Cryogenic detectors bring unprecedented accuracy to X-ray and gamma-ray photon emission resolution
One mission of LIST’s Laboratoire National Henri Becquerel (LNHB) is to improve the accuracy of the X-ray and gamma-ray photon emission measurements required in radionuclide metrology. X-ray and gamma-ray spectrometry is normally used to measure these emissions by means of silicon detectors, whose accuracy mainly depends on the detector’s performance in terms of energy resolution and detection efficiency. Silicon detectors have now reached their theoretical limits in terms of energy resolution, however.
The LNHB has chosen to considerably improve detector performance by developing new types: cryogenic detectors incorporating a magnetic spectrometer, or magnetic bolometers. A magnetic bolometer consists ofa gold absorber, in which the photons interact, tightly linked to a magnetic heat sensor. When photons interact in the absorber, this causes the temperature to rise in inverse proportion to the sensor’s heat capacity. The magnetic sensor translates this temperature change into a magnetization change. This physical size can then be measured extremely accurately using dual-SQUID (Superconducting Quantum Interference Device) electronics, making it possible to construct the energy spectrum of the source’s photon emissions. The detector operates at a very low temperature, resulting in optimized energy spectrum resolution (T < 50 mK).
This principle was adopted in the development of the MG2 detector (see right) so that energy levels ranging from 1 to 100 keV can be detected. It consists of a gold absorber 1.1 mm in diameter and 0.34 mm thick that provides 70% detection efficiency at 100 keV. The magnetic sensor is magnetically coupled to the dual-SQUID electronics (see left) through a reading reel of thin film made in collaboration with the University of Heidelberg.
The detector has been characterized with a 133Ba source (see graphs below) at a temperature of 12.5 mK, the lowest temperature reached using the LNHB’s dilution refrigerator. The mid-height width measured by deconvoluting the energy spectrum is between 58 eV and 81 keV, offering 1,400 resolution power and performance nearly 7 times better than is possible when using a planar germanium detector (between 400 eV and 81 keV).
The next phases in the detector’s development will address the following points:
- Approaching the theoretical energy resolution limit for this type of detector (25 eV);
- Increasing the count rate (3 impacts per second);
- Characterizing detection efficiency so that quantitative analysis of measured energy spectra is possible.
The next phases in the detector’s development will address the following points:
- Approaching the theoretical energy resolution limit for this type of detector (25 eV);
- Increasing the count rate (3 impacts per second);
- Characterizing detection efficiency so that quantitative analysis of measured energy spectra is possible.
Energy spectrum of 133Ba with the MG2 magnetic bolometer |
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