M. K. Bacrania

Large-Area Microcalorimeter Detectors for Ultra-High-Resolution X-Ray and Gamma-Ray Spectroscopy

We discuss recent developments in using cryogenic microcalorimeter detectors for x- and gamma-ray spectroscopy. We are currently operating a detector array consisting of thirteen pixels with time-domain multiplexed readout. With a single pixel from this detector, we have measured 97.43-keV gamma rays from 153-Gd with 22-eV resolution (FWHM). We have also made the first multiplexed array measurements of plutonium x- and gamma-rays with 45-eV resolution. We are currently testing a 66-pixel next-generation detector chip. Preliminary measurements with the new detector indicate improved energy linearity and single-pixel energy resolution of 50-100 eV at 100 keV. We present preliminary calibration data from this chip, and a high-statistics multiplexed 21-pixel spectrum of the Pu x-ray region between 90 and 130 keV.

A Comprehensive Technique for Determining the Intrinsic Light Yield of Scintillators

Precise knowledge of the intrinsic light yield of scintillating materials is important for characterizing, optimizing, and simulating scintillating detectors for radiation detection applications. We present here a comprehensive method for determining the intrinsic light yield of scintillating materials. By maintaining complete control over all aspects of photodetector response measurement, and through the use of a novel method for direct measurement of the light collection efficiency of the scintillator, we have successfully demonstrated that our technique allows for new levels of precision and understanding in the measurement of the intrinsic light yield of scintillating materials.

Large-scale synthesis of CexLa1−xF3 nanocomposite scintillator materials

Transparent nanocomposites have been developed which consist of nanocrystals embedded in an organic matrix. The materials are comprised of up to 60% by volume of 7–13 nm crystals of the phosphor CexLa1−xF3, and are greater than 70% transparent in the visible region at a thickness of 1 cm. Consistencies of the nanocomposites range from a solid polymer to a wax to a liquid, depending on the workup conditions of the nanoparticle synthesis. These transparent nanophosphor composite materials have potential applications in radiation detection as scintillators, as well as in other areas such as imaging and lighting, and can be produced on large scales up to near-kilogram quantities at near ambient conditions, much lower in temperature than typical nanoparticle syntheses.

Superconducting Transition-Edge Sensor Microcalorimeters for Ultra-High Resolution Alpha-Particle Spectrometry

Alpha-particle spectrometry is a powerful analytical tool for nuclear forensics and environmental monitoring. Superconducting transition-edge sensor microcalorimeters have been shown to yield unsurpassed energy resolution for alpha spectrometry. With nearly an order of magnitude better energy resolution (1.06 keV FWHM at 5.3 MeV) than the current state-of-the-art silicon detectors (8-10 keV at 5.3 MeV), it is possible to measure samples containing multiple radioisotopes that would require expensive and time-consuming radiochemical separation prior to measurement with a silicon detector. This paper presents recent results from the Los Alamos four-channel microcalorimeter alpha spectrometer. We have prepared a source from weapons-grade plutonium and demonstrated the ability of microcalorimeter alpha spectrometry to simultaneously resolve alpha energies from 239Pu, 240Pu, 238Pu, and 241Am. The low-energy performance of the spectrometer system has been improved to allow measurement of energies as low as 5 keV, which gives a dynamic range of 1000. We have demonstrated this capability by simultaneously measuring the alpha particles and low-energy x-rays and internal conversion electrons emitted by an electroplated 240Pu source.

LaF3:Ce nanocomposite scintillator for gamma-ray detection

Nanophosphor LaF3:Ce has been synthesized and incorporated into a matrix to form a nanocomposite scintillator suitable for application to γ-ray detection. Owing to the small nanocrystallite size (sub-10 nm), optical emission from the γ / nanophosphor interaction is only weakly Rayleigh scattered (optical attenuation length exceeds 1 cm for 5-nm crystallites), thus yielding a transparent scintillator. The measured energy resolution is ca. 16% for 137Cs γ rays, which may be improved by utilizing brighter nanophosphors. Synthesis of the nanophosphor is achieved via a solution-precipitation method that is inexpensive, amenable to routine processing, and readily scalable to large volumes. These results demonstrate nanocomposite scintillator proof-of- principle and provide a framework for further research in this nascent field of scintillator research.

High Rate Pulse Processing Algorithms for Microcalorimeters

Microcalorimeters, cryogenic radiation detectors measuring the energy of photons by the increase of temperature in an absorber, can achieve energy resolutions more than an order of magnitude better than HPGe detectors. However, due to the thermal nature of the pulse generation, the active volume has to be small to maintain good resolution, and pulse decay times are in the order of milliseconds. Consequently, the detection efficiency is low and count rates are limited, especially for commonly used “optimum filter” algorithms that require isolated pulses to measure pulse heights. This is typically solved by building systems with multiple detector elements (arrays). Large arrays, however, require that as much pulse processing as possible be performed at the front end of the electronics to avoid transferring large amounts of waveform data to a host computer for processing. Pulse processing algorithms developed by XIA LLC for use in digital spectrometers with HPGe detectors, suitably modified for the slower time scale, meet this requirement. In the work reported here, we offline-processed microcalorimeter pulse streams with modified HPGe filter algorithms to provide an initial engineering evaluation of their performance as “practical” filters, capable of achieving sufficiently good energy resolution for most applications while being a) simple enough to be implemented in the readout electronics and b) capable of processing overlapping pulses and thus of achieving higher count rates. In the course of this work, a new filter was developed that uses only a fraction of a pulse while still achieving good energy resolution for very high count rates. The success of this work suggests that future microcalorimeter read-out systems can indeed be built with electronics on which these filters are implemented in multiplexed form, taking advantage of high speed digital signal processing elements to process many channels in parallel at a large reduction in processing cost per channel.

Application of GEANT4 to the Simulation of High Energy-Resolution Microcalorimeter Detectors

GEANT4 is a versatile Monte Carlo code for simulating the interactions of radiation with matter. GEANT4 has proven to be an effective toolkit for the simulation of a wide variety of detectors. We are interested in the application of GEANT4 to a new type of sensor technology being developed for X-ray and gamma-ray measurements. Microcalorimeter detectors based on transition-edge sensors coupled to bulk absorbers are an emerging technology for hard X-ray and soft gamma-ray measurements with unprecedented energy resolution. In this work, we assess the ability of the GEANT4 electromagnetic physics package to reproduce measured microcalorimeter data. We also use the simulations to explore the design space of absorber materials and cryostat design.

Cryogenic Microcalorimeter System for Ultra‐High Resolution Alpha‐Particle Spectrometry

Microcalorimeters have been shown to yield unsurpassed energy resolution for alpha spectrometry, up to 1.06 keV FWHM at 5.3 MeV. These detectors use a superconducting transition‐edge sensor (TES) to measure the temperature change in an absorber from energy deposited by an interacting alpha particle. Our system has four independent detectors mounted inside a liquid nitrogen/liquid helium cryostat. An adiabatic demagnetization refrigerator (ADR) cools the detector stage to its operating temperature of 80 mK. Temperature regulation with ∼15‐μK peak‐to‐peak variation is achieved by PID control of the ADR. The detectors are voltage‐biased, and the current signal is amplified by a commercial SQUID readout system and digitized for further analysis. This paper will discuss design and operation of our microcalorimeter alpha‐particle spectrometer, and will show recent results.

Issues in energy calibration, nonlinearity, and signal processing for gamma‐ray microcalorimeter detectors

Issues regarding the energy calibration of high dynamic range microcalorimeter detector arrays are presented with respect to new results from a minor actinide‐mixed oxide radioactive source. The need to move to larger arrays of such detectors necessitates the implementation of automated analysis procedures, which turn out to be nontrivial due to complex calibration shapes and pixel‐to‐pixel variability. Some possible avenues for improvement, including a more physics‐based calibration procedure, are suggested.

High rate pulse processing algorithms for microcalorimeters

Microcalorimeters, cryogenic radiation detectors measuring the energy of photons by the increase of temperature in an absorber, can achieve energy resolutions more than an order of magnitude better than HPGe detectors. However, due to the thermal nature of the pulse generation, the active volume has to be small to maintain good resolution, and pulse decay times are in the order of milliseconds. Consequently, the detection efficiency is low and count rates are limited, especially for commonly used “optimum filter” algorithms that require isolated pulses to measure pulse heights. This is typically solved by building systems with multiple detector elements (arrays). Large arrays, however, require that as much pulse processing as possible be performed at the front end of the electronics to avoid transferring large amounts of waveform data to a host computer for processing. Pulse processing algorithms developed by XIA LLC for use in digital spectrometers with HPGe detectors, suitably modified for the slower time scale, meet this requirement. In the work reported here, we offline-processed microcalorimeter pulse streams with modified HPGe filter algorithms to provide an initial engineering evaluation of their performance as “practical” filters, capable of achieving sufficiently good energy resolution for most applications while being a) simple enough to be implemented in the readout electronics and b) capable of processing overlapping pulses and thus of achieving higher count rates. In the course of this work, a new filter was developed that uses only a fraction of a pulse while still achieving good energy resolution for very high count rates. The success of this work suggests that future microcalorimeter read-out systems can indeed be built with electronics on which these filters are implemented in multiplexed form, taking advantage of high speed digital signal processing elements to process many channels in parallel at a large reduction in processing cost per channel.