Semiconductor-Based Radiation Detectors: Industrial Sensor Development at Lawrence Berkeley National Laboratory

Description:
The Ernest Orlando Lawrence Berkeley National Laboratory Semiconductor Detector Group (SDG) focuses on the development of semiconductor-based radiation detectors and their applications. The basic function of the detectors is to convert the energy of absorbed photons or charged particles into electrical signals that provide a measure of each particle's energy and, depending on the application, interaction location within the detector.

Broad Fields of Use:
The types of radiation that can be measured with semiconductor detectors includes a large portion of the electromagnetic spectrum, with photon energies ranging from <1 eV (electron-volt, near infrared) to ~10 MeV (million electron-volts, gamma rays), and charged particles with energies from keV to GeV (thousand to billion eV). Depending on the type of radiation to be measured and the end application, different semiconductor materials and/or device structures are employed. The SDG has unique expertise in the areas of silicon (Si), germanium (Ge), and cadmium zinc telluride (CdZnTe) detectors, covering the full range of detector-related issues: material properties, device physics, fabrication processes, system development, and applications. The SDG collaborates with government and academic institutions as well as commercial companies to develop detector systems and technologies that meet specific user requirements. The detectors and detector systems produced by Berkeley Lab's SDG are used in scientific research as well as environmental, medical, and industrial applications.

Comparison with Current Technologies:
Semiconductor radiation detectors have unique capabilities and provide superior performance in many respects over other kinds of detectors. Typical competing technologies are those based on gas-filled detectors or on scintillators. The energy resolution achieved with semiconductor-based detectors is superior to that of these other technologies. Furthermore, the semiconductor detectors can be efficient, compact, and rugged. As a result of these advantages, a wide variety of such detectors are commercial available. The SDG develops unique semiconductor-based detectors and systems that are not available commercially.

Lithium-drifted silicon detectors:
The current focus of the SDG in the lithium-drifted Si detector area is in the development and production of large area and/or segmented detectors. These detectors are used in astrophysics, nuclear physics, national security, nuclear non-proliferation, medical imaging, and x-ray spectroscopy for material analysis.

A 20 by 20 orthogonal-strip lithium-drifted Si detector for imaging and high-resolution spectroscopy measurements. The active part of the
detector is 46 mm by 46 mm by 3.5 mm thick. These detectors may be operated at relatively high temperatures (above 200K) while still
maintaining low noise performance.

Figure 1. A 20 by 20 orthogonal-strip lithium-drifted Si detector for imaging and high-resolution spectroscopy measurements. The active part of the detector is 46 mm by 46 mm by 3.5 mm thick. These detectors may be operated at relatively high temperatures (above 200K) while still maintaining low noise performance.

A 100 mm diameter wafer and fabricated lithium-drifted Si detectors for the Cosmic Ray Isotope Spectrometer (CRIS) on NASA's Advanced
Composition Explorer (ACE) spacecraft launched in 1997.

Figure 2. A 100 mm diameter wafer and fabricated lithium-drifted Si detectors for the Cosmic Ray Isotope Spectrometer (CRIS) on NASA's Advanced Composition Explorer (ACE) spacecraft launched in 1997.

High-resistivity silicon detectors:
Detector arrays fabricated on high-resistivity Si wafers are being developed for a variety of applications, such as x-ray spectrometers for high-count-rate synchrotron radiation detection, portable instrumentation, and planetary exploration. Photodiode arrays are also being developed for medical imaging applications. The common feature of these devices is their extremely low noise, which results from the use of high-resistivity Si substrates and a low-leakage-current fabrication process that was originally developed for producing Si strip detectors for high-energy physics applications.

Fully-depleted silicon charge coupled devices:
Fully-depleted charge coupled devices (CCDs) are being developed for astronomy and astrophysics applications. Unlike conventional CCDs, which have a thin depletion layer only about 20 microns thick, fully depleted CCDs use high-resistivity Si and a substrate bias voltage to achieve full depletion of their 300-micron wafer thickness. This increased thickness translates into significantly higher quantum efficiency in the near infrared as well as in x-rays out to 20 keV or more. Because the device is fully depleted, it is sensitive to radiation absorbed at the back surface. This results in high sensitivity in the blue region of the spectrum without the need for the expensive back thinning process that is employed with conventional CCDs. For more information on fully-depleted CCDs, please visit http://www-ccd.lbl.gov/.

Germanium detectors:
The current focus of the SDG in the Ge detector area is in the development and production of detectors with segmented electrodes fabricated using the SDG-developed amorphous-semiconductor electrical contact technology. This technology offers the advantages of: excellent energy resolution, potentially high spatial resolution, large active volumes leading to high detector efficiencies, simplified fabrication, and enabling unique detector geometries and detection schemes. Application areas for these detectors include gamma-ray astronomy, medical imaging, environmental remediation, x-ray spectroscopy using synchrotron radiation, nuclear physics, and nuclear material detection.

A 25 by 25 orthogonal-strip Ge detector for gamma-ray imaging and high-resolution spectroscopy measurements. The active part of the
detector is 60 mm by 60 mm by 10 mm thick and was produced using the amorphous-semiconductor electrical contact technology. The
three-dimensional position and deposited energy for each gamma-ray interaction in such a detector can be accurately measured.

Figure 3. A 25 by 25 orthogonal-strip Ge detector for gamma-ray imaging and high-resolution spectroscopy measurements. The active part of the detector is 60 mm by 60 mm by 10 mm thick and was produced using the amorphous-semiconductor electrical contact technology. The three-dimensional position and deposited energy for each gamma-ray interaction in such a detector can be accurately measured.

A 40 by 40 pixel array Ge detector developed for hard x-ray astronomy. The pixels are 0.3 mm by 0.3 mm in size with a 0.5 mm
center-to-center spacing. The detector was produced using the amorphous-semiconductor electrical contact technology.

Figure 4. A 40 by 40 pixel array Ge detector developed for hard x-ray astronomy. The pixels are 0.3 mm by 0.3 mm in size with a 0.5 mm center-to-center spacing. The detector was produced using the amorphous-semiconductor electrical contact technology.

Cadmium zinc telluride detectors:
The current focus of the SDG in the CdZnTe detector area is in the development of compact, field-portable spectroscopy systems based on coplanar-grid detectors. The coplanar-grid technique was invented by the SDG and enables high spectral resolution combined with high detection efficiency. Currently, the SDG is developing these detectors for nuclear safeguard applications. Other potential applications include environmental remediation, well logging, medical diagnostics, and gamma-ray astronomy.

Figure 5. Photograph of a coplanar-grid CdZnTe gamma-ray detector combined with a schematic diagram of the front-end measurement electronics.

Measured 137Cs spectrum obtained with a 1 cm3 CdZnTe-based detector. Left: conventional planar geometry.
This demonstrates the spectroscopic performance improvement achieved with the coplanar-grid technique.

Measured 137Cs spectrum obtained with a 1 cm3 CdZnTe-based detector. Right: Coplanar-grid geometry.
This demonstrates the spectroscopic performance improvement achieved with the coplanar-grid technique.

Figure 6. Measured ¹³⁷Cs spectrum obtained with a 1 cm³ CdZnTe-based detector. Left: conventional planar geometry. Right: Coplanar-grid geometry. This demonstrates the spectroscopic performance improvement achieved with the coplanar-grid technique.

A 2 by 2 detector array assembled from four detector modules. Each module consists of a front-end electronics assembly and a 1 cm<sup>3</sup>
coplanar-grid CdZnTe detector contained in a compliant mount. Large detector arrays can be formed in this fashion in order to achieve the
high detection efficiencies required in some applications.

Figure 7. A 2 by 2 detector array assembled from four detector modules. Each module consists of a front-end electronics assembly and a 1 cm³ coplanar-grid CdZnTe detector contained in a compliant mount. Large detector arrays can be formed in this fashion in order to achieve the high detection efficiencies required in some applications.