# Vibration Reduction in a Pulse Tube-Cooled High Purity Germanium Detector
## 1 Introduction  
HPGe detectors are the laboratory standard for high-resolution gamma-ray spectroscopy and are used extensively in a wide range of application areas, such as environmental monitoring, medicine, geology and scientific research [1]. In order to reduce the leakage of current-induced noise from thermal generation of charge carriers, HPGe detectors need to operate at a temperature range of 90-120 K. To achieve these temperatures, liquid nitrogen (LN2), a cryogen, was commonly used with a cryostat. However, cryogen use is confronted with several health risks and operational problems [2]. Electromechanical cooling is an attractive choice, especially for portable HPGe detectors. Advances in Joule-Thomson technology coupled with HPGe detectors have been reported [3]. Electromechanical cooling systems (cryocooler) have become common for HPGe detectors. HPGe detector resolution is a measure of the width, specifically the full width at half maximum (FWHM), of a single energy peak, expressed in absolute keV. Better (lower FWHM value) resolution enables the system to more clearly separate the peaks within a spectrum. Mechanically cooled HPGe detector resolution performance is typically specified as degradation by some percentage compared to LN2 performance. The cryocooler essentially needs pressure oscillation to generate low temperatures. Gas pressure oscillation intrinsically causes vibrations at the cold stage of the cryocooler. Mechanical vibrations inherent in EMC cause electrical noise induced at the preamplifier input and periodic changes of structural capacitances between the HPGe detector and components. The vibration alters the capacitances in input circuits of the preamplifier, causing electrical interference, and causing the detector performance to degrade. Many technologies have been developed to reduce the micro-vibration induced by coolers [4; 5; 6; 7; 8]. [PERSON] et al. [5] developed the ultra-low vibration cryogenic cooler byutilizing a combination of passive and active suppression techniques. The effectiveness of the techniques is mainly reflected in the axial direction of the cooler. [PERSON] et al. [6] proposed a multidimensional, triple-stage, passive vibration isolator on the cold tip of a small cryogenic cooler. The isolator is used to attenuate cold-tip vibration; however, vibration induced by the compressor inevitably passes to the cold tip when the compressor and cold finger are fixed in the same platform. [PERSON] et al. [7] proposed an SMA blade isolator that ensures desirable vibration isolation performance, except in the gravitational axis, regardless of 1G and 0G conditions. The isolator has high stiffness in order to support the cooler in the gravitational axis. Thus, the isolator cannot isolate vibration in the gravitational axis. [PERSON] et al. [8] proposed a multilayered blade isolator with double-sided adhesive tape, and this isolator can provide a constant vibration isolation capability irrespective of gravitational effects. Although the applicability and survivability of double-sided adhesive tapes were evaluated through qualification-level TV tests, the long-term stability and reliability of double-sided adhesive tape is unconvincing. [PERSON] et al. [4] proposed a passive isolation system named PLOVIS, and PLOVIS is used to isolate vibration from the cooler. However, the isolator requires an additional 1G compensation device to obtain an ideal micro-vibration isolation capability on the ground. In this paper, a portable PTC, suitable for HPGe detectors, was developed. A split PTC was used to maintain the cryogenic temperature. The split PTC consisted of a linear compressor and cold head. Degradation of detector performance was suppressed in two ways. A portable pulse tube cryocooler (PTC) was expected to function as a low-vibration cryocooler. Due to the absence of moving components in the cold head, the split PTC could provide a cryogenic temperature with low vibration. Compressors are well-known sources of harmonic disturbance.