The pure alpha emitter 148Gd may have a significant radiological impact in terms of internal dose to exposed humans in case of accidental releases from a spallation source using a tungsten target, such as the one to be used in the European Spallation Source (ESS). In this work we aim to present an approach to indirectly estimate the whole-body burden of 148Gd and the associated committed effective dose in exposed humans, by means of high-resolution gamma spectrometry of the gamma-emitting radiogadolinium isotopes 146Gd and 153Gd that are accompanied by 148Gd generated from the operation of the tungsten target. Theoretical minimum detectable whole-body activity (MDA) and associated internal doses from 148Gd are calculated using a combination of existing biokinetic models and recent computer simulation studies on the generated isotope ratios of 146Gd/148Gd and 153Gd/148Gd in the ESS target. Of the two gamma-emitting gadolinium isotopes, 146Gd is initially the most sensitive indicator of the presence of 148Gd if whole-body counting is performed within a month after the release, using the twin photo peaks of 146Gd centered at 115.4 keV (MDA < 1 Bq for ingested 148Gd, and < 25 Bq for inhaled 148Gd). The corresponding minimum detectable committed effective doses will be less than 1 µSv for ingested 148Gd, but substantially higher for inhaled 148Gd (up to 0.3 mSv), depending on operation time of the target prior to the release. However, a few months after an atmospheric release, 153Gd becomes a much more sensitive indicator of body burdens of 148Gd, with a minimum detectable committed effective doses ranging from 18 to 77 µSv for chronic ingestion and between 0.65 to 2.7 mSv for acute inhalation in connection to the release. The main issue with this indirect method for 148Gd internal dose estimation, is whether the primary photon peaks from 146 and 153Gd can be detected undisturbed. Preliminary simulations show that nuclides such as 182Ta may potentially create perturbations that could impair this evaluation method, and which impact needs to be further studied in future safety assessments of accidental target releases.
In this study, we demonstrate the significant increase in the hard X-ray yield (more than 1011 photons/s in 4π solid angle in 6 - 40 keV range) that can be achieved in an ambient air environment when solid targets are irradiated by sequences of high average power (90 W) bursts of femtosecond laser pulses, generated in GHz burst laser amplifier operated at high repetition rate (100 kHz). The combination of the prepulse and ∼ 10 times greater driving pulse not only enhances X-ray generation efficiency (∼ 10−6) by more than two orders of magnitude compared to the single pulse regime but also protects a target allowing continuous operation for 3 hours with only 30% predictable and gradual drop of X-ray yield. In addition, we show that X-ray yield enhancement becomes around 6 times more pronounced at higher repetition rates (100 kHz compared to < 5 kHz). The simplicity and relative cost-effectiveness of the presented X-ray source makes it an attractive solution for future applications in ultrafast X-ray imaging and spectroscopy.
The ionizing radiation created by high intensity and high repetition rate lasers can cause significant radiological hazard. Earlier defined electron temperature scalings are used for dose characterization and prediction using Monte Carlo modeling. Dosimetric implications of different electron temperature scalings are investigated and the resulting equivalent doses are compared. It was found that scaling defined by Beg et al.(1997) predicts the highest electron temperatures for given intensities, and subsequently the highest doses. The atomic number of the target, x-ray generation efficiency and interaction volume are the other parameters necessary for the dose evaluation. The set of these operational parameters should be sufficient to characterize radiological characteristics of ultrashort laser pulse based x-ray generators and evaluate radiological hazards of the laser processing facilities.
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