Practical X-ray beam spectrometry with cadmium telluride detector in 10–300 kVp range at Czech Metrology Institute. Part I. Instrumentation

Abstract

The paper describes instrumentation used at Czech Metrology Institute for measurement of photon fluence spectra directly in X-ray beams. Detector with a cadmium telluride (CdTe) sensor was characterized by means of energy resolution and precise energy calibration. The hole trapping effect in CdTe causing full-energy peak deformations in measured spectra was adopted consisting of analytical models of incomplete charge collection and carrier trapping based on Hecht equation. The detector collimator provided by the manufacturer was modified with additional tungsten shielding that effectively absorbs the majority of photons scattered in the collimator body resulting in significant improvement of the consistency of measured and Monte Carlo simulated detector response. The purpose-made alignment platform increased the accuracy of orientation of the collimator axis improving the measurement in high photon fluence rate beams using collimation disks with very small apertures (down to 100 μm in diameter). The adopted instrumentation and methods for X-ray beam spectrometry enabled to fulfil the requirements of the new ISO 4037:2019-ed.2.0 standard for reference metrology laboratories. Software-focused part dealing with Monte Carlo simulations and procedures for photon fluence spectra unfolding is summarized in [1].

Practical X-ray beam spectrometry with cadmium telluride detector in 10–300 kVp range at Czech Metrology Institute. Part II. Unfolding

“…The paper does not aim to give a review of possible software solutions for X-ray beam spectrometry but rather provides the reader a description of practical solutions adopted at CMI. The instrumentation part for the X-ray beam spectrometry at CMI and the modelling of the charge carrier transport (CCT) in the CdTe spectrometer are detailed in [1].…”

“…where 𝑘 is the channel number and 𝐸 is the energy absorbed in the sensor. Determination of the calibration function is described in detail in [1].…”

“…Also, higher count rates result in the decreased energy resolution (higher full-width at half maximum of peaks in the detector spectrum). The number of pile-up counts was minimized by the following methods: 1) the pile-up rejection was enabled in the detector setting [1]; 2) measurement conditions were set such to keep the count rate below about 1.5 × 10 4 s −1 and simultaneously the dead time below 10% which results in very small ( 0.2%) pile-up correction; 3) an iterative algorithm based on [19] was implemented to accurately re-sample the remaining pile-up counts occurring behind the detector spectrum end-point into the correct detector spectrum.…”

“…The matrix was obtained by MC simulations (section 2.4) because it is not possible to determine it experimentally with monoenergetic photons. The validated MC model of the detector (section 2.4.4) with the optimized collimator [1] was used. The original EXVC Collimator Kit [24] bought from the detector manufacturer included a set of seven tungsten collimator disks.…”

“…The adopted methods and tools enable to fulfil requirements of the ISO 4037 standard dealing with the necessity of direct measurement of X-ray beam photon fluence spectra whose parameters do not comply with the ones listed in the standard. The instrumentation part for the X-ray beam spectrometry at CMI is described in a separate paper [1].…”