Large-area Si(Li) orthogonal-strip detectors
Segmented lithium drifted silicon detectors are being developed for use in large Compton cameras and for medical imaging. We have successfully fabricated 3.5 mm thick crossed-strip detectors with active areas of 40 /spl times/ 40 mm/sup 2/ with a 2 mm strip pitch. These detectors utilize new contact technology consisting of a boron implanted p-type contact and an amorphous silicon (/spl alpha/-Si) n-type contact. Good energy resolution and excellent strip separation was obtained from both contacts at relatively high operating temperatures. An energy resolution of 2.1 keV FWHM at 122 keV was obtained for the /spl alpha/-Si strip at temperature up to 200 K and for the B strip at temperature up to 240 K. Measurements of the charge sharing between the strips were also performed. When the detector was flood illuminated with 60 keV gamma-rays, a small signal deficit was observed for events in which the signal was shared between two adjacent strips. The amount of deficit depended on the contact type, strip geometry and the operating temperature.
Evaluation of Si(Li) detectors for use in Compton telescopes
Si(Li) detectors are currently being developed for use in large Compton telescopes. A major advantage of silicon when compared with germanium is its ability to operate at significantly higher temperature. To determine the feasibility of using Si(Li) detectors in a Compton telescope, their performance as a function of temperature has been studied. We present leakage current, noise data and gamma-ray spectral performance at various temperatures for single 6-mm thick planar devices. It has been determined that for detectors without a guard ring, the noise began to rise significantly around 210K. Adding a guard ring improved the leakage current by about an order of magnitude and reduced the total noise (detector plus electronics) by about 25 percent. The noise of the detectors with {approx}130 mm2 area and a guard ring did not exceed our performance goal of 2 keV FWHM until the temperature was approximately 240K. For 122 keV gamma rays, no evidence of ballistic deficit was seen at 8 ms peaking time and bias voltages corresponding to an internal electric field of {approx}1150 V/cm. Some evidence of ballistic deficit was seen for 662 keV gamma rays at temperatures above 220K.
Position resolution in a Ge-strip detector
We have investigated, both experimentally and theoretically, how to reconstruct in 3D the interaction positions for (gamma) -rays penetrating into a double-sized Ge cross trip detector. We found that when a suitable geometry is used, the 3D-reconstruction problem can be reduced to three 1D ones, which greatly simplifies the task. We report measurements on a 10mm thick detector with 2mm strip pitch, showing that at least 2mm position resolution can easily be achieved perpendicular to the detector plane. While the in- plane resolution is presently limited to the strip pitch we present work on progress in developing algorithms to improve this. This includes in particular the expected effects of the electronics and the interstrip capacitance on the signal shapes. Finally, we present captured waveforms that indicate the possibility of reconstructing more complex events such as Compton scattering.