A graphite calorimeter for absolute measurements of absorbed dose to water: application in medium-energy x-ray filtered beams. (3rd February 2016)

• Record Type: Journal Article
• Authors: Pinto, M; Pimpinella, M; Quini, M; D'Arienzo, M; Astefanoaei, I; Loreti, S; Guerra, A S
• Is Part Of: Physics in medicine & biology. Volume 61:Number 4(2016:Feb.)
• Page Start: 1738
• Page End: 1764
• Publication Date: 2016-02-03
• Subjects: graphite calorimetry -- calorimetry -- medium-energy x-rays -- reference dosimetry -- primary standard -- x-ray dosimetry; Biophysics -- Periodicals; Medical physics -- Periodicals; 610.153
• DOI: 10.1088/0031-9155/61/4/1738
• Languages: English
• ISSNs: 0031-9155

Abstract

Abstract: The Italian National Institute of Ionizing Radiation Metrology (ENEA-INMRI) has designed and built a graphite calorimeter that, in a water phantom, has allowed the determination of the absorbed dose to water in medium-energy x-rays with generating voltages from 180 to 250 kV. The new standard is a miniaturized three-bodies calorimeter, with a disc-shaped core of 21 mm diameter and 2 mm thickness weighing 1.134 g, sealed in a PMMA waterproof envelope with air-evacuated gaps. The measured absorbed dose to graphite is converted into absorbed dose to water by means of an energy-dependent conversion factor obtained from Monte Carlo simulations. Heat-transfer correction factors were determined by FEM calculations. At a source-to-detector distance of 100 cm, a depth in water of 2 g cm −2, and at a dose rate of about 0.15 Gy min −1, results of calorimetric measurements of absorbed dose to water, D w, were compared to experimental determinations, D wK, obtained via an ionization chamber calibrated in terms of air kerma, according to established dosimetry protocols. The combined standard uncertainty of D w and D wK were estimated as 1.9% and 1.7%, respectively. The two absorbed dose to water determinations were in agreement within 1%, well below the stated measurement uncertainties. Advancements are in progress to extend the measurement capability of the new in-water-phantom graphite calorimeter to other filtered medium-energy x-ray qualities and to reduce the D w uncertainty to around 1%. The new calorimeter represents the first implementation of in-water-phantom graphite calorimetry in the kilovoltage range and, allowing independent determinations of D w, it will contribute to establish a robust system of absorbed dose to water primary standards for medium-energy x-ray beams.