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FINAL REPORT
NASA GRANT NAGW-5146

NEW TECHNOLOGY CZT DETECTORS FOR HIGH-ENERGY FLARE SPECTROSCOPY:

THE ROOM TEMPERATURE SEMICONDUCTOR SPECTROMETER FOR JAWSAT

Prepared by:

W. Thomas Vestrand, Principal Investigator

Physics Department and Space Science Center
University of New Hampshire, Durham, NH 03824

Prepared for:

William Wagner
NASA HEADQUARTERS
WASHINGTON, DC
 RTSS Final Report Phase A-1

Final Report Phase A: The Room Temperature Semiconductor Spectrometer (RTeSS)

Introduction.

Today it is generally accepted that high-purity germanium (HPGe) detectors are the best

choice for construction of high resolution solar flare spectrometers operating at energies from 30

keV up to 10 MeV. HPGe instruments typically yield spectra with very high resoution (_1 keV)

at MeV energies. However, a major disadvantageof HPGe detectors is that they require cooling

to _80 K which, for spacecraft operation in near orbit, demands either stored cryogen or a me-

chanical cryogenic cooler. Both cooling techniques have inherent drawbacks. Stored cryogens
are short-lived and often severely limit the mission lifetime. Mechanical coolers add substantial

volume, mass, and power requirements and can generate microphonics problems. Solar flare

spectrometers that employ HPGe detectors are therefore large and expensive instruments.

The goal of our Room Temperature Semiconductor Spectrometer (RTeSS) project is to

develop a small high-energy solar flare spectrometer employing semiconductor detectors that do

not require significant cooling when used as high-energy solar flare spectrometers. Specifically,

the goal is to test Cadmium Zinc Telluride (CZT) detectors with coplanar grid electrodes

as x-ray and gamma-ray spectrometers and to design an experiment that can be flown as a

"piggy-back" payload on a satellite mission during the next solar maximum.

CZT Detectors.

CZT detectors were selected for RTeSS because they show great promise for the construction

of the next generation of spacecraft borne hard x-ray and gamma-ray instruments. The high

average Z and large bandgap energy of CZT makes it an attractive material for detector
construction. However, until recently, the thickness of CZT detectors for spectroscopy was

limited to a few millimeters. The limit was imposed by the low mobility of holes in CZT

material and the resultant incomplete charge collection for high-energy photons interacting

more than about a millimeter from the detector cathode. The interaction depth dependence

for the charge induction efficiency produces a tail on the low energy side of the photopeak that

degrades the detector energy resolution. Recently, P. M. Luke of Lawrence Berkeley Laboratory
demonstrated a new method of unipolar charge sensing that reduces the tailing problem and

allows one to achieve very good energy resolution at high energies from much thicker detectors.

The technique employs two sets of interdigitated coplanar electron grids with slightly different

bias whose difference in induced charge signal is only significant when electrons are moving in

the region near the collecting electrodes. As a result, electrons dominate the induced charge
 RTSS Final Report Phase A-2

difference signal and the effects of poor hole transport are nearly eliminated. By tuning the

relative gains of the two grid signals one can generate a relatively constant charge induction

efficiency for interactions that occur throughout the detector volume.

We ordered a large volume (1 cm 3) CZT with co-planar grid electrodes from eV products

in Saxonburg, Pennsylvania, for evaluation as a high energy spectrometer. A test jig was

constructed and laboratory NIM electronics were connected for testing. A large number of
measurements with radioactive sources were undertaken to characterize and calibrate the de-

tector. We also explored the influence of grid bias potentials with the goal of optimizing the

charge collection and spectral resolution. By tuning the grid potentials we were able to acheive

a spectral resolution at room temeperature of 4% for 662 keY from Cs 137. That spectral reso-
lution, which is a factor of two better than the best scintillation spectrometers, is comparable

to what we need to resolve the cz - c_ line complex in flare spectra.

A key issue for our instrument design is the determination of the the best mounting technique

for assuring survival of the CZT crystals under launch loads. We undertook a study of possible

techniques for mounting the sensors. As per our specifications, eV products fabricated four test

CZT detectors for us that are mounted using hard mountings (conductive silver base-mount;

epoxy side-mount) and soft mounting (conductive silicone base-mount; silicone base-mount).

A special test jig which will allow shake testing was constructed and a test plan was written.

The shake testing for the RTeSS sensors will be done in conjunction with CATSAT sensor

testing during the next phase of the project.

Instrument Electronics.

A first order design of the analog flight electronics was completed. The flight electronics can

be subdivided into three functional subsystems: I) preamplifiers, 2) shaping amplifiers, and 3)

high voltage power supplies.

Induced charge signals on the two interdigitated electrode grids of each CZT detector are

measured on At-coupled Amptex A250 charge-sensitive preamplifiers. The charge signals

are differenced using a simple circuit employing two op-amps and a potentiometerwhich allows

adjustment of the relative gain for the grid signals. The relative gain adjustment allows tuningof

the net charge induction efficiency to achieve uniformity for interactions throughout the detector
volume. The differenced signals are subsequently amplied and shaped by multipole Gaussian

shaping amplifers (A275) with shaping time constant selected for the optimum signal to noise

ratio. The Gaussian shape provides a quick return to baseline for instrument robustness at high

counting rates. Designs were developed and tested for the peak track and hold circuit which

permits digitization by the analog-to-digital converter. In the current design, signal pulses are
 RTSS Final Report Phase A-3

extracted from the output of an intermediate pole amplifier in Gaussian shaping amplifiers to

test against the lower level threshold. Only pulses that exceed the commandable threshold

level are processed by the pulse height sampling circuit. A list of flight qualified parts for this

circuit was compiled.

Thick CZT detectors require a high voltage bias in the range from -i00 to -1500 Volts.

A new design for a high voltage power supply that is capable of providing the voltage was

developed. This student designed power-supply is a derivative of an earlier design developed in

the Small Satellite Laboratory at UNH. The supply employs pulse width modulation to control

an IC oscillator which drives a step-up transformer whose output feeds a three stage multiplier.

Active regulation is employed through feedback to the modulation controller and filtering is
used to ensure that the ripple constraints are met.

Microcomputer.

A feasibility study for the use of a COTS (commercial off-the-shelf) PC/104 card with an

embedded 80386 system was completed. Part of the study, performed by two graduate engi-

neering students, is documented on-line at http://www.ece.unh/links/as/project.htm. It was

concluded that a ruggedized PC/104 standard card could be modified for use in the RTSS.

Our current design employs a Real Time Devices CMi386SX33 cpu module with all electrolytic

capacitors replaced with tantalum versions. The ICs will be shielded with tantalum foils that

are epoxied to the IC packaging. In addition to the 386SX processor this system provides on-
board ROM and RAM, serial and parallel I/O, a watchdog timer, real-time clock, solid-state

disk support, and a data acquisition card. Power consumption for the system is approximately
5 watts.

Data Storage.

A trade-off study of mass storage devices was completed. Solid state devices such as IDE Flash
Drives and PCMCIA Flash Disks were determined to be better suited to our application than

a standard hard disk in a pressurized housing. The study also concluded that the FTL (Flash

Translation Layer) PCMCIA cards, because of their robustness, minimal power requirements,

low cost, and capacity of 2-85 MB, are well studied for our application. Thermal issues are

a potential problem but it was found that minor modifications that add a high-conductivity
from the device to its case can solve the problem. An industrial grade card, the Raymond

Engineering Sentinel card, which is upgraded for thermal conductivity and hermeticity, and has

an operating temperature range of-40 to +85C was identified. The Radiation tolerance of
 RTSS Final Report Phase A-4

these devices is still an open issue. The experience with PCMCIA cards in the shuttle program
indicates that the use of something like tantalum foils may be required for shielding.

Radiation Environment Study.

The radiation environment for the proposed 650 km sun-synchronous orbit of JAWSAT was

studied using the software package "Space Radiation" produced by Severn Communications

Corporation. With 2ram of AI shielding provided by the spacecraft skin, we find a total dose
rate within the spacecraft of about 4K rad/year at solar maximum. The addition of 2ram of AI

shielding at the electronics box level was found to reduce the dose rate to about lk rad/year.

Telemetry Format Design.

The telemetry format was designed and optimized for using S-Band capabilities. In the new

format each gamma-ray that generates a pulse height above the LLD will be recorded as a

single event. The stored event data will be organized into 16 second long major frames which

carry 32 bit synchronization and 32 bit absolute UT time markers. Each event in the frame
requires two 8 bit words composed of 5 bits for 0.5 second time resolution, 2 bits for detector

identification, 8 bits for the event amplitude and 1 bit for parity.

Mechanical Housing.

Design of a hermetic electronic housing was completed. The housing will employ a seated

gasket and a pinch tube. For flight the housing will be filled with 100 mb of dry nitrogen and
sealed. A 10% admixture of helium will be included in the fill gas to allow use of a sniffer

to test the hermeticity of the container. A re-design of the sensor housing was begun due to
concerns about CZT crystal cleaving during launch. A study of the optimal detector mounting

technique is currently underway.