Calorimetry in Computed Tomography Beams.

A portable calorimeter for direct realization of absorbed dose in medical computed tomography (CT) procedures was constructed and tested in a positron emission tomography (PET) CT scanner. The calorimeter consists of two small thermistors embedded in a polystyrene (PS) cylindrical "core" (1.5 cm diameter) that can be inserted into a cylindrical high-density polyethylene (HDPE) phantom (30 cm diameter). The cylindrical design of core and phantom allows coaxial alignment of the system with the scanner rotation axis, which is necessary to minimize variations in dose that would otherwise occur as the X-ray source is rotated during scanning operations. The core can be replaced by a cylindrical ionization chamber for comparing dose measurement results. Measurements using the core and a calibrated thimble ionization chamber were carried out in a beam of 6 MV X-rays from a clinical accelerator and in 120 kV X-rays from a CT scanner. Doses obtained from the calorimeter and chamber in the 6 MV beam exhibited good agreement over a range of dose rates from 0.8 Gy/min to 4 Gy/min, with negligible excess heat. For the CT beam, as anticipated for these X-ray energies, the calorimeter response was complicated by excess heat from device components. Analyses done in the frequency domain and time domain indicated that excess heat increased calorimetric temperature rise by a factor of about 15. The calorimeter's response was dominated by dose to the thermistor, which contains high-atomic-number elements. Therefore, for future construction of calorimeters for CT beams, lower-atomic-number temperature sensors will be needed. These results serve as a guide for future alternative design of calorimeters toward a calorimetry absorbed dose standard for diagnostic CT.

Assessing Radiation Hardness of Silicon Photonic Sensors.

In recent years, silicon photonic platforms have undergone rapid maturation enabling not only optical communication but complex scientific experiments ranging from sensors applications to fundamental physics investigations. There is considerable interest in deploying photonics-based communication and science instruments in harsh environments such as outer space, where radiation damage is a significant concern. In this study, we have examined the impact of cobalt-60 γ-ray radiation up to 1 megagray (MGy) absorbed dose on silicon photonic devices. We do not find any systematic impact of radiation on passivated devices, indicating the durability of passivated silicon devices under harsh conditions.

Large area alpha sources with a lip: Integral counting and spectral distortions.

The detection efficiency for large area alpha sources with adjustable heights of a raised lip around the edge were measured by 2π gas-filled proportional counter. The variations in low-energy spectral shape were modeled using a Geant4 radiation and charge transport Monte Carlo simulation, to enable extrapolation of the spectrum to zero energy. COMSOL Multiphysics finite-element analysis was used to explore changes in the spectrum gain in the presence of a lip. It qualitatively reproduced an increase in peak height due to an increasing height of the source lip. A spectrum analysis procedure was developed to perform integral counting on sources with a lip. The experimental results were used to validate the model, which was then used to predict the changes in 2π counting efficiency for other source-lip geometries.

Testing the Image Quality of Cabinet X-ray Systems for Security Screening: The Revised ASTM F792 Standard.

ASTM F792, Standard Practice for Evaluating the Imaging Performance of Security X-ray Systems, provides test objects and methods for measuring the imaging performance of cabinet X-ray systems used at security checkpoints. The standard is widely used, with many thousands of ASTM F792 test objects utilized throughout the world. The last major revision of the standard was more than 15 years ago (2001), and since that time, several deficiencies have been noted when using the standard for testing modern systems employing multiple-view and multiple-energy configurations. Accordingly, the present work describes a new revision of the ASTM F792 standard realized as a trifurcation into three parts, each with its own separate test object and associated test method. The three parts of the standard are intended for routine testing, human-perception testing, and objective technical testing, and represent a major update to this venerable standard.

Measurements and standards for bulk-explosives detection.

Recent years have seen a dramatic expansion in the application of radiation and isotopes to security screening. This has been driven primarily by increased incidents involving improvised explosive devices as well as their ease of assembly and leveraged disruption of transportation and commerce. With global expenditures for security-screening systems in the hundreds of billions of dollars, there is a pressing need to develop, apply, and harmonize standards for x-ray and gamma-ray screening systems used to detect explosives and other contraband. The National Institute of Standards and Technology has been facilitating the development of standard measurement tools that can be used to gauge the technical performance (imaging quality) and radiation safety of systems used to screen luggage, persons, vehicles, cargo, and left-behind objects. After a review of this new suite of national standard test methods, test objects, and radiation-measurement protocols, we highlight some of the technical trends that are enhancing the revision of baseline standards. Finally we advocate a more intentional use of technical-performance standards by security stakeholders and outline the advantages this would accrue.

High-resolution ultrasonic thermometer for radiation dosimetry.

This paper describes recent developments in the area of high-precision ultrasonic thermometry with the potential to provide on-site direct determination of radiation doses administered for cancer treatment. Conventional calorimeters used for this purpose measure radiation-induced heating in a water phantom at one point in space by means of immersed thermistors and are subject to various thermal disturbances due to Ohmic heating and interactions of the radiation with the sensor probes. By contrast, the method described here is based on a high-resolution ultrasonic system that determines the change of the speed of sound due to small temperature changes in an acoustic propagation path in the radiation-heated water, thereby avoiding such undesired thermal effects. The thermometer is able to measure tens of microkelvin changes in the water temperature averaged over the acoustic path of about 60 cm at room temperature, with root-mean-squared noise of about 5 microK. Both incandescent and ionizing radiation heating data are presented for analog and digital implementations of a laboratory prototype. This application of the ultrasonic technique opens up possibilities for a new approach to performing therapy-level radiation dosimetry for medical clinics and standards laboratories.