Craig Tindall

Silicon Detectors for Low Energy Particle Detection

Silicon detectors with very thin entrance contacts have been fabricated for use in the IMPACT SupraThermal Electron (STE) instrument on the STEREO mission and for the Solid State Telescopes on the THEMIS mission. The silicon diode detectors were fabricated using a 200 Aring thick phosphorous doped polysilicon layer that formed the thin entrance window. A 200 Aring thick aluminum layer was deposited on top of the polysilicon in order to reduce their response to stray light. Energy loss in the entrance contact was about 350 eV for electrons and about 2.3 keV for protons. The highest detector yield was obtained using a process in which the thick polysilicon gettering layer was removed by chemical etching rather than chemical mechanical polishing.

New contact development for Si(Li) orthogonal-strip detectors

At present, the contacts generally used for lithium-drifted silicon detectors consist of a diffused lithium layer (n-type) and a gold surface barrier (p-type). These contacts work well for unsegmented detectors. However, they both have disadvantages if used for segmented detectors. For this reason, we are developing new types of contacts that will be more robust and easier to segment. To replace the lithium n-type contact, we are using a thin layer of amorphous silicon (a-Si) with metalization on top. The new p-type contact consists of boron implanted into the silicon and annealed at the relatively low temperature of 500 degrees C. The implantation and annealing is carried out as the first step in the process, prior to lithium drifting. Detectors have been fabricated using the new contacts both with and without a guard ring. They performed as well as detectors with standard contacts at operating temperatures between 80K and 240K. We will present data on the leakage current vs. temperature, isolation resistance between the guard ring and the center contact versus temperature and bias voltage, electronic noise and energy resolution versus temperature, as well as 57Co spectra.

Low-noise low-mass front end electronics for low-background physics experiments using germanium detectors

A low-noise resistive-feedback front-end electronics assembly has been developed for use with p-type point contact (PPC) Ge detectors in low background experiments. The front end was designed to have a low mass and potentially low radioactivity. It is fabricated on a fused silica substrate, and consists of a low-noise JFET, a feedback resistor formed from an amorphous Ge thin film, and feedback capacitor based on the stray capacitance between circuit traces. The substrate provides the appropriate thermal impedance to allow the FET to operate at the optimal temperature from self-heating when one side of the substrate is held at liquid nitrogen temperature. A noise level of 85 eV FWHM at 20 us peaking time has been observed in combination with a small PPC detector, the 1/f contribution being as low as 30 eV for the front end alone. This approach of employing ultra-low-mass and low-noise front-end electronics in combination with larger-size PPC detectors can be an enabling technology towards the observation of particles and processes such as neutrino-less double-beta, coherent neutrino scattering or cold dark matter.

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.

Proximity Charge Sensing With Semiconductor Detectors

Semiconductor radiation detectors are routinely used for the detection, imaging, and spectroscopy of gamma-ray, x-ray, and charged particles. In basic form, a detector is comprised of a semiconductor crystal with two or more electrodes formed on its surfaces. Besides allowing for the application of bias voltage, one or more of the electrodes on a detector also serve as readout electrode. Charge carriers drifting across the detector induce a charge signal on the electrode, which can then be measured by a charge-sensitive amplifier connected to the electrode. Although in general the readout electrodes of a detector are formed on the detector itself, charge can be induced on any electrode, even if the electrode is not physically in contact with the semiconductor. Such proximity charge sensing effects can be utilized to achieve a variety of advantages in applications involving semiconductor detectors. In this paper, we report on the experimental verification of signal readout using proximity electrodes and demonstrate several possible applications of this technique, including the position-sensitive readout of detectors and the sensing of incomplete charge collection in detectors as a means to reduce spectral background.

Performance of the LBNL FastCCD for the European XFEL

The European X-ray Free Electron Laser (XFEL.EU) is currently being commissioned in Schenefeld, Germany. From 2017 onwards it will provide spatially coherent X-rays of energies between 0.25 keV and 25keV with a unique timing structure. One of the detectors foreseen at XFEL.EU for the soft X-ray regime (energies below 6keV) is a quasi column-parallel readout FastCCD developed by Lawrence Berkeley National Lab (LBNL) specifically for the XFEL.EU requirements. Its sensor has 1920×960 pixels of 30μm ×30μm size with a beam hole in the middle of the sensor. The camera can be operated in full frame and frame store mode. With the FastCCD a frame rate of up to 120 fps can be achieved, but at XFEL.EU the camera settings are optimized for the 10Hz XFEL bunch-mode. The detector has been delivered to XFEL.EU. Results of the performance tests and calibration done using the XFEL.EU detector calibration infrastructure are presented quantifying noise level, gain and energy resolution.

Multi-Correlated Double Sampling vs. analog shaper: Low power ASIC for pixelated CdTe

Comparison of continuous time and discrete time noise filtering is presented. Two sets of measurements have been performed: one with the classical analog semi-Gaussian shaper (CR-RC2), the other with M-CDS (Multi-Correlated Double Sampling). The Charge Sensitive Amplifier used in the measurements is optimized for input capacitance of 1 pF and detector dark current below 5 pA. With the analog shaper the Equivalent Noise Charge is characterized as a function of peaking time. ENC in the M-CDS method is characterized against the sampling frequency and the number of samples. The two methods have been compared. They show very similar noise performances: in each case the ENC as low as 30 electrons rms was achieved with the input capacitance of 0.3 pF and CSA power consumption of 14 μW. Spectroscopy measurements with the CSA connected to Si diode were performed using the 57CO source. The achieved FWHM resolutions were 390 eV at 14 keY with the analog shaper and 440 eV at 14 keY with the M-CDS method.

Characterisation of a thin, fully-depleted, back-illuminated SOI pixel sensor with soft X-ray radiation

THE availability of the Silicon on Insulator (SOI) process at OKI Inc. (now Lapis Semiconductor) with an handle wafer of moderate resistivity and contacts through the buried oxide layer has promoted a significant R&D on monolithic Si pixel sensors for charged particle tracking and imaging. The SOI technology has a number of potential advantages compared to bulk CMOS processes for the fabrication of pixel sensors. Past the first proof of principle of beam particle detection with an SOI pixel sensor [1], the R&D had to solve the back-gating effect, which limited the practical depletion voltage and thus the depleted thickness. The use of a buried p-well to protect the CMOS electronics from the potential on the handle wafer has successfully solved this problem and SOI pixels developed by KEK and by our group (LBNL, UCSC and INFN, Padova) have demonstrated full functionality up to 90 V and above [2], [3], [4]. This corresponds to a depleted thicknesses of ≃130 µm for a nominal resistivity of 700 O·cm.

A 5- μ m pitch charge-coupled device optimized for resonant inelastic soft X-ray scattering

We have developed a charge-coupled device (CCD) with 5 μm × 45 μm pixels on high-resistivity silicon. The fully depleted 200 μm-thick silicon detector is back-illuminated through a 10 nm-thick in situ doped polysilicon window and is thus highly efficient for soft through >8 keV hard X-rays. The device described here is a 1.5 megapixel CCD with 2496 × 620 pixels. The pixel and camera geometry was optimized for Resonant Inelastic X-ray Scattering (RIXS) and is particularly advantageous for spectrometers with limited arm lengths. In this article, we describe the device architecture, construction and operation, and its performance during tests at the Advance Light Source (ALS) 8.0.1 RIXS beamline. The improved spectroscopic performance, when compared with a current standard commercial camera, is demonstrated with a ∼280 eV (CK) X-ray beam on a graphite sample. Readout noise is typically 3-6 electrons and the point spread function for soft CK X-rays in the 5 μm direction is 4.0 μm ± 0.2 μm. The measured quantum efficiency of the CCD is greater than 75% in the range from 200 eV to 1 keV.