A. Loshak

The "nuclear car wash": a scanner to detect illicit special nuclear material in cargo containers

There is an urgent need to improve the reliability of screening cargo containers for illicit nuclear material that may be hidden there for terrorist purposes. A screening system is described for the detection of fissionable material hidden in maritime cargo containers. The system makes use of a low-intensity neutron beam for producing fission and the detection of the abundant high-energy /spl gamma/ rays emitted in the /spl beta/-decay of short-lived fission products and /spl beta/-delayed neutrons. The abundance of the delayed /spl gamma/ rays is almost an order of magnitude larger than that of the delayed neutrons normally used to detect fission, and they are emitted on about the same time scale as the delayed neutrons, i.e., /spl sim/1 min. The energy and temporal distributions of the delayed /spl gamma/ rays provide a unique signature of fission. Because of their high energy, these delayed /spl gamma/ rays penetrate low-Z cargoes much more readily than the delayed neutrons. Coupled with their higher abundance, the signal from the delayed /spl gamma/ rays escaping from the container is predicted to be as much as six decades more intense than the delayed neutron signal, depending upon the type and thickness of the intervening cargo. The /spl gamma/ rays are detected in a large array of scintillators located along the sides of the container as it is moved through them. Measurements have confirmed the signal strength in somewhat idealized experiments and have also identified one interference when 14.5-MeV neutrons from the D, T reaction are used for the interrogation. The interference can be removed easily by the appropriate choice of the neutron source.

High-resolution hard x-ray and gamma-ray spectrometers based on superconducting absorbers coupled to superconducting transition edge sensors

We are developing detectors based on bulk superconducting absorbers coupled to superconducting transition edge sensors (TES) for high-resolution spectroscopy of hard X-rays and soft gamma-rays. We have achieved an energy resolution of 70 eV FWHM at 60 keV using a 1 X 1 X 0.25 mm3 Sn absorber coupled to a Mo/Cu multilayer TES with a transition temperature of 100 mK. The response of this detector is compared with a simple model using only material properties data and characteristics derived from IV-measurements. We have also manufactured detectors using superconducting absorbers with a higher stopping power, such as Pb and Ta. We present our first measurements of these detectors, including the thermalization characteristics of the bulk superconducting absorbers. The differences in performance between the detectors are discussed and an outline of the future direction of our detector development efforts is given.

High-Resolution Gamma-Ray Spectrometers using Bulk Absorbers Coupled to Mo/Cu Multilayer Superconducting Transition-Edge Sensors

In x-ray and gamma-ray spectroscopy, it is desirable to have detectors with high energy resolution and high absorption efficiency. At LLNL, we have developed superconducting tunnel junction-based single photon x-ray detectors with thin film absorbers that have achieved these goals for photon energies up to 1 keV. However, for energies above 1 keV, the absorption efficiency of these thin-film detectors decreases drastically. We are developing the use of high-purity superconducting bulk materials as microcalorimeter absorbers for high-energy x-rays and gamma rays. The increase in absorber temperature due to incident photons is sensed by a superconducting transition- edge sensor (TES) composed of a Mo/Cu multilayer thin film. Films of Mo and Cu are mutually insoluble and therefore very stable and can be annealed. The multilayer structure allows scaling in thickness to optimize heat capacity and normal state resistance. We measured an energy resolution of 70 eV for 60 keV incident gamma-rays with a 1 X 1 X 0.25 mm3 Sn absorber. We present x-ray and gamma-ray results from this detector design with an Sn absorber. We also propose the use of an active negative feedback voltage bias to improve the performance of our detector and show preliminary results.

Gamma-ray spectrometers using superconducting transition edge sensors with external active feedback bias

We are developing X-ray and gamma-ray spectrometers with high absorption efficiency and high energy-resolution for X-ray and gamma-ray spectroscopy. They are microcalorimeters consisting of a bulk Sn absorber coupled to a Mo/Cu multilayer superconducting transition edge sensor (TES). We have operated these microcalorimeters with an external active feedback bias to linearize the detector response, improve the count rate performance, and extend the detection energy range. We measured an energy resolution of 120 eV FWHM for 60 keV incident gamma-rays with no degradation of resolution from active bias. We present X-ray and gamma-ray results and operation of this detector design in both bias modes.

DC and AC biasing of a transition edge sensor microcalorimeter

We are developing AC-biased transition edge sensor (TES) microcalorimeters for use in large arrays with frequency-domain multiplexing. Using DC bias, we have achieved a resolution of 17 eV FWHM at 2.6 keV with a decay time of 90 μs and an effective detector diameter of 300 μm. We have successfully measured thermal pulses with a TES microcalorimeter operated with an AC bias. We present here preliminary results from a single pixel detector operated under DC and AC bias conditions.

A multichannel cryogenic detector system for synchrotron-based x-ray spectroscopy

Fluorescence-detected x-ray absorption spectroscopy probes the fine structure of electronic energy levels with sub-eV resolution by scanning a monochromatic synchrotron beam through the corresponding absorption edge and measuring the intensity of the resulting x-ray fluorescence. For dilute samples, grating spectrometers lack the detection efficiency and conventional Si(Li) or Ge detectors often lack the energy resolution to separate the weak fluorescence signal from strong nearby emission lines. We have built a high-resolution, high-efficiency cryogenic detector system for synchrotron-based soft x-ray spectroscopy. The sensor is a 3×3 array of 200 μm×200 μm superconducting Nb-Al-AlOx-Al-Nb tunnel junctions with an energy resolution of ≈15 eV below 1 keV and a total count rate capability of ≈100,000 counts/second. This sensor array is cooled to below 0.4 K by a two-stage adiabatic demagnetization refrigerator while held at the end of a 40-cm-long cold finger that can be inserted into a UHV sample chamber ...