Tissue-equivalent Plastic Research Articles

CRaTER: The Cosmic Ray Telescope for the Effects of Radiation Experiment on the Lunar Reconnaissance Orbiter Mission

The Cosmic Ray Telescope for the Effects of Radiation (CRaTER) on the Lunar Reconnaissance Orbiter (LRO) characterizes the radiation environment to be experienced by humans during future lunar missions. CRaTER measures the effects of ionizing energy loss in matter due to penetrating solar energetic protons (SEP) and galactic cosmic rays (GCR), specifically in silicon solid-state detectors and after interactions with tissue-equivalent plastic (TEP), a synthetic analog of human tissue. The CRaTER investigation quantifies the linear energy transfer (LET) spectrum in these materials through direct measurements with the lunar space radiation environment, particularly the interactions of ions with energies above 10 MeV, which penetrate and are detected by CRaTER. Combined with models of radiation transport through materials, CRaTER LET measurements constrain models of the biological effects of ionizing radiation in the lunar environment as well as provide valuable information on radiation effects on electronic systems in deep space. In addition to these human exploration goals, CRaTER measurements also provide new insights on the spatial and temporal variability of the SEP and GCR populations and their interactions with the lunar surface. We present here an overview of the CRaTER science goals and investigation, including: an instrument description; observation strategies; instrument testing, characterization, and calibration; and data analysis, interpretation, and modeling plans.

Tissue equivalency of phantom materials for neutron dosimetry in proton therapy

Previous Monte Carlo and experimental studies involving secondary neutrons in proton therapy have employed a number of phantom materials that are designed to represent human tissue. In this study, the authors determined the suitability of common phantom materials for dosimetry of secondary neutrons, specifically for pediatric and intracranial proton therapy treatments. This was achieved through comparison of the absorbed dose and dose equivalent from neutrons generated within the phantom materials and various ICRP tissues. The phantom materials chosen for comparison were Lucite, liquid water, solid water, and A150 tissue equivalent plastic, These phantom materials were compared to brain, muscle, and adipose tissues. The magnitude of the doses observed were smaller than those reported in previous experimental and Monte Carlo studies, which incorporated neutrons generated in the treatment head. The results show that for both neutron absorbed dose and dose equivalent, no single phantom material gives agreement with tissue within 5% at all the points considered. Solid water gave the smallest mean variation with the tissues out of field where neutrons are the primary contributor to the total dose. Of the phantom materials considered, solid water shows best agreement with tissues out of field.

Simulation of the Charge Produced by Protons inside a Tissue Equivalent Ionization Chamber in a Mixed Neutron/Gamma Field in BNCT

The neutron and gamma dose in boron neutron capture therapy (BNCT) can be determined by using ionization chambers of different materials. However, inexplicable results, such as negative doses, are sometimes obtained. Computer simulations using MCNPX can help one to understand the behavior of ionization chambers. This paper deals with a part of this investigation: the contribution of protons to the total measured charge in a tissue equivalent (TE) ionization chamber that is flushed with methane-based TE gas. The inherent problem is that the Monte Carlo code MCNPX cannot track protons below 1 MeV.A custom-made program, called Proton Produced Ionization Chamber Charge (PPICC), calculates the deposited energy and thus the charge in the TE gas per proton. For this, it uses the stopping powers for protons in TE plastic and gas. MCNPX provides the total number of protons produced by all neutron interactions near the gas. To check this new procedure, measurements and simulations have been performed using a validated mixed beam of neutrons and gammas. The neutron fluence consists of 12% fast neutrons and 87% epithermal neutrons. In one setup the chamber is free-in-air (epithermal/fast neutron field) and in the other is in a cubic polymethylmethacrylate phantom at 25 mm depth (thermal/epithermal neutron field).The total charge is the sum of the charges due to electrons, originating from primary and neutron-induced gammas, and protons from 1H(n,n)1H and 14N(n,p)14C reactions. The total measured and calculated charges in the two setups have acceptable uncertainties and are in good agreement. The charge collected in a TE ionization chamber can be simulated in a mixed field of neutrons and gammas. The charge resulting from proton recoil in the gas is unexpectedly large.

Study of a Solid-State Microdosimeter Based on Micrometric-Size Diodes Coupled to a Residual Energy Measurement Stage

An array of micrometric diodes coupled to a residual energy measurement stage was proposed for solid-state microdosimetry. Each diode has a sensitive volume of a cylindrical shape (9 μm in nominal diameter) in order to reproduce that simulated by a cylindrical tissue-equivalent proportional counter (TEPC). The silicon microdosimeter was irradiated coupled to a polyethylene converter with monoenergetic neutrons of several energies. The spectra of the energy imparted to the segmented telescope were corrected for tissue equivalence through an optimized procedure that exploits the information from the residual energy stage. A geometrical correction was also applied. The dose-mean lineal energy values were qualitatively compared with literature data. The silicon microdosimeter was also covered with a tissue-equivalent plastic (A150) and with a nylon converter. The results showed a contribution of heavy recoils (mainly carbon and nitrogen nuclei) generated in the tissue-equivalent plastic lower than that measured by the TEPC, owing to their stopping in the titanium-based dead layer of the silicon device.

Design, Construction, and Implementation of Spherical Tissue-Equivalent Proportional Counter

In this work a new single anode spherical tissue-equivalent proportional counter (TEPC) was developed for mixed field radiation dosimetry. The approach used was to divide the cathode into several rings with different thicknesses and adjust the potential difference between each ring and the anode to produce an electric field that is nearly constant along the length of the anode. A-150 tissue-equivalent plastic is used for the detector walls, the insulator material between the cathode rings is low-density polyethylene, and the gas inside the detector is propane. The detector, along with the charge sensitive preamplifier, is encased in a stainless steel vacuum chamber. This 1.8-cm-diam TEPC provides low electronic noise, a gas gain of 550 at 782 V, good spectral resolution (26 keV/μm), and a full-width at half-maximum value of 17%. This spherical TEPC detector system will improve the accuracy of dosimetry in space and as a result improve radiation safety for astronauts.

TU-D-BRB-07: Determination of Neutron Dose Due to a Therapeutic Proton Beam Incident On a Closed Tungsten MLC Using the Dual Hydrogenous/non-Hydrogenous Ionization Chamber Method

Purpose: To measure neutron dose around a tungsten MLC used for proton beam therapy using the dual ionization chamber (IC) method. Method and Materials: ICs were constructed with large 10cc volumes to enable low dose measurements in the mGy range. The chambers are identical aside from wall, electrode, and guard-ring material with one constructed of A150 tissue-equivalent plastic (T) and filled with methane based tissue-equivalent gas and the other magnesium (U) and filled with argon gas. IC(T) has approximately the same sensitivity to neutrons and photons while the non-hydrogenous IC(U) exhibits a lower sensitivity to neutrons. The dose from neutrons and photons can be expressed as DN=(RT-RU)/(1-kU) and DG=(RU- kURT)/(1-kU) where RU,RT are IC readings and kU is the relative sensitivity of IC(U) to neutrons which varies with energy. Measurements were made with 230MeV proton beams incident on a closed MLC with the ICs on the beam axis 21cm from the MLC. Open field measurements were made with attenuation by 0, 0.5, 2, 5, 9, and 10cm of solid water. The ICs were calibrated against a Farmer chamber in air with a 6MV beam and suitable buildup caps. Results: Initial analysis of the measurements suggests an average neutron energy of 2.6MeV corresponding to kU=0.033. Measured neutron dose with the MLC closed and detector geometry stated above is 1.67mGy/Gy where IC(T) was used to determine dose in the open field. Conclusion: Initial measurements of neutron dose from a proton therapy beam incident on a MLC were performed which indicate the validity of the dual chamber method for determining neutron dose around a proton beam. This work was supported by the US Army Medical Research and Materiel Command under Contract Agreement No. DAMD17-W81XWH-07-2-0121. Opinions, interpretations, conclusions and recommendations are those of the author and are not necessarily endorsed by the US Army.

A comparison of the measured responses of a tissue-equivalent proportional counter to high energy heavy (HZE) particles and those simulated using the Geant4 Monte Carlo code

Monte Carlo simulations of heavy ion interactions using the Geant4 toolkit were compared with measurements of energy deposition in a spherical tissue-equivalent proportional counter (TEPC). A spherical cavity with a physical diameter of 12.7 mm was filled with propane-based tissue-equivalent gas surrounded by a wall of A-150 tissue-equivalent plastic that was 2.54 mm to thick. Measurements and Monte Carlo simulations were used to record the energy deposition and the trajectory of the incident particle on an event-by-event basis for ions ranging in atomic number from 2 ( 4He) to 26 ( 56Fe) and in energy from 200 MeV/nucleon to 1000 MeV/nucleon. In the simulations, tracking of secondary electrons was terminated when the range of an electron was below a specified threshold. The effects of range cuts for electrons at 0.5 μm, 1 μm, 10 μm, and 100 μm were evaluated. To simulate an energy deposition influenced by large numbers of low energy electrons with large transverse momentum, it was necessary to track electrons down to range cuts of 10 μm or less. The Geant4 simulated data closely matched the measured data acquired using a TEPC for incident particles traversing the center of the detector as well as near the gas-wall interface. Values of frequency mean lineal energy and dose mean lineal energy were within 8% of the measured data. The production of secondary particles in the aluminum vacuum chamber had no effect on the response of the TEPC for 56Fe at 1000 MeV/nucleon. The results of this study confirm that Geant4 can simulate patterns of energy deposition for existing microdosimeters and is valuable for improving the design of a new generation of detectors used for space dosimetry and for characterizing particle beams used in hadron radiotherapy.

SU‐HH‐AUD C‐02: Carbon Foam Collimators for Proton Therapy

Purpose: The purpose of this study was to test the effectiveness of a new collimating material made of carbon foam and low‐density polyethylene. The hypothesis is that the carbon foam material will produce fewer neutrons and be more affordable to manufacture than conventional materials. The study includes comparison with spallations produced from a tungsten target and a combination between a new material and tungsten, as well as the effect of the products in Tissue Equivalent Plastic, and the energy deposition contribution to the total dose. The study also includes incorporating a new Multi‐leaf Collimator concept. Method and Materials The collimation material developed in this study is based on a space radiation shielding materials NASA project, and it is a low density Polyethylene that was impregnated into Carbon Foam (PELowFOAM). The material has a density of 1.071 g/cm3, a hydrogen atomic composition of 10.5% and a carbon atomic composition of 89.5%. Monte Carlo analyses for neutron and secondary protons production was performed (using HETC‐HEDS). Results: The number of neutrons produced with Tungsten is 65.78 % of the primary proton beam. PELowFoam and PELowFoam with tungsten produce almost 3.5 times fewer neutrons than tungsten alone. Secondary protons produced are not of concern. Almost 45.43 % of the primary beam deposited its energy as a scattering neutron into the Tissue equivalent Plastic. This number is alarming and needs to definitely be taken into consideration. Conclusion: A novel material was tested that is made of low density polyethylene impregnated in carbon foam to be used for collimating proton beams instead of using high Z materials like tungsten. Out of the three materials, tungsten performed the worst producing 66% of the primary beam in the form of neutrons, while the PELowFoam produced about 16% neutrons, and the PELowFoam combined with tungsten produced about 19% neutrons.

SU‐GG‐AUD‐07: Development of a Novel Detector for Measuring Proton Energy Spectra

Purpose: The purpose of this study was to design an instrument for measuring proton energy spectra, and present energy deposition results that compare calibration runs with two state of the art Monte Carlo codes, HETC‐HEDS, and GEANT. Method and Materials: The proton energy spectrum analyzer developed in this study is based on a telescope called the Cosmic Ray telescope of the Effects of radiation (CRaTER), originally designed to achieve characterization of the global lunar radiation environment and its biological impacts. It employs a stack of silicon detectors and tissue‐equivalent plastic (TEP) to establish the linear energy transfer (LET) spectra of cosmic radiation relevant for human and electronic parts considerations. Measured and calculated spectra were obtained using dE/dX= (E0−Ef)/L, where E0 and Ef are the energies of the particles as measured by the silicon detectors at either end of each TEP section and L is the length of the material. Results: Results show a very good agreement between experimental and computer models. A comparison of the peak position shows that HETC‐HEDS simulation of the instrument in a clinical proton beam yield a peak at channel 50. In comparison, the peak channel was 55 with Geant and 57 from the MGH data. The reason for the discrepancy is that the HETC‐HEDS analysis was performed for energies down to 0.2 MeV, while the analyses with GEANT and the MGH data included around 20 more energy channels. Conclusion: An LET energy spectrum was designed and computational testing for proton therapy. Comparisons between calibration runs and two simulation runs using GEANT and HETC‐HEDS show promising results in being able to use the instrument for proton therapy calibration purposes. Future investigations will include analyses for different geometrical configurations, as well as investigating the detector's response to other charged particles.

Lunar Reconnaissance Orbiter Overview: The Instrument Suite and Mission

NASA’s Lunar Precursor Robotic Program (LPRP), formulated in response to the President’s Vision for Space Exploration, will execute a series of robotic missions that will pave the way for eventual permanent human presence on the Moon. The Lunar Reconnaissance Orbiter (LRO) is first in this series of LPRP missions, and plans to launch in October of 2008 for at least one year of operation. LRO will employ six individual instruments to produce accurate maps and high-resolution images of future landing sites, to assess potential lunar resources, and to characterize the radiation environment. LRO will also test the feasibility of one advanced technology demonstration package. The LRO payload includes: Lunar Orbiter Laser Altimeter (LOLA) which will determine the global topography of the lunar surface at high resolution, measure landing site slopes, surface roughness, and search for possible polar surface ice in shadowed regions, Lunar Reconnaissance Orbiter Camera (LROC) which will acquire targeted narrow angle images of the lunar surface capable of resolving meter-scale features to support landing site selection, as well as wide-angle images to characterize polar illumination conditions and to identify potential resources, Lunar Exploration Neutron Detector (LEND) which will map the flux of neutrons from the lunar surface to search for evidence of water ice, and will provide space radiation environment measurements that may be useful for future human exploration, Diviner Lunar Radiometer Experiment (DLRE) which will chart the temperature of the entire lunar surface at approximately 300 meter horizontal resolution to identify cold-traps and potential ice deposits, Lyman-Alpha Mapping Project (LAMP) which will map the entire lunar surface in the far ultraviolet. LAMP will search for surface ice and frost in the polar regions and provide images of permanently shadowed regions illuminated only by starlight. Cosmic Ray Telescope for the Effects of Radiation (CRaTER), which will investigate the effect of galactic cosmic rays on tissue-equivalent plastics as a constraint on models of biological response to background space radiation. The technology demonstration is an advanced radar (mini-RF) that will demonstrate X- and S-band radar imaging and interferometry using light weight synthetic aperture radar. This paper will give an introduction to each of these instruments and an overview of their objectives.

Development and characterization of a tissue equivalent plastic scintillator based dosimetry system

High precision techniques in radiation therapy, such as intensity modulated radiation therapy, offer the potential for improved target coverage and increased normal tissue sparing compared with conformal radiotherapy. The complex fluence maps used in many of these techniques, however, often lead to more challenging quality assurance with dose verification being labor-intensive and time consuming. A prototype dose verification system has been developed using a tissue equivalent plastic scintillator that provides easy-to-acquire, rapid, digital dose measurements in a plane perpendicular to the beam. The system consists of a water-filled Lucite phantom with a scintillator screen built into the top surface. The phantom contains a silver coated plastic mirror to reflect scintillation light towards a viewing window where it is captured using a charge coupled device camera and a personal computer. Optical photon spread is removed using a microlouvre optical collimator and by deconvolving a glare kernel from the raw images. A characterization of the system was performed that included measurements of linear output response, dose rate dependence, spatial linearity, effective pixel size, signal uniformity and both short- and long-term reproducibility. The average pixel intensity for static, regular shaped fields between 3 cm X 3 cm and 12 cm x 12 cm imaged with the system was found to be linear in the dose delivered with linear regression analysis yielding a correlation coefficient r2 > 0.99. Effective pixel size was determined to be 0.53 mm/pixel. The system was found to have a signal uniformity of 5.6% and a long-term reproducibility/stability of 1.7% over a 6 month period. The system's ability to verify a dynamic treatment field was evaluated using 60 degrees dynamic wedged fields and comparing the results to two-dimensional film dosimetry. Results indicate agreement with two-dimensional film dosimetry distributions within 8% inside the field edges. With further development, this system promises to provide a fast, directly digital, and tissue equivalent alternative to current dose verification systems.

Sci‐PM Sat ‐ 04: Characterization of a tissue equivalent plastic scintillator dosimetry system

High precision techniques in radiation therapy such as IMRT offer the potential for improved target coverage and increased normal tissue sparing. The complex fluence maps used in many of these techniques, however, lead to more challenging quality assurance with dose verification being labor‐intensive and time consuming. We have developed a dose verification system using tissue equivalent plastic scintillator that provides easy to acquire, rapid electronic and directly digital dose measurements in a plane perpendicular to the beam. The prototype system consists of a water‐filled Lucite phantom with a scintillator screen built into the top surface. The phantom contains a plastic mirror to reflect scintillation light towards a viewing window where it is captured using a CCD camera and a personal computer. Optical photon spread is removed using a micro‐louvre optical collimator and by deconvolving a glare kernel from the raw images. System characterization tests indicate excellent dose and spatial linearity. The system was found to have good signal uniformity and to be independent of dose rate. Spatial resolution was determined to be 0.53 mm/pixel. The system's ability to verify a dynamic treatment field was evaluated using a 60° enhanced dynamic wedge and comparing the results to 2‐D film dosimetry. Results indicate agreement within 5% of 2‐D film dosimetry. With further development this system promises to provide a fast, directly digital, and tissue equivalent alternative to current dose verification systems.

SU‐CC‐J‐6C‐11: Verification of IMRT Dose Distributions Using a Tissue Equivalent Plastic Scintillator Based Dosimetry System

Purpose: Intensity modulated radiation therapy (IMRT) offers the potential for improved target coverage and increased normal tissue sparing compared with conformal radiotherapy. The complex fluence maps used in IMRT, however, lead to more challenging quality assurance with dose verification being labor‐intensive and time consuming. An IMRT dose verification system has been developed using tissue equivalent plastic scintillator that provides easy to acquire, rapid electronic and directly digital dose measurements of a 2D plane perpendicular to the beam. Method and Materials: The prototype system consists of a water‐filled Lucite phantom with a scintillator screen built into the top surface. The phantom contains a plastic mirror to reflect scintillation light towards a viewing window where it is captured using a CCD camera and a personal computer. Optical photon spread is removed using a micro‐louvre optical collimator and by deconvolving a glare kernel from the raw images. System calibration is performed using radiation fields of known dose distributions. The system was evaluated by verifying a 5‐field IMRT plan and comparing the results to a 2‐D film dosimetry verification of the same plan. Resultant distributions were compared using 1‐D dose profiles, 2‐D dose difference maps and gamma factor analysis. A comparison of the time requirements for verification with each system was also performed. Results: Preliminary results acquired with the new verification system indicate agreement within 5% of 2‐D film dosimetry. Gamma factor analysis shows excellent agreement over most of the distributions. The timing comparison of the verification systems highlights the efficiency of the new scintillator system for IMRT verification with a 50% reduction in time. Conclusion: Results from a 5‐field IMRT plan verification using the new scintillator based system are very promising. With further development this system promises to provide a fast, directly digital, and tissue equivalent alternative to current IMRT verification systems.

Differential absorbed dose distributions in lineal energy for neutrons and gamma rays at the mono-energetic neutron calibration facility

Absorbed dose distributions in lineal energy for neutrons and gamma rays of mono-energetic neutron sources from 140 keV to 15 MeV were measured in the Fast Neutron Laboratory at Tohoku University. By using both a tissue-equivalent plastic walled counter and a graphite-walled low-pressure proportional counter, absorbed dose distributions in lineal energy for neutrons were obtained separately from those for gamma rays. This method needs no knowledge of energy spectra and dose distributions for gamma rays. The gamma-ray contribution in this neutron calibration field >1 MeV neutron was <3%, while for <550 keV it was >40%. The measured neutron absolute absorbed doses per unit neutron fluence agreed with the LA150 evaluated kerma factors. By using this method, absorbed dose distributions in lineal energy for neutrons and gamma rays in an unknown neutron field can be obtained separately.

Neutron irradiation field produced by 25 MeV deuterons bombarding on thick beryllium target for radiobiological study

A fast neutron irradiation field for biological studies was established at the cyclotron facility in the National Institute of Radiological Sciences. Neutron spatial distributions and absorbed dose distributions were measured to characterize the neutron field. By using a simple technique that an aluminum plate activated by neutrons was exposed to the imaging plates, the width of the neutron-irradiated area was measured to be 23.6 cm with 2.5 mm spatial resolution. The absorbed dose distributions of neutrons and γ rays were obtained separately by using a tissue-equivalent plastic- and a graphite-walled low-pressure proportional counter. The neutron absorbed dose in a tissue-equivalent plastic was measured to be 144.6 (Gy/C). It was confirmed that this neutron field has only 1.8% contamination of γ -ray dose and has good quality for neutron irradiation to biological samples.

05/00109 Thermo-hydrodynamic design and safety parameter studies of the TRIGA MARK II research reactor: Huda, M. Q. and Rahman, M. Annals of Nuclear Energy, 2004, 31, (10), 1101–1118

A heavily filtered fast neutron irradiation system (FNIS) was developed for a variety of applications, including the study of long-term health effects of fast neutrons by evaluating the biological mechanisms of damage in cultured cells and living animals such as rats or mice. This irradiation system includes an exposure cave made with a lead–bismuth alloy, a cave positioning system, a gamma and neutron monitoring system, a sample transfer system, and interchangeable filters. This system was installed in the irradiation cell of the Texas A&M University Nuclear Science Center Reactor (NSCR). For a realistic modeling of the NSCR, the irradiation cell, and the FNIS, this study used the Monte Carlo N-Particle (MCNP) code and a set of high-temperature ENDF/B-VI continuous neutron cross-section data. Sensitivity analysis was performed to find the characteristics of the FNIS as a function of the thickness of the lead–bismuth alloy. A paired ion chamber system was constructed with a tissue-equivalent plastic (A-150) and propane gas for total dose monitoring and with graphite and argon for gamma dose monitoring. This study, in addition, tested the Monte Carlo modeling of the FNIS system, as well as the performance of the system by comparing the calculated results with experimental measurements using activation foils and paired ion chambers.

Dose inhomogeneities for photons and neutrons near interfaces

Perturbations of charged particle equilibrium (CPE) at interfaces of materials of different atomic composition can lead to considerable differences in the energy deposition by photons and neutrons. Specific examples of these interface perturbations are encountered during irradiation of body cavities and soft tissue adjacent to bone or metallic implants and irradiation of cells in monolayer on the bottom of culture dishes. Another example is the build-up of CPE at air-tissue interfaces, referred to in radiotherapy as the skin sparing effect. For photon irradiation excess production of secondary electrons in high-Z materials, such as glass, bone or gold, will induce appreciably higher doses and decreased cell survival compared to the equilibrium situation. The energy dissipation of fast neutrons in biological materials occurs through recoil protons, heavy recoil nuclei and products of nuclear reactions. Owing to the large contribution from recoil protons to the neutron kerma, the hydrogen content of the biological material mainly determines the energy deposition. For neutron irradiation of cells in monolayer, CPE can be established or deliberately avoided by mounting tissue-equivalent plastic or carbon discs in front of the cells, respectively. This approach makes it possible to distinguish the biological effects of the low- and high-LET radiation components.

The role of nonelastic reactions in absorbed dose distributions from therapeutic proton beams in different medium

Many new techniques for delivering radiation therapy are being developed for the treatment of cancer. One of these, proton therapy, is becoming increasingly popular because of the precise way in which protons deliver dose to the tumor volume. In order to achieve this level of precision, extensive treatment planning needs to be carried out to determine the optimum beam energies, energy spread (which determines the width of the spread-out Bragg peak), and angles for each patient's treatment. Due to the level of precision required and advancements in computer technology, there is increasing interest in the use of Monte Carlo calculations for treatment planning in proton therapy. However, in order to achieve optimum simulation times, nonelastic nuclear interactions between protons and the target nucleus within the patient's internal structure are often not accounted for or are simulated using less accurate models such as analytical or ray tracing. These interactions produce high LET particles such as neutrons, alpha particles, and recoil protons, which affect the dose distribution and biological effectiveness of the beam. This situation has prompted an investigation of the importance of nonelastic products on depth dose distributions within various materials including water, A-150 tissue equivalent plastic, ICRP (International Commission on Radiological Protection) muscle, ICRP bone, and ICRP adipose. This investigation was conducted utilizing the GEANT4.5.2 Monte Carlo hadron transport toolkit.

Filtered fast neutron irradiation system using Texas A&M University Nuclear Science Center Reactor

A heavily filtered fast neutron irradiation system (FNIS) was developed for a variety of applications, including the study of long-term health effects of fast neutrons by evaluating the biological mechanisms of damage in cultured cells and living animals such as rats or mice. This irradiation system includes an exposure cave made with a lead–bismuth alloy, a cave positioning system, a gamma and neutron monitoring system, a sample transfer system, and interchangeable filters. This system was installed in the irradiation cell of the Texas A&M University Nuclear Science Center Reactor (NSCR). For a realistic modeling of the NSCR, the irradiation cell, and the FNIS, this study used the Monte Carlo N-Particle (MCNP) code and a set of high-temperature ENDF/B-VI continuous neutron cross-section data. Sensitivity analysis was performed to find the characteristics of the FNIS as a function of the thickness of the lead–bismuth alloy. A paired ion chamber system was constructed with a tissue-equivalent plastic (A-150) and propane gas for total dose monitoring and with graphite and argon for gamma dose monitoring. This study, in addition, tested the Monte Carlo modeling of the FNIS system, as well as the performance of the system by comparing the calculated results with experimental measurements using activation foils and paired ion chambers.

Measurement of the tissue to A-150 tissue equivalent plastic kerma ratio at two p(66)Be neutron therapy facilities

The ICRU tissue to A-150 tissue equivalent plastic kerma ratio is needed for neutron therapy dosimetry. The current ICRU protocol for neutron dosimetry recommends using a common conversion factor of 0.95 at all high-energy neutron therapy facilities. In an effort to determine facility specific ICRU tissue to A-150 plastic kerma ratios, an experimental approach was pursued. Four low pressure proportional counters that differed in wall materials (i.e. A-150, carbon, zirconium and zirconium-oxide) were used as dosimeters and integral kerma ratios were determined directly in the clinical beam. Measurements were performed at two p(66)Be facilities: iThemba LABS near Cape Town and Fermilab near Chicago. At the iThemba facility the clinical neutron beam is routinely filtered by a flattening and hardening filter combination. The influence of beam filtration on the kerma ratio was evaluated. Using two recent gas-to-wall dose conversion factor (rm,g value) evaluations a mean ICRU tissue to A-150 plastic kerma ratio of 0.93 ± 0.05 was determined for the clinical beam at iThemba LABS. The respective value for the Fermilab beam is 0.95 ± 0.05. The experimentally determined ICRU tissue to A-150 plastic kerma ratios for the two clinical beams are in agreement with theoretical evaluations. Beam filtration reduces the kerma ratio by 3 ± 2%.