Energy 60Co Research Articles

Improving the resolution and light yield in CsI(Tl) scintillators

In the search for Dark Matter, very low-energy sensitive detectors are needed that would be able to distinguish minute differences in closely residing energy peaks. To increase the ability of CsI(Tl) scintillator detectors to do so, a different wrapping material will be used on the innermost layer of the detector. This wrapping material is a 3MTM Enhanced Specular Reflector (ESR) material which is hypothesized to produce a higher light yield with better resolution than the PTFE wrapping of plumber’s tape due to its high reflectivity. Two different types of measurements were carried out to test this hypothesis. Each test was run on a different half of a CsI(Tl) rectangular crystal of volume 48 in3. First, a source of 60Co was used to test the difference between the wrappings. The test was run an hour for the PTFE and the ESR wrapping. We observe a 35% increase in the ADC count for each 60Co energy peak and 15% decrease in the value of the standard deviation around those peaks. Secondly, a source of 241Am was used to test the low-energy capabilities of the ESR material. Here also we see a significant increase in the ADC count for the two energy peaks of the 241Am source. Due to the high photon reflectivity capability of the ESR film, our detector is more capable of collecting photons created by low-energy interactions. As we know Dark Matter produces very low-energy recoils, while interacting with the detector target material, so the ability to contain a maximum number of photons within the detector volume will help us move one step closer to detecting the rare interaction. Because of this significant increase in light yield and general decrease in standard deviation, it is clear that the ESR material has a greater ability to gather higher resolution data than its predecessor. This could lead to a greater understanding of low-energy particles, which in turn could help us understand what Dark Matter really is.

The effect of fluorine replacement on the physical properties of oxyfluorotellurite glasses TeO2-Li2O-ZnO-ZnF2

Oxyfluorotellurite glasses in the composition 65TeO2–15Li2O-(20-x)ZnO-xZnF2 (x = 0, 5, 10 mol%) were prepared by melting quenching technique and their physical, thermal and structural properties were investigated. The results showed that the density and the oxygen packing density values decrease with increasing x. By the differential scanning calorimetry, the crystallization kinetic for each sample was evaluated and showed that activation energy decreases with increasing x, which can be related to the exchange of simple Te-O bonds for Te-F bonds. The variation on the quantity of these bonds were confirmed by FTIR and Raman spectroscopy. The mass attenuation coefficient values were found to reduce exponentially with increasing the energy. The radiation protection efficiency of glasses with 10 mm was 100, 50, and 30% for X-ray (40 KeV), 60Co (2.5MeV), and 137Cs energy lines (284 KeV & 66 KeV), respectively.

Enhanced time response of 1-in. LaBr3(Ce) crystals by leading edge and constant fraction techniques

We have characterized in depth the time response of three detectors equipped with cylindrical LaBr3(Ce) crystals with dimensions of 1-in. in height and 1-in. in diameter, and having nominal Ce doping concentration of 5%, 8% and 10%. Measurements were performed at 60Co and 22Na γ-ray energies against a fast BaF2 reference detector. The time resolution was optimized by the choice of the photomultiplier bias voltage and the fine tuning of the parameters of the constant fraction discriminator, namely the zero-crossing and the external delay. We report here on the optimal time resolution of the three crystals. It is observed that timing properties are influenced by the amount of Ce doping and the crystal homogeneity. For the crystal with 8% of Ce doping the use of the ORTEC 935 CFD at very shorts delays in addition to the Hamamatsu R9779 PMT has made it possible to improve the LaBr3(Ce) time resolution from the best literature value at 60Co photon energies to below 100ps.

Development and characterization of a three-dimensional film stack dosimeter

A three-dimensional (3D) film stack dosimeter (FSD) using Gafchromic® EBT2 film was characterized for use in external-beam radiotherapy. The FSD was found to have negligible energy dependence and orientation dependence less than 2% using Monte Carlo simulations. Percent-depth-dose measurements with the FSD aligned parallel and perpendicular to the beam axis agreed with Monte Carlo simulations within 2%. Measurements of a 60Co slit field with the FSD and thermoluminescent dosimeters agreed with a gamma passing rate of 97.6% using 1.5%/1.5 mm criteria. Edge artifacts were smaller than 2 mm, minimally affecting the usable measurement volume. The FSD is water equivalent, energy independent and orientation independent within measurement uncertainty at 60Co energies, providing a 3D dosimeter that can be analyzed with a desktop scanner.

Study of the time response of a LuAG(Pr) crystal for fast timing applications

The recently developed praseodymium-doped lutetium aluminum garnet, LuAG(Pr), holds a strong potential for fast timing applications. In this study we report on the time response of LuAG(Pr) at 22Na and 60Co photon energies. The measurements were performed using a small crystal cube of 1cm3 coupled to a Hamamatsu R5320 photomultiplier tube. The full width at half maximum (FWHM) time resolution is found to be 147±2ps at 60Co energies, and 238±2ps at 22Na.

Fast timing study of a CeBr3 crystal: Time resolution below 120 ps at 60Co energies

We report on the time response of a novel inorganic scintillator, CeBr3. The measurements were performed using a cylindrical crystal of 1-in. in height and 1-in. in diameter at 22Na and 60Co photon energies. The time response was measured against a fast reference BaF2 detector. Hamamatsu R9779 and Photonis XP20D0 fast photomultipliers (PMTs) were used. The PMT bias voltages and Constant Fraction Discriminator settings were optimized with respect to the timing resolution. The Full Width at Half Maximum (FWHM) time resolution for an individual CeBr3 crystal coupled to Hamamatsu PMT is found here to be as low as 119ps at 60Co energies, which is comparable to the resolution of 107ps reported for LaBr3(Ce). For 511keV photons the measured FWHM time resolution for CeBr3 coupled to the Hamamatsu PMT is 164ps.

New information on the characteristics of 1 in.×1 in. cerium bromide scintillation detectors

In view of highly demanded new and accurate data on prompt γ ‐ ray emission in nuclear fission a major part of investigations is directed towards the selection of suitable detector systems. Here we have studied a new type of crystal scintillation detectors made from cerium bromide (CeBr 3). For the first time a full characterization of such a detector is presented in terms of energy resolution, pulse-height linearity, intrinsic activity and intrinsic timing resolution. In particular the latter one is very important for prompt fission γ ‐ ray studies, because the presence of fast neutrons, emitted in fission too, requires the time-of-flight method for their discrimination. The energy resolution has been found to be comparable to that of cerium-doped LaCl 3:Ce detectors at an efficiency comparable to the one of a LaBr 3:Ce detector of the same size. The intrinsic activity of the CeBr 3 crystal was observed to be much lower compared to lanthanum halide crystals. The intrinsic timing resolution of a coaxial 1 in.×1 in. sized detector was measured relative to that of a previously characterized LaCl 3:Ce detector and found to be (326±7) ps at 60Co energies, which is in between those of a LaBr 3:Ce and a LaCl 3:Ce detector of same size.

Effect of liquid embolic agents on Gamma Knife surgery dosimetry for arteriovenous malformations

The effectiveness of Gamma Knife stereotactic surgery to obliterate brain arteriovenous malformations (AVMs) may be diminished by the preoperative adjunctive use of endovascular liquid embolic agents. The purpose of the present investigation was to determine if commercially available liquid embolic agents reduce the radiation dose to the target because of attenuation of the (60)Co beam. The apparent linear attenuation coefficients for 120- to 140-keV radiographs in embolized regions were retrieved from CT scans for several patients with AVMs who had undergone embolization procedures with liquid embolic agents to reduce nidal volumes. Based on these coefficients and a virtual model of Gamma Knife surgery (GKS) with basic ray tracing, the authors obtained the path lengths and densities of the embolized regions. The attenuation of (60)Co beams was then calculated for various sizes and positions of embolized AVM regions and for the number of beams used for treatment. Published experiments for several high-atomic-number materials were used to estimate the effective (60)Co beam attenuation coefficients for the N-butyl cyanoacrylate (NBCA, suspended in ethiodized oil) and ethylene vinyl alcohol copolymer (EVOH, with suspended micronized tantalum powder, Onyx) used in the AVM embolizations. Dose reductions during GKS were calculated for a theoretical model based on the CT-documented apparent linear attenuation coefficients and for the (60)Co energy attenuation coefficient. Dose measurements were obtained in a phantom study with EVOH for comparison with the estimates generated from the two attenuation coefficients. Based on CT (keV) apparent attenuation coefficients, the authors' theoretical model predicted that the cumulative effect of either of the embolic agents decreased the number of kilovoltage photons in an embolized nidus by -8% to -15% because of the increased atomic number and density of NBCA and Onyx. However, in using the effective attenuation coefficient for the (60)Co energies as is used in GKS, the authors' theoretical model yielded only a 0.2% dose reduction per beam and a < 0.01%-0.2% dose reduction in total. These theoretical results were validated by measurements in a head phantom containing Onyx. Dose reduction due to attenuation of the (60)Co beam by the AVM embolization material was negligible for both NBCA and EVOH because of the high-energy (60)Co beam.

SENSITIVITY COMPARISON OF TWO L-ALANINE DOPED BLENDS TO DIFFERENT PHOTON ENERGIES

Blends of L-alanine (85% weight proportion) with KI (10%) and with PbI2 (10%), these last two compounds acting as dopants, and with PVA (5%) acting as binder, were prepared in water at 80 degrees C. A blend of pure L-alanine (95%) with PVA (5%) was also prepared. The three blends were irradiated with photon beams of different energies (120 kV, Co, and 10 MV) to a unique dose of 30 Gy to compare their sensitivities for those three energies. EPR spectra of the three irradiated blends were recorded in a K-Band spectrometer (24 GHz) taking aliquots of about 4 mg for each blend. The energy sensitivity of a blend was defined as the peak-to-peak amplitude of its EPR spectrum central line. For the Co energy (1.25 MeV) the blends presented practically the same sensitivity, indicating that the presence of the dopants does not affect the sensitivity of L-alanine. For 10 MV x-rays, there was an increment (around 10%-20%) in sensitivity for the two L-alanine doped blends compared with the pure L-alanine blend (not doped). In the case of 120 kV x-rays, the blends ala+KI and ala+PbI showed increments of 10 and 20 times more sensitivity than the pure L-alanine blend. It is concluded that the dopants KI and PbI2 produce a great enhancement of the L-alanine sensitivity to low-energy photons. For the same dopant's content (10%) in the blend, PbI2 showed a better performance. Increasing the PbI2 proportion (30%) in the blend allows the detection of radiation dose as low as 10 mGy for 120 kV x-rays. These results encourage the authors to try to enhance the sensitivity of L-alanine even more by increasing the dopant's content in the blend and diminishing the lower limit detection. Application of these L-alanine doped blends in the dosimetry in diagnostic radiology could be possible.

Response of an implantable MOSFET dosimeter to HDR radiation

New in vivo dosimetry methods would be useful for clinical HDR brachytherapy. An implantable MOSFET Dose Verification System designed by Sicel Technologies, Inc. was examined for use with 192Ir HDR applications. This investigation demonstrated that varying the dose rate from 22 to 84 cGy/min did not change detector response. The detectors exhibited a higher sensitivity to 192Ir energies than 60Co energies. A nonlinear accumulated dose effect was characterized by three third-order polynomials fit to data from detectors placed at three different distances from the source. The detectors were found to have minimal rotational angular dependence. A strong longitudinal angular dependence was found when the detector's copper coil and electronics assembly were aligned between the MOSFETs and incident radiation. This orientation showed a 16% decrease in response relative to other orientations tested.

Accuracy of EGSnrc calculations at energies for the response of ion chambers configured with various wall materials and cavity dimensions

In this investigation, five experimental data sets are used to evaluate the ability of the EGSnrc Monte Carlo code to calculate the change in chamber response associated with changes in wall material and cavity dimension at 60Co energies. Calculations of the ratios of response per unit mass of air as a function of cavity volume for walls ranging from polystyrene to lead are generally within 1%-3% of experiments. A few exceptions, which are discussed, include 20%-30% discrepancies with experiments involving lead-walled chambers used by Attix et al. [J. Res. Natl. Bur. Stand. 60, 235-243 (1958)] and Cormack and Johns [Radiat. Res. 1, 133-157 (1954)], and 5% discrepancies for the graphite chamber of Attix et al. (relative to data for other wall materials). Simulations of the experiment by Whyte [Radiat. Res. 6, 371-379 (1957)], which varied cavity air pressure in a large cylindrical chamber, are generally within 0.5% (wall/electrode materials ranging from beryllium to copper). In all cases, the agreement between measurements and EGSnrc calculations is much better when the response as a function of cavity height or air pressure is considered for each wall material individually. High-precision measurements [Burns et al., Phys. Med. Biol. 52, 7125-7135 (2007)] of the response per unit mass as a function of cavity height for a graphite chamber are also accurately reproduced, and validate previous tests of the transport mechanics of EGSnrc. Based on the general agreement found in this work between corresponding experimental results and EGSnrc calculations it can be concluded that EGSnrc can reliably be used to calculate changes in response with changes in various wall materials and cavity dimensions at 60Co energies within a accuracy of a few percent or less.

Energy and angular anisotropy optimisation of a p-type diode for in vivo dosimetry in photon-beam radiotherapy

We present simulation work using the Monte Carlo code MCNPX that shows that there is a possibility of improving the silicon p-type diode as a radiation dosemeter, by altering the construction of the diode. Altering the diode die thickness can reduce the inherent angular anisotropy of the diode, with little effect on its energy response. Conversely, the contact material and geometry have a large impact on the energy response with little effect on the inherent angular anisotropy. By correct choice of contact material, the typical over-response -100 keV relative to the response at 60Co energy can be reduced from approximately 20 to 4. It is expected that further enhancements may be made with different geometries and materials.

SU-FF-T-232: Characterization and Use of MOSFET as In Vivo Dosimeters under 192Ir Irradiation for Real-Time Quality Assurance

Purpose: To characterize the new microMOSFET and MOSFET linear array from Thomson‐Nielson for use under 192Ir irradiation and evaluate their use as dosimeters to verify the delivered dose in high dose rate (HDR) brachytherapy.Method and Materials: MicroMOSFETs and MOSFET linear array (5 dose points/MOSFET) response to 192Ir photons were characterized in terms of the intrinsic error, the absolute calibrations, the anisotropy and the catheter factor. Such exercise is essential since the MOSFETs are not water equivalent at these low energies. Secondly, an HDR implant (13 catheters) and treatment plan (Plato, Nucletron) were performed on a realistic prostate phantom. During the CT exams, MOSFETs are inserted in a catheter near the urethra. At the same time as catheter reconstruction, a dose point was associated in Plato to the dosimeter position. Comparisons between the dose calculations of Plato and the measured values were performed. Results: Intrinsic errors were found to be 1.05 cGy for the MOSFET array and 0.71 cGy for the microMOSFETs. The calibration factors for 192Ir are higher than for 60Co or higher energies as expected for a Si‐based device. The calibration factors are also found to decrease as the cumulative dose to the dosimeters increases. The variation is evaluated at 5 % at 192Ir energy over the course of a treatment (around 10 Gy). The dosimeteranisotropy is within the manufacturer specification and was found to have no impact on in vivo dose measurements. The resulting overall difference between the planned dose and the experimental measurements agreed within 7 %. Conclusion: A proof of principle that new microMOSFETs and linear array could be used as in vivo dosimeter for dosimetric QA of HDR brachytherapy treatment was made. At this stage, the major uncertainty might well be the real target volume density as compared to water.

Feasibility of a Nal(Tl) Scintillation Bar Detector for Measurement of Radioactive Contamination

Feasibility of a Nal(Tl) scintillation bar detector for the measurement of radioactive contamination has been studied. The scintillator has dimensions of 6.35×6.35×71.12cm3. Its surface is polished to obtain the information of position. Two photomultiplier tubes (PMTs) are mounted on both sides of the bar scintillator. The position where an incident gamma-ray interacts with the scintillator is determined from the difference of light output between the two PMTs. The energy distribution deposited in the scintillator is obtained by summing up the output signals. For a collimated 60Co source, the position and energy resolutions at the center of bar detector were 3.1cm ( FWHM ) and 6.3% at the energy of 1.33MeV, respectively. For a collimated 137Cs source, the resolutions were 3.4cm and 9.8%, respectively. For the 60Co point source, the lead plates of lcm thick and 10cm length were used as a collimator with the intervals of 5cm in order to prevent the spatial extent of radiation. Then, the source position placed at a distance of 10cm was identified within the intervals. These results show that the information of energy and position of radioactive contamination can be obtained using the simple bar detector. K

Comparison of dosimetric standards of Canada and France for photons at 60Co and higherenergies

We report the results of a comparison of the dosimetric standards of Canadaand France for photon beams at 60Co and a few higher energies. Thepresent primary standard of absorbed dose to water for NRC, Canada is based onmeasurements made with a sealed water calorimeter. The corresponding standardof the LNHB, France is based on measurements made with a graphite calorimeterat 60Co energy and transferred to absorbed dose to water for 60Coand higher-energy photon beams using both ion chambers and Fricke dosemetersas transfer instruments. To make this comparison, we used three graphite-walledNE2571 Farmer chambers. The absorbed dose to water determined by the LNHB wasgreater than that determined by NRC by 0.20% at 60Co energy. Thisdifference is not significant given the uncertainties on the standards. Inorder to do the comparison for higher-energy photons, we interpolated the NRCdata set at the beam qualities used at the LNHB. When %dd(10)x is used as themethod of specifying beam quality, the determination of absorbed dose to waterby the LNHB is about 0.2% greater than that determined by NRC and consistent withthe results at 60Co. However, when using TPR20,10 as the beam qualityspecifier, the LNHB determination is greater than the NRC's determination by 0.8%and 1.2% at 12 and 20 MV respectively. This discrepancy, which systematicallyincreases with increasing energy, eventually exceeds the uncertainties in theratio of the standards, estimated to be 0.7%. This underscores the importanceof selecting the method of specifying beam quality, either %dd(10)x orTPR20,10, at least for the `soft' beams used by NRC in this comparison. In thecase of the air kerma standards, which were also compared at 60Co energy,the LNHB determination was greater than NRC's by 0.14%, which is notsignificant given the uncertainties on the standards.

The relative response of fricke, dichromate and alanine dosimeters to 60Co and high energy electron beam radiation

The relative response of Fricke, dichromate and alanine dosimeters to 60Co and high energy electron beam radiation has been determined directly using graphite calorimeters. The response of Fricke and alanine dosimeters is not significantly different in 60Co or high energy electron beams. The response of dichromate dosimeters is, however, approximately 3% lower in electron beams than in 60Co. Differences between the effective density of the dosimeter and the material surrounding it have been shown to be a significant source of error in electron beam dosimetry.

Timing properties of Philips XP2020UR photomultiplier

The timing properties of scintillation counters consisting of small NE111 plastic scintillators coupled to the XP2020UR photomultipliers have been studied in comparison to that based on the XP2020. The time jitter of both the photomultipliers, their photoelectron yield for the NE111 plastics and the time resolution with 60Co and 22Na sources were measured. A high time resolution was achieved with the XP2020UR equal to 90 ps for 60Co (at 1 MeV) and 140 ps for 22Na (at 290 keV) energy settings, about 30% and 20% better respectively than those observed with the XP2020. The large improvement of the time resolution is due to: 30% lower time jitter, about 25% higher photoelectron yield and for the 60Co energy settings also to a reduced influence of the space charge effect in the XP2020UR. The normalised time resolution to the photoelectron number, measured with 22Na source, is better by a factor of 1.14 which corresponds well to the improvement of the time jitter in the new XP2020UR photomultiplier.

Use of the FBX dosimeter for the quality assurance of 60Co and high energy teletherapy machines by mail

The concentrations of the solutes in the standard FBX dosimeter were changed in order to reduce the rate of thermal oxidation of ferrous ions in the solution. The modified solution contained 10-4 mol dm-3 ferrous ions. 10-3 mol dm-3 benzoic acid and 5*10-5 mol dm-3 xylenol orange in 0.04 mol dm-3 aqueous aerated sulphuric acid. The rates of thermal oxidation of ferrous ions do not differ significantly when the solution is stored either in glass or polypropylene containers. The error due to the post-irradiation delay in dose estimation depends on the dose and a dose of about 5 Gy is most suitable. When the solution stored up to about one month before irradiation and about one month after irradiation, the error in dose estimation is less than +or-2%. The blank and the irradiated solutions should be stored in similar containers.

Calculated buildup curves for photons with energies up to 60Co.

This report investigates the buildup region of the depth-dose curve for broad parallel beams of 60Co and lower energy photons incident on slabs of tissue as defined by the International Commission on Radiation Units and Measurements (ICRU) and other materials. The EGS Monte Carlo system was used to simulate the transport of photons and electrons in the materials. Calculated buildup regions of the depth-dose curves are presented for photons incident on ICRU tissue. For 60Co beams, the primary and scatter dose components are presented, both for the buildup region and at depth in the phantom. Effects of beam radius were investigated and found to have only a small effect in the buildup region of the central axis depth-dose curve. The calculated effects of beam radius are larger at depth and in good agreement with experiment. Effects of scatter and attenuation in a small miniphantom of water were investigated as a function of photon energy and were found to cancel within 0.8%. The mean distance of electron transport was calculated for 60Co beams incident on carbon, water, polystyrene, aluminum, and lead.

AtomicK-shell photoionization cross sections forSc46andCo60γrays

Photoelectron spectra resulting from the interaction of 46Sc (energy 889.2 and 1120.4 keV) and 60Co (energy 1173.2 and 1332.5 keV) gamma rays with tin, tantalum, gold, lead, and thorium targets have been measured by the coincidence method developed by Ranganath et al., for the direct estimation of K-shell cross sections in the case of radioisotopes emitting monoenergetic gamma rays. Since the two coincidence peaks (in both the cases) are not well resolved, K-shell photoionization cross sections are estimated by fitting a Gaussian to each peak. Good agreement is observed between the present cross sections and the theoretical cross sections.