Detector Holder Research Articles

TU‐A‐116‐01: Phantom Measurement of Surface and Organ Dose Using Optically Stimulated Luminescence Detectors in Diagnostic Radiology

Purpose: To measure the organ and surface dose in a phantom using aluminum oxide doped with carbon (Al2O3:C) and beryllium oxide (BeO) optically stimulated luminescence (OSL) detectors for the purpose of evaluating patient organ dose in diagnostic radiology procedures. Methods: The Al2O3 :C detectors used in this study were cut from Luxel™ tape into 4 mm diameter circles of thickness 0.3 mm. The BeO detectors used in this study were ∼4.7 mm × ∼4.7 mm × 0.5 mm chips. The liver dose was measured by placing detectors into the custom‐made detector holders and placing the holders in the 19 locations corresponding to the liver in an anthropomorphic phantom (RANDO). Each measurement was repeated three times. An AP abdomen protocol with AEC was used to determine the technical parameters for the exam. After irradiations, the BeO signal was read using a Riso TL/OSL‐DA‐15 automated reader and the Al2O3 :C signal was read using a custom‐built light emitting diode (LED)‐based reader system. The results were compared to those obtained from LiF:Mg,Ti thermoluminescence detectors (TLD). Results: The liver dose measured by BeO and Al2O3:C for a standard AP abdomen exam was found to be 208 and 222 μGy, respectively. The surface dose measured by BeO and Al2O3:C exam was both found to be 540 μGy. For comparison, the TLDs measured an organ and surface dose of 257 and 630 μGy, respectively. Conclusion: The proposed method of using BeO and Al2O3:C OSL detectors to measure organ and surface dose in an anthropomorphic phantom yielded comparable results to that of TLDs and shows improved reproducibility of the measurements when compared to TLDs. Given the flexibility and accuracy of OSL detectors for various readout techniques, OSL detectors have a great potential to allow for clinical organ dose measurement of diagnostic x‐ray exams.

SU-E-T-54: Dosimetric Verification of Monte Carlo Dose Calculations for a GammaMed HDR 192Ir Brachytherapy Source Using Various Detector Types, and Non-Reference Condition Correction Factors KNR

Purpose: To dosimetrically verify a Monte Carlo model of a GammaMed HDR brachytherapy source using various detector types, namely an unshielded diode (EDD‐5), a semiflex air‐filled ion chamber (PTW Freiburg), a TLD type 100H (LiF:Mg,Cu,P) and a radiochromic EBT3 film. The applicability of the sensitivity correction factor kNR, correcting the spectral dependence of the detector response, was investigated for the diode and TLD detectors. Methods: The EGSnrc Monte Carlo program was used to model the source and to compute lateral dose profiles and photon fluence spectra within a water phantom of dimensions (Radius = 20 cm, Height =40 cm). Special detector holders were constructed for the dosimetric verification of the source using four different detector types. Factor kNR was calculated using the ratio of the weighted responses Yt of a given detector t at radial distance r = 1 cm from the source and that under non‐reference conditions x (other off‐axis points); kNR = Yt(xref)/Yt(x). Gamma index 3%, 3mm was used to compare measured against Monte Carlo profiles. Results: Between radial positions from 1–10 cm from the source, the average deviation of measurements from Monte Carlo simulations using the gamma criterion were (values in braces); diode (0.46), semiflex (0.69), TLD 100H (1.6) and EBT3 (0.44). Considering measurement points closer to the source, the lowest average deviations were obtained with the semiflex chamber despite possible dose averaging effects. Precise dosimetry at the source surface is challenging, for example due to problems by water diffusion into the film. Conclusion: The modeled GammaMed HDR has been successfully validated in a medium‐sized cylindrical water phantom. This study demonstrates that the semiflex chamber is suitably applicable without kNR corrections, while the unshielded diode or TLD 100H are equivalently applicable for high‐precission measurements on application of kNR.

SU‐E‐J‐215: A Two‐Dimensional Deformable Head and Neck Phantom With In‐Vivo Dosimetry for Adaptive Radiotherapy Verification

Purpose: To design and construct a deformable head and neck (HN) phantom with in‐vivo dosimetry for adaptive radiotherapy (ART) verification purposes. Methods: The two‐dimensional deformable phantom recreates a single CT slice of a HN patient. It consists of two parallel acrylic plates with deformable material sandwiched in between. Gypsum plaster is used for bony anatomy and is rigidly attached to the plates. Soft tissue is mimicked with TX151 solidifying powder mixed with water. Areas of denser tissue are obtained by adding plaster powder into the mixture of TX151 and water. Parotids are represented with properties in between less dense and denser tissue. Tissue is distributed around the bony anatomy and constrained by an elastic rubber band around it. The pharynx is created with a hollow nylon tube. An inflatable silicone balloon filled with diluted barium sulfate acts as tumor. Thin plastic markers are glued on the phantom surfaces adjacent to the clear acrylic plates, providing quantifiable visual information of the deformation. Fixed detector holders are placed inside the cord and near the mandible while holders on rails that move with phantom deformation are placed close to the tumor and inside one parotid. Results: The deformable phantom has been tested for the validation of ART. We have studied the symmetric property and the response of an SRS diode that is used for dose measurements. The four point doses are obtained before and after deformation with the original plan and the re‐plan irradiated. The deformable image registration (DIR) algorithm accuracy is also verified by the deformation ground truth provided by surface markers. Conclusion: We have designed and constructed a deformable and durable HN phantom. This phantom can be used for DIR algorithm accuracy tests as well as ART verification.

Determination of the calibration factor for CR-39 based indoor radon detector

CR-39 based radon detectors are widely used in measuring indoor radon. In this regard, different groups have developed their own systems. However, before using any system for indoor radon measurements, it has, first, to be calibrated with a known source of radon. In the current study, CR-39 based NRPB type radon detector has been calibrated and presented. In this regard, about 200 holders for CR-39 were obtained from the Radiation Protection Division of the Health Protection Agency (former NRPB), UK and several thousand more similar detector holders, hereafter called NRPB type holders, were fabricated locally in Pakistan. Uranium ore samples of known grade were placed into the plastic containers of volume 5.4 × 103 cm3 and CR-39 detectors were placed in the NRPB type holders and were then installed into the containers at a distance of 25 cm from the surface of the known grade ore samples. The containers were hermetically sealed and the detectors were allowed to expose to radon for 3 weeks. After 16 h etching in 25 % NaOH at 80 °C, the measured track densities were related to the radon concentration. The calibration factor of 2.563 tracks cm−2 h−1/kBq m−3 was obtained.

SU‐D‐BRCD‐02: A Novel Cylindrical 3D Water Scanner for Beam Data Collection: I. Validation of the Automatic‐Setup

To determine the accuracy and reproducibility of the automatic- setup procedure of a 3D water scanner (3D SCANNER, Sun Nuclear Corp). The procedure automatically aligns the scanner's coordinate system to the Linac coordinate system. A unique off-center detector holder extends the scan range to 630 mm, eliminating the need for scanner shifts for large-field scans. The automatic-setup procedure provides means to automatically (1) level the tank; (2) zero the detector position to water surface; (3) align the scanner's coordinate system to the Linac system; and (4) correct motor hysteresis. After automatic-setup, (1) and (2) were evaluated visually by three physicists. Step (3) was tested by moving one of the jaws to the isocenter and scans were made along the jaw edge. Angular misalignment between the scanner and the Linac would Result in a noticeably tilted profile. Step (4) was tested by scanning with the chamber moving forward and then backward five times. The accuracy of the off-center detector holder was tested by comparing scans made using the regularand the off-center detector holders. All tests were repeated three times. All tests of the water surface and horizontal chamber movementwere acceptable from the nine independent evaluations. Scans along the jawedge resulted in < 1% in profile tilt, corresponding to < 0.05 degree in angular misalignment. Same direction scans resulted in field edge differences < 0.05 mm, and < 0.2 mm when compared to scans in thereverse direction. There was no difference in accuracy when different scan speeds were used. Profiles obtained using the regular and the off-centerdetector holders were identical. The automatic-setupprocedure resulted in accurate alignment of the 3D SCANNER to the Linac. These results were user-independent. It also provides the ability to scan large fields without the need to change tank setup. This work was partially supported by a research grant from Sun Nuclear Corp., Melbourne, FL.

Automated Systems.

Automated Systems.

A whole-body dosimetry system for personal monitoring based on hot-pressed thin layer TLD

We are introducing a new high-capacity thermoluminescent dosemeter (TLD) system to measure the whole body values of H(p)(10) and H(p)(0.07) from photons for use in individual monitoring services. Small and light-weight badges allow a convenient application in a wide variety of workplaces with photon radiation from 20 keV to at least 7 MeV. The main advantage of this system will be the large capacity of ∼ 100,000 dosemeters per month at costs equivalent to the current film monitoring. The hot-pressed thin-layer TL detector (LiF:Mg,Ti) is welded onto an aluminium substrate and provided with a data matrix code for automatic processing. The detector holder has been optimised, that no additional filter is necessary. The new designed TLD reader with readout times <10 s will allow a large throughput and a considerable degree of automation.

Anomalous results with the widely used NRPB/SSI-type passive radon dosemeter

In an industrial hall, with large variations of radon concentration within minutes, simultaneous measurements were done with two types of passive radon detectors and an active radon measuring device. The widely used passive radon detector of the National Radiological Protection Board (NRPB) [Health Protection Agency (former NRPB) (HPA)]/Statens strålskyddsinstitut (Swedish Radiation Protection Institute) (SSI) type produced anomalous results, seemingly uncorrelated to the radon concentration which was in the order of hundreds of becquerels per metre, usually underestimating but occasionally overestimating. We tried to reproduce similar exposure characteristics in our laboratory, but failed to reproduce the anomalous readings. We suspected, but could not prove, that the anomalous results were due to the combination of high radon concentration gradients, with pressure-driven air exchange between the inside of the detector holder and the outside atmosphere. Moreover, this theory was at least partly contradicted when we drilled holes in the detector holder. Although of interest, this effect is not likely to have substantially influenced any radon surveys, given the unusual nature of the exposure that caused the effect.

Quality Control Of High-Dose-Rate Brachytherapy: Treatment Delivery Analysis Using Statistical Process Control

Purpose: Statistical process control (SPC) is a quality control method used to ensure that a process is on-target and operating with minimal variation. The focus of this study is to determine if SPC is a viable technique for evaluating the proper operation of a high-dose-rate (HDR) brachytherapy treatment delivery system. Materials and Methods: Thomson-Nielsen MOSFET Dosimetry System (MOSFET model 502RD) was used as the dosimeter. MOSFET dosimeters (MD) were calibrated against a Farmer-type ionization chamber at a distance of 1.6 cm from 192Ir in water. A surrogate prostate patient was developed utilizing Vyse Ordance™ gelatin. Needle placement was performed according to our prostate HDR clinical protocol. A total of 10 MD were placed from prostate base to apex. Three detector trains of three MD each, spaced 2.5 cm apart, were inserted into two detector holders at the prostate periphery as well as in the urethra. The remaining MD was placed at the apex on the periphery. CT guidance was used to accurately position the first detector in each train at the base. The plan consisted of 12 needles with 129 dwell positions spaced 0.5 cm apart, delivering a prescribed peripheral dose of 200 cGy. Marks were placed on the needles at the template, and the distance from the template to the needle hub was recorded to allow accurate repositioning of each needle. Sixteen accurate treatment trials were delivered as planned. Subsequently a number of treatments were delivered with errors introduced. The errors were: wrong patient (i.e., a clinical plan for a different patient with 12 needles), wrong source calibration (3 & 7 day source decay inaccuracy), wrong sequence (i.e. two needles connected to incorrect location in turret), single needle displaced inferiorly 5mm, and entire implant displaced 2mm and 4mm inferiorly. Each MD constituted an evaluation subgroup size of one (bias correction factors d2 & d3 determined for n = 2 for consistency with established SPC methodology). The range was therefore determined by the difference between sequential treatment delivery measurements. Two process behavior charts (PBC), an average and range chart, were developed for each MD. Results: There were 4 false positives resulting from 160 measurements from 16 accurately delivered treatments. Generally speaking, data points resulting from accurately delivered treatments were well within PBC limits. The PBC indicate that measurements made at the base and apex were much more sensitive to treatment delivery errors. Each of the errors introduced was correctly identified by either the average or range PBC for measurements made at the base (2 out of 3 locations) and apex (all 4 locations). The PBC for measurements made at mid-gland (3 locations) were less sensitive to errors in treatment delivery. The only errors detected at these locations were “wrong patient” and “wrong source calibration.” Conclusions: SPC is a viable methodology to assess the quality of HDR treatment delivery. Further development is necessary to determine the most effective positioning of the MD for detecting potential treatment delivery errors.

SU‐FF‐I‐171: New High‐Resolution Detector Changer for a Clinical Fluoroscopic C‐Arm Unit

Purpose: To investigate a means to alternate image acquisition between a high‐resolution region‐of‐interest (ROI) detector and a standard flat‐panel detector (FPD) when used for neurovascular image‐guided interventions with a clinical fluoroscopic C‐arm so that the larger area detector could be used during initial parts of a procedure and the high‐resolution detector used during critical parts of the intervention when better images are needed. Method and Materials: A new detector changer was designed, fabricated, and mounted on a clinical fluoroscopic unit to allow a ROI detector such as the Micro‐Angiographic Fluoroscope (MAF) or Solid‐State X‐ray Image Intensifier (SSXII) to be introduced in front of the standard FPD to enable high‐resolution fluoroscopy, DSA, and conebeam computed tomography. The x‐ray collimation automatically adjusts the field‐of‐view (FOV) when the changer inserts or retracts the ROI detector so that the correct FOV, smaller for the ROI detector or larger for the FPD, respectively, is implemented. A touch sensor at the front of the ROI detector holder causes the entire imaging unit (FPD and ROI detector) to retract upon physical contact in order to avoid collision with a patient or other objects on or near the table. Results: The new detector changer has been installed onto a Toshiba Infinix clinical fluoroscopic C‐arm unit. The changer provides stable support for the ROI detector, allowing operator selection of high‐resolution ROI or standard FPD. The collision avoidance and automatic collimation features have worked reliably during laboratory evaluation and should provide a patient‐safe imaging capability. Conclusion: The new detector changer mounted on a commercial clinical fluoroscopic C‐arm unit provides a safe and rapid means to exchange imaging detectors to accommodate the resolution requirements of angiographic and interventional vascular imaging during the course of a procedure.(Partial support: NIH Grants R01‐EB008425, R01‐EB002873)

Recent validation tests of the Edelweiss II detector holders

A very good understanding of noise sources is highly important for the sensitivity of a dark matter search experiment. One of these noise contributions in the Edelweiss experiment could be the microphonics and temperature variations caused by vibrations of the cryostat. Recent studies complement an earlier work in describing the effect of the Edelweiss II detector holders on the amplitude of the vibrations seen by the detector. The results of these studies will be presented in this article.

The temperature stabilization system of CUORICINO: an array of macro bolometers

We present our circuit solution for the implementation of a very simple temperature stabilization system of an array of 62 large mass bolometric detectors, CUORICINO. The implemented instrument exploits one of the front-end channels to close the feedback loop between a thermistor and a heather, both of them located on the detector holder in the refrigerating system. A very compact layout was developed that avoids any possible presence of EMI interference, which would otherwise be introduced if a commercial instrument were adopted. The stabilized temperature is obtained exploiting the offset correcting circuit of the remotely programmable front-end channel to autogenerate the reference operating point for the feedback loop. Thanks to the adopted configuration, the instability of the cryogenic apparatus, which induced a drift corresponding to about 50 keV/day, was reduced to less than about 0.5 keV/day. To model the system behavior, we have reformulated the bolometer theory of operation using the concept of a feedback network. The obtained results will be shown.

Development of a size-selected gas cluster ion beam system for low-damage processing

A high-current gaseous cluster ion beam (GCIB) system with a magnetic size separation function has been developed. This system was equipped with a permanent magnet with a magnetic flux density of 1.2T. The system consists of a magnetic spectrometer with sliding detector and sample holder on a guiding rail perpendicular to the incoming cluster beam axis. Depending on the momentum of the cluster ions, positioning of the sample on the guiding rail allows bombardment of the sample with the desired cluster size. In this study, preliminary results demonstrating cluster mass spectra and mass resolution are presented. Finally, scanning tunneling microscopy is used to characterize the morphology of individual cluster impacts after irradiation.

Alternative procedure for LR 115 chemical etching and alpha tracks counting

An improved procedure for etching and analysis of alpha tracks induced in LR-115 detectors is proposed with the advantages of simplicity and its relatively low cost. A new type of detector holder was designed to etch and rinse efficiently up to 100 detectors. We develop a simple and reliable methodology with a semiautomatic track count using a Nikon digital camera coupled to a PC and employing software “SCION” freely available on the Internet. Track images are binarized prior the application of software “SCION” so that original track shapes are not distorted, space resolution is improved and track counting has low dependence on focus and illumination level. High discrimination for tracks is achieved when marks and rips perturb the detector surface. An image generator of nuclear tracks is included to study the effect of track overlapping effect on counting.

An irradiation chamber for experimental studies on SSNTD response to alpha particles

An irradiation apparatus was designed in our laboratory to study the detection efficiency dependence of SSNTDs (CR-39, LR115, Makrofol) on alpha particles of different energy and incident angle. The system was designed to irradiate up to nine detectors in a short time, allowing to obtain reproducible conditions on many detectors and therefore to reduce random variations of the results. The system is composed of a pressure-controlled stainless-steel chamber containing a 241 Am source and a circular rotating table with 10 detector holders, one of which is devoted to an ion-implanted silicon detector for irradiation energy monitoring. The table rotation is controlled electronically via a photodiode-based system, so that the position of the detectors under the source is known with a maximum uncertainty of 0.5 mm. The detector holders allow to change the detector (both passive and active) to source inclination angle continuously and with an uncertainty better than 1 ∘ . The source-to-detector distance is controlled electronically and can be varied from 5 to 30 cm with an uncertainty of about 0.1 mm. Some simulations, obtained using the transport code TRIM, to project the irradiation chamber and its main characteristics are reported and discussed.

Comparative measurements of soil gas radon concentration using thermoluminescent and track detectors

Track etch detectors are extensively used for Rn measurement in the environment because of their excellent ability to detect heavy charged particles. However, sometimes their application requires additional equipment and evaluation without automatic image analyzer can be time consuming. Moreover, due to track overlapping, track detectors are best suited for relatively low Rn concentrations. Thermoluminescent detectors (TLDs) are routinely used for detection of β and γ radiation, and in some cases they can be successfully used to measure other kinds of radiation, e.g. neutrons and α. In this study we compare the performance of TLDs and track detectors during field measurement of soil gas Rn concentration. A specially designed detector holder determines the irradiation geometry and contains either CR-39 track detectors or two sets of several (6–14) TLDs. The first set of TLDs (bare) detects both α and γ and the second set is hermetically sealed and detects γ radiation only. The response of two kinds of Bulgarian thermoluminescent materials, CaSO4:Dy and a CaSO4:Dy/graphite mixture was compared to the response of reference track detectors. Measurements were carried out at three distinct sites with different Rn concentration and two types of detector sealing were tested. The obtained results suggest that TLDs can successfully be used to measure medium or higher (>50kBqm−3) soil gas Rn concentrations. Compared to track detectors TLDs extend the upper limit of detection by as much as two orders of magnitude.

Fabrication of ultra-low radioactivity detector holders for Edelweiss-II

In its first stage, the EDELWEISS-II dark matter experiment will use 21×320g ionization-heat bolometers with NTD thermal sensors. To improve the present sensitivity of EDELWEISS to WIMP interactions by a further factor 100, ultra-low radioactivity detector holders exclusively made of copper and Teflon have been designed. The new design is using the relative expansion coefficients of copper, Teflon and germanium to hopefully ensure a dissipation-free detector holding.In order to validate this new holder, we need to compare the vibration behavior of the old holder and the new one. The amplitude of the bolometer vibrations is measured using the variation of capacitance between electrodes sputtered on the Ge crystal and reference electrodes fixed above the detector holders. We present noise measurements and compare them to the equivalent measurements using the detector holders of the Edelweiss-I experiment, which gave very satisfying results.

Experimental Study of Cluster Size Effect with Size-selected Cluster Ion Beam System

ABSTRACTA size-selected gas cluster ion beam (GCIB) system has been developed to study the size effects of energetic large cluster ion bombardments on a solid surface for the first time. This system equipped a permanent magnet with a magnetic flux density of 1.2 T. There is a sliding detector and sample holder on a guiding rail perpendicular to the incoming cluster beam axis. By locating a sample at a certain position, particular size of cluster ion can be irradiated continuously with affordable ion current density. When the total acceleration energy of Ar-GCIB was 5keV, both amorphous and oxide thickness on Si substrate increased with decreasing cluster size. This result showed good agreement with that obtained from molecular dynamics simulations.

A New TL Extremity Dosimetry System Optimised for Routine Personnel Monitoring

A new TL extremity dosimetry system optimised for routine personnel monitoring is presented. It was developed at GSF, based on several GSF patents, and consists of a flexible plastic strip type detector holder with permanent ID code containing the phosphor and a PC controlled standard TL reader. The holder is prepared for single use and designed as a finger ring of adjustable size. It is comfortable to wear even under surgical gloves, water and solvent proof, heat resistant up to autoclave temperatures and can be used with all common TL detectors, limited only by their size. Each phosphor is labelled with an automated readable permanent ID code using a newly developed technique for direct inscription of TL crystals, to allow an individual calibration before application. The software controlling the TL reader includes a computerised glow curve deconvolution (CGCD) algorithm for data analysis and a database system. The CGCD algorithm replaces all annealing procedures in additional furnaces and saves up to 65% processing time at the GSF personnel monitoring service, when using LiF:Mg,Ti (TLD-100) TL detectors. The modular system design gives the possibility of modifying all equipment parts, e.g. detector holder, phosphor or TL reader, for the combined use with components from TL systems commonly used at monitoring services. The extremity dosemeter is type tested by the Physikalisch Technische Bundesanstalt (PTB) and is in routine use at the official personnel monitoring service of GSF.

Triton and alpha-particle contribution from LiF converter for neutron dosimeter

A personnel neutron dosimeter prototype based on chemical and electrochemical etched CR-39 detector, combined with LiF converter, has been calibrated using an ICRP-like phantom, under a heavy-water moderated Californium source neutron spectra; A conversion factor of 1.052±126 spots cm −2 mSv −1 was obtained. The sealing properties of the detector holder showed a ten-fold reduction in radon background when it was tested in a high radon atmosphere. A convenient mechanical shock resistance was achieved in LiF converters by sintering to 11 tons pressure LiF powder at 650°C, during one hour.