A liquid-helium cooled large-area silicon PIN photodiode x-ray detector

An x-ray detector using a liquid-helium cooled large-area silicon PIN photodiode has been developed along with a tailor-made charge sensitive preamplifier whose firststage JFET has been cooled. The operating temperature of the JFET has been varied separately and optimized. The xand γ-ray energy spectra for an Am source have been measured with the photodiode operated at 13 K. An energy resolution of 1.60 keV (FWHM) has been obtained for 60-keV γ rays and 1.30 keV (FWHM) for the pulser. The energy threshold could be set as low as 3 keV. It has been shown that a silicon PIN photodiode serves as a low-cost excellent x-ray detector which covers large area at 13 K. We have developed an x-ray detector to be employed in our solar axion experiment. The axion is a light pseudoscalar particle introduced to solve the strong CP problem [1]. Sikivie [2] proposed an experiment to detect the axions emitted by the sun using a system of a strong magnetic field and an x-ray detector, called the axion helioscope. In the magnetic field, solar axions convert to x rays of black body radiation spectrum with an average energy of 4.2 keV. The conversion can be enhanced by filling the conversion region with dense gas [3]. We are going to adopt cold helium gas as conversion medium with a temperature just above the boiling point at one atmosphere. In the experiment, an x-ray detector with the following specification is needed: • sensitive to x rays of 3–10 keV • energy resolution better than a few keV • operable at low temperature • sensitive area larger than 10 cm • low cost Preprint submitted to Elsevier Preprint 7 February 2008 • extremely low background Although gaseous detectors are often used for the soft x-ray detection, they are unusable at low temperature. By contrast, low temperature is a favorable environment for semiconductor detectors since their leakage currents decrease with temperature, hence high energy resolution is achieved. Among various semiconductor detectors, Si (Li) detectors are commonly used for low energy photon detection, but they are very expensive so that they are not appropriate for a large-area x-ray detector. Silicon PIN photodiodes usually used as light detectors are known to work also as good radiation detectors at liquid nitrogen temperature [4]. They are commercially available at relatively low price in various size and shapes. The thickness of their active layers is typically 200–500 μm, which is sufficient for the detection of soft x ray, but is so thin that silicon PIN photodiodes are almost insensitive to γ rays of higher energy. The latter feature, the low sensitivity to the background γ rays, makes silicon PIN photodiodes even advantageous over the expensive Si (Li) detectors because lower background is expected for them. In this letter, we present the result of a measurement we have performed to see whether a silicon PIN photodiode possesses the required performance, namely, sensitivity to the x rays and enough energy resolution, even at the temperature far below liquid-nitrogen temperature. The experimental setup is illustrated in Fig. 1. Since electronic noise dominates the energy resolution of the detector system, the first-stage junction field-effect transistor (JFET) and the feedback resistor of the charge sensitive preamplifier were cooled together with the PIN photodiode in order to reduce the thermal noise. They were put into a vacuum vessel and the vessel was immersed in a liquid-helium dewar. An Am source was also put inside the vessel. A silicon PIN photodiode with a thickness of 500 μm, Hamamatsu S3204-06, was used. It is a windowless photodiode with an active area of 18mm×18mm and is supplied on a ceramic base. The typical capacitance value is specified to be 80 pF at a reverse bias voltage of 100 V by the manufacturer. Between the source and the PIN photodiode is a copper collimator of 1-mm thick and with a hole of 1.5 mm in the center. In order to reduce the microphonic noise the PIN photodiode was softly supported with a phosphor bronze ribbon to a liquid-helium cooled copper plate soldered on the end cap. Its temperature was monitored with a carbon resistance thermometer (Allen-Bradley 56-Ω solid carbon resistor) stuck on its back. Other electronic components, such as the JFET and the feedback resistor, were mounted on a glass-epoxy printed circuit board and cooled with a 1mm-thick copper-plate cold finger fixed on the end flange at its base. A low-