D. Landis

Emission-Line Intensity Ratios in Fe XVII Observed with a Microcalorimeter on an Electron Beam Ion Trap

We report new observations of emission line intensity ratios of Fe XVII under controlled experimental conditions, using the National Institute of Standards and Technology electron beam ion trap (EBIT) with a microcalorimeter detector. We compare our observations with collisional-radiative models using atomic data computed in distorted wave and R-matrix approximations, which follow the transfer of the polarization of level populations through radiative cascades. Our results for the intensity ratio of the 2p6 1S0-2p53d 1P1 15.014 A line to the 2p6 1S0-2p53d 3D1 15.265 A line are 2.94 ± 0.18 and 2.50 ± 0.13 at beam energies of 900 and 1250 eV, respectively. These results are not consistent with collisional-radiative models and support conclusions from earlier EBIT work at the Lawrence Livermore National Laboratory that the degree of resonance scattering in the solar 15.014 A line has been overestimated in previous analyses. Further observations assess the intensity ratio of the three lines between the 2p6-2p53s configurations to the three lines between the 2p6-2p53d configurations. Both R-matrix and distorted wave approximations agree with each other and our experimental results much better than most solar and stellar observations, suggesting that other processes not present in our experiment must play a role in forming the Fe XVII spectrum in solar and astrophysical plasmas.

Laboratory Astrophysics Survey of Key X-Ray Diagnostic Lines Using A Microcalorimeter on an Electron Beam Ion Trap

Cosmic plasma conditions created in an electron beam ion trap (EBIT) make it possible to simulate the dependencies of key diagnostic X-ray lines on density, temperature, and excitation conditions that exist in astrophysical sources. We used a microcalorimeter for such laboratory astrophysics studies because it has a resolving power ≈1000, quantum efficiency approaching 100%, and a bandwidth that spans the X-ray energies from 0.2 keV to 10 keV. Our microcalorimeter, coupled with an X-ray optic to increase the effective solid angle, provides a significant new capability for laboratory astrophysics measurements. Broadband spectra obtained from the National Institute of Standards and Technology EBIT with an energy resolution approaching that of a Bragg crystal spectrometer are presented for nitrogen, oxygen, neon, argon, and krypton in various stages of ionization. We have compared the measured line intensities to theoretical predictions for an EBIT plasma.

Background recognition in Ge detectors by pulse shape analysis

Abstract A method of event identification that distinguishes single and multiple-site events by determining the number of interactions in a high purity germanium detector is reported. The selectivity of the method has been experimentally verified.

Long-term radiation damage to a spaceborne germanium spectrometer

Abstract The Transient Gamma-Ray Spectrometer aboard the Wind spacecraft in deep space has observed gamma-ray bursts and solar events for four years. The germanium detector in the instrument has gradually deteriorated from exposure to the ≈10 8 p / cm 2 / yr (>100 MeV ) cosmic-ray flux. Low-energy tailing and loss of efficiency, attributed to hole trapping and conversion of the germanium from n- to p-type as a result of crystal damage, were observed. Raising the detector bias voltage ameliorated both difficulties and restored the spectrometer to working operation. Together, these observations extend our understanding of the effects of radiation damage to include the previously unsuccessfully studied regime of long-term operation in space.

X-ray and gamma-ray astronomy with NTD germanium-based microcalorimeters

We report on the performance of our NTD-Ge microcalorimeters. To date, the spectral resolution for x-ray and gamma-ray lines from radioactive sources and laboratory plasmas is 4.8 eV in the entire 1–6 keV band and 52 eV at 60 keV. Technical details responsible for this performance are presented as well as an innovative electro-thermal approach for enhancing count-rate capability.

An improved operating mode for a Si(Li) X-ray spectrometer

A novel technique has been developed for restoring the charge to the feedback capacitor in charge-sensitive preamplifiers. the method, transient-reset, was developed for use in very-low-noise Si(Li) X-ray spectrometers. It has proved valuable as a diagnostic tool for evaluating the noise components of the input circuit in these systems. The technique is described and compared with the pulsed-light and transistor-reset methods. To complement the charge restoration technique, a new configuration of Si(Li) X-ray detector with a very thin aluminum entrance window has been developed. Measurements of the effective window thickness are compared with similar results obtained on gold, nickel, and ion-implanted boron contacts. >

GAMMASPHERE-elimination of ballistic deficit by using a quasi-trapezoidal pulse shaper

Gammasphere uses an spherical array of very large (7.2 cm dia.) germanium detectors and only high-multiplicity events are studied. To achieve a reasonable coincidence rate, the individual detector channels must handle high rates with minimum pile-up losses. Ten microseconds was chosen as the total processing time for a signal which means that the shaped signal peaks in about 4 us. The combination of short pulse shaping and the fluctuating long charge collection times (up to 400 ns) in the detectors exaggerates the energy resolution degradation due to ballistic deficit effects. We describe a method of producing a flat-topped pulse with a simple time-invariant network that satisfies GAMMASPHERE requirements and eliminates ballistic deficit effects. >

NTD germanium-based microcalorimeters for hard x-ray spectroscopy

Composite microcalorimeters using neutron transmutation-doped germanium (NTD) thermistors have been tested at hard x-ray energies. We present a broad band spectrum showing the energy resolution at 60 keV to be approximately 50 eV. The application of these microcalorimeters to the field of nuclear line astrophysics is discussed.

A new microcalorimeter concept for photon counting x-ray spectroscopy

Abstract We present an innovative approach for performing photon counting X-ray spectroscopy with cryogenic microcalorimeters. The detector concept takes advantage of the temperature dependence of the dielectric constant in ferroelectric materials. A dielectric calorimeter has many potential advantages over traditional resistive devices, particularly in the reduction of Johnson noise. This makes the energy resolution for photon counting spectroscopy limited only to the noise produced by the intrinsic temperature fluctuations of the device. The detector concept is presented and its predicted performance is compared with resistive calorimeters. Calculations have shown that practical instruments operating with an energy resolution less than 20 eV may be possible at 300 mK.

Automated temperature regulation system for adiabatic demagnetization refrigerators

Abstract A temperature control system was constructed for an adiabatic demagnetization refrigerator operating between 50 mK and 4 K. The control algorithm is discussed and the system performance is presented. A temperature stability of 2 μK RMS at 100 mK has been maintained for more than 8 h. To improve the thermodynamic efficiency, the magnetic field is used as the control parameter, rather than resistive heating.