactory for radiation protection purposes. However, in neutron fields including neutrons above about 7 MeV, where the major neutron dose contribution is from neutrons between 10 keV and several MeV (like those at CERF behind iron shielding), the doses provided by ELDO might be too small and care must be taken in interpreting the results.

Measurement of neutron dose in the compensator IMRT treatment

A radiation treatment delivery technique, intensity modulated radiation therapy (IMRT), has found widespread use in the treatment of cancers. One of IMRT implementing methods is IMRT compensator based, which the modulation are done by high Z materials. When photons with energies higher than 8MV interact with high Z material in path, Photoneutrons are produced. In this study, the effect of compensator on photoneutron production was investigated. The Monte Carlo code MCNPX was used to calculate the neutron dose equivalent as a function of the depth in phantom with and without compensator. Measurements were made using CR-39 track-etched detectors. CR-39 detectors, were cut in dimensions of 2.5×2.5 cm2 by laser, placed in different depths of slab phantom and then irradiated by 18MV photons. Same procedure was performed with the compensator present and absent. The measured data were compared with MCNP calculations. In both experimental and simulation results, neutron dose equivalent when compensator used, was less than non-compensator field. The calculated neutron dose equivalent was maximum at surface and decreased exponentially by increasing depth, but in experimental data, the neutron dose equivalent reached a maximum at approximately 3cm depth in the phantom and beyond which decreased with depth.CR-39 calibration was carried out in air, by considering that neutron energy spectrum changes toward thermal neutrons by depth in phantom increasing, it is suggested that for measuring equivalent neutron dose at phantom depth, should have proper neutron calibration in terms of energy spectrum.

Uncertainty evaluation of fluorescent nuclear track detectors (FNTDs) for neutron dose measurements

Fluorescent nuclear track detectors (FNTDs) developed by Landauer Inc. are used as passive dosimeters for neutron dose measurements. Commercial FNTD readers are operated in a track counting mode for low dose neutrons (up to 50 mSv). In order to evaluate the ability of the reader to detect recoil proton tracks, the relationship between track densities of FNTDs and etch pit densities of CR-39 plastic nuclear track detectors (PNTDs) for 241Am-Be neutrons was compared. The relationship was linear in the range of the measured doses from 0.63 mSv to 13.30 mSv. The correlation indicates that both neutron dosimeters similarly detect the recoil proton tracks. The low limit of detection (LLD) using the reader was 1.44 tracks/mm2. In this measurement, the uncertainty of track counting was also evaluated. The relationship between track counts and coefficients of variation (CVs) agreed well with the Poisson distribution. According to the theoretical estimation and the LLD, more than 11.11 counts per 2.08 mm2 are needed to obtain CVs below 30%. In addition, decreasing the CV value was demonstrated by expanding the scan area. Therefore, there is a possibility to improve the measurement accuracy for the LLD with a wider scan area.

Measurement of stray neutron doses inside the treatment room from a proton pencil beam scanning system

PurposeTo measure the environmental doses from stray neutrons in the vicinity of a solid slab phantom as a function of beam energy, field size and modulation width, using the proton pencil beam scanning (PBS) technique. MethodMeasurements were carried out using two extended range WENDI-II rem-counters and three tissue equivalent proportional counters. Detectors were suitably placed at different distances around the RW3 slab phantom. Beam irradiation parameters were varied to cover the clinical ranges of proton beam energies (100–220MeV), field sizes ((2×2)–(20×20)cm2) and modulation widths (0–15cm). ResultsFor pristine proton peak irradiations, large variations of neutron H∗(10)/D were observed with changes in beam energy and field size, while these were less dependent on modulation widths. H∗(10)/D for pristine proton pencil beams varied between 0.04μSvGy−1 at beam energy 100MeV and a (2×2)cm2 field at 2.25m distance and 90° angle with respect to the beam axis, and 72.3μSvGy−1 at beam energy 200MeV and a (20×20) cm2 field at 1m distance along the beam axis. ConclusionsThe obtained results will be useful in benchmarking Monte Carlo calculations of proton radiotherapy in PBS mode and in estimating the exposure to stray radiation of the patient. Such estimates may be facilitated by the obtained best-fitted simple analytical formulae relating the stray neutron doses at points of interest with beam irradiation parameters.

A Monte Carlo model for photoneutron generation by a medical LINAC

For an optimal tuning of the radiation protection planning, a Monte Carlo model using the MCNPX code has been built, allowing an accurate estimate of the spectrometric and geometrical characteristics of photoneutrons generated by a Varian TrueBeam Stx© medical linear accelerator. We considered in our study a device working at the reference energy for clinical applications of 15MV, stemmed from a Varian Clinac©2100 modeled starting from data collected thanks to several papers available in the literature. The model results were compared with neutron and photon dose measurements inside and outside the bunker hosting the accelerator obtaining a complete dose map. Normalized neutron fluences were tallied in different positions at the patient plane and at different depths. A sensitivity analysis with respect to the flattening filter material were performed to enlighten aspects that could influence the photoneutron production.

Analysis of the relationship between neutron dose and Cerenkov photons under neutron irradiation through Monte Carlo method

To theoretically explore the feasibility of neutron dose characterized by Cerenkov photons, the relationship between Cerenkov photons and neutron dose in a water phantom was quantified using the Monte Carlo toolkit Geant4. Results showed that the ratio of the neutron dose deposited by secondary electrons above Cerenkov threshold energy to the total neutron dose is approximately a constant for monoenergetic neutrons from 0.01 eV to 100 eV. With the initial neutron beam energy from 0.01 eV to 100 eV, the number of Cerenkov photons has a good correlation with the total neutron dose along the central axis of the water phantom. The changes of neutron energy spectrum and mechanism analysis also explored at different depths. And the ratio of total neutron dose to the intensity of Cerenkov photons is independent of neutron energy for neutrons from 0.01 eV to 100 eV. These findings indicate that Cerenkov radiation also has potential in the application of neutron dose measurement in some specific fields.

SU‐F‐T‐657: In‐Room Neutron Dose From High Energy Photon Beams

Purpose:To estimate neutron dose inside the treatment room from photodisintegration events in high energy photon beams using Monte Carlo simulations and experimental measurements.Methods:The Monte Carlo code MCNP6 was used for the simulations. An Eberline ESP‐1 Smart Portable Neutron Detector was used to measure neutron dose. A water phantom was centered at isocenter on the treatment couch, and the detector was placed near the phantom. A Varian 2100EX linear accelerator delivered an 18MV open field photon beam to the phantom at 400MU/min, and a camera captured the detector readings. The experimental setup was modeled in the Monte Carlo simulation. The source was modeled for two extreme cases: a) hemispherical photon source emitting from the target and b) cone source with an angle of the primary collimator cone. The model includes the target, primary collimator, flattening filter, secondary collimators, water phantom, detector and concrete walls. Energy deposition tallies were measured for neutrons in the detector and for photons at the center of the phantom.Results:For an 18MV beam with an open 10cm by 10cm field and the gantry at 180°, the Monte Carlo simulations predict the neutron dose in the detector to be 0.11% of the photon dose in the water phantom for case a) and 0.01% for case b). The measured neutron dose is 0.04% of the photon dose. Considering the range of neutron dose predicted by Monte Carlo simulations, the calculated results are in good agreement with measurements.Conclusion:We calculated in‐room neutron dose by using Monte Carlo techniques, and the predicted neutron dose is confirmed by experimental measurements. If we remodel the source as an electron beam hitting the target for a more accurate representation of the bremsstrahlung fluence, it is feasible that the Monte Carlo simulations can be used to help in shielding designs.

Secondary Neutron Dose Measurement for Proton Line Scanning Therapy

Secondary Neutron Dose Measurement for Proton Line Scanning Therapy

Measured Neutron Spectra and Dose Equivalents From a Mevion Single-Room, Passively Scattered Proton System Used for Craniospinal Irradiation

To measure, in the setting of typical passively scattered proton craniospinal irradiation (CSI) treatment, the secondary neutron spectra, and use these spectra to calculate dose equivalents for both internal and external neutrons delivered via a Mevion single-room compact proton system. Secondary neutron spectra were measured using extended-range Bonner spheres for whole brain, upper spine, and lower spine proton fields. The detector used can discriminate neutrons over the entire range of the energy spectrum encountered in proton therapy. To separately assess internally and externally generated neutrons, each of the fields was delivered with and without a phantom. Average neutron energy, total neutron fluence, and ambient dose equivalent [H* (10)] were calculated for each spectrum. Neutron dose equivalents as a function of depth were estimated by applying published neutron depth-dose data to in-air H* (10) values. For CSI fields, neutron spectra were similar, with a high-energy direct neutron peak, an evaporation peak, a thermal peak, and an intermediate continuum between the evaporation and thermal peaks. Neutrons in the evaporation peak made the largest contribution to dose equivalent. Internal neutrons had a very low to negligible contribution to dose equivalent compared with external neutrons, largely attributed to the measurement location being far outside the primary proton beam. Average energies ranged from 8.6 to 14.5 MeV, whereas fluences ranged from 6.91 × 10(6) to 1.04 × 10(7) n/cm(2)/Gy, and H* (10) ranged from 2.27 to 3.92 mSv/Gy. For CSI treatments delivered with a Mevion single-gantry proton therapy system, we found measured neutron dose was consistent with dose equivalents reported for CSI with other proton beamlines.

The Impact of Neutrons in Clinical Proton Therapy.

In proton therapy, high-energy proton beams cause the production of secondary neutrons. This leads to an unwanted dose contribution, which can be considerable for tissues outside of the target volume regarding the long-term health of cancer patients. Due to the high biological effectiveness of neutrons with regard to cancer induction, small neutron doses can be important. Published comparisons of neutron dose measurements and the corresponding estimates of cancer risk between different treatment modalities differ over orders of magnitude. In this report, the controversy about the impact of the neutron dose in proton therapy is critically discussed and viewed in the light of new epidemiological studies. In summary, the impact of neutron dose on cancer risk can be determined correctly only if the dose distributions are carefully measured or computed. It is important to include not only the neutron component into comparisons but also the complete deposition of energy as precisely as possible. Cancer risk comparisons between different radiation qualities, treatment machines, and techniques have to be performed under similar conditions. It seems that in the past, the uncertainty in the models which lead from dose to risk were overestimated when compared with erroneous dose comparisons. Current risk models used with carefully obtained dose distributions predict a second cancer risk reduction for active protons vs. photons and a more or less constant risk of passive protons vs. photons. Those findings are in general agreement with newly obtained epidemiologically results.

Novel detectors for silicon based microdosimetry, their concepts and applications

This paper presents an overview of the development of semiconductor microdosimetry and the most current (state-of-the-art) Silicon on Insulator (SOI) detectors for microdosimetry based mainly on research and development carried out at the Centre for Medical Radiation Physics (CMRP) at the University of Wollongong with collaborators over the last 18 years. In this paper every generation of CMRP SOI microdosimeters, including their fabrication, design, and electrical and charge collection characterisation are presented. A study of SOI microdosimeters in various radiation fields has demonstrated that under appropriate geometrical scaling, the response of SOI detectors with the well-known geometry of microscopically sensitive volumes will record the energy deposition spectra representative of tissue cells of an equivalent shape. This development of SOI detectors for microdosimetry with increased complexity has improved the definition of microscopic sensitive volume (SV), which is modelling the deposition of ionising energy in a biological cell, that are led from planar to 3D SOI detectors with an array of segmented microscopic 3D SVs. The monolithic ΔE−E silicon telescope, which is an alternative to the SOI silicon microdosimeter, is presented, and as an example, applications of SOI detectors and ΔE−E monolithic telescope for microdosimetery in proton therapy field and equivalent neutron dose measurements out of field are also presented. An SOI microdosimeter “bridge” with 3D SVs can derive the relative biological effectiveness (RBE) in 12C ion radiation therapy that matches the tissue equivalent proportional counter (TEPC) quite well, but with outstanding spatial resolution. The use of SOI technology in experimental microdosimetry offers simplicity (no gas system or HV supply), high spatial resolution, low cost, high count rates, and the possibility of integrating the system onto a single device with other types of detectors.

SU‐E‐T‐542: Measurement of Internal Neutrons for Uniform Scanning Proton Beams

Purpose:In proton radiotherapy, the production of neutrons is a wellknown problem since neutron exposure can lead to increased risk of secondary cancers later in the patient's lifetime. The assessment of neutron exposure is, therefore, important for the overall quality of proton radiotherapy. This study investigates the secondary neutrons created inside the patient from uniform scanning proton beams.Methods:Dose equivalent due to secondary neutrons was measured outside the primary field as a function of distance from beam isocenter at three different angles, 45, 90 and 135 degree, relative to beam axis. Plastic track nuclear detector (CR‐39 PNTD) was used for the measurement of neutron dose. Two experimental configurations, in‐air and cylindrical‐phantom, were designed. In a cylindrical‐phantom configuration, a cylindrical phantom of 5.5 cm diameter and 35 cm long was placed along the beam direction and in an in‐air configuration, no phantom was used. All the detectors were placed at nearly identical locations in both configurations. Three proton beams of range 5 cm, 18 cm, and 32 cm with 4 cm modulation width and a 5 cm diameter aperture were used. The contribution from internal neutrons was estimated from the differences in measured dose equivalent between in‐air and cylindrical‐phantom configurations at respective locations.Results:The measured ratio of neutron dose equivalent to the primary proton dose (H/D) dropped off with distance and ranged from 27 to 0.3 mSv/Gy. The contribution of internal neutrons near the treatment field edge was found to be up to 64 % of the total neutron exposure. As the distance from the field edge became larger, the external neutrons from the nozzle appear to dominate and the internal neutrons became less prominent.Conclusion:This study suggests that the contribution of internal neutrons could be significant to the total neutron dose equivalent.

SU‐E‐T‐566: Neutron Dose Cloud Map for Compact ProteusONE Proton Therapy

Purpose:To establish the base line of neutron cloud during patient treatment in our new compact Proteus One proton pencil beam scanning (PBS) system with various beam delivery gantry angles, with or without range shifter (RS) at different body sites. Pencil beam scanning is an emerging treatment technique, for the concerns of neutron exposure, this study is to evaluate the neutron dose equivalent per given delivered dose under various treatment conditions at our proton therapy center.Methods:A wide energy neutron dose equivalent detector (SWENDI‐II, Thermo Scientific, MA) was used for neutron dose measurements. It was conducted in the proton therapy vault during beam was on. The measurement location was specifically marked in order to obtain the equivalent dose of neutron activities (H). The distances of 100, 150 and 200 cm at various locations are from the patient isocenter. The neutron dose was measured of proton energy layers, # of spots, maximal energy range, modulation width, field radius, gantry angle, snout position and delivered dose in CGE. The neutron dose cloud is reproducible and is useful for the future reference.Results:When distance increased the neutron equivalent dose (H) reading did not decrease rapidly with changes of proton energy range, modulation width or spot layers. For cranial cases, the average mSv/CGE was about 0.02 versus 0.032 for pelvis cases. RS will induce higher H to be 0.10 mSv/CGE in average.Conclusion:From this study, neutron per dose ratio (mSv/CGE) slightly depends upon various treatment parameters for pencil beams. For similar treatment conditions, our measurement demonstrates this value for pencil beam scanning beam has lowest than uniform scanning or passive scattering beam with a factor of 5. This factor will be monitored continuously for other upcoming treatment parameters in our facility.