# Vibration Reduction in a Pulse Tube-Cooled High Purity Germanium Detector
## 3 Evaluation Performance of the PTC Vibration Isolation System  
The PTC vibration isolation system was evaluated both on the PTC and the HPGe detector. A three-dimensional dynamometer force sensor (Kistler 9257B) was used to measure compressor vibration [14]. A PCB three-dimensional piezoelectric accelerometer (PCB 356B18) was installed to measure detector interface vibration acceleration. Meanwhile, detrimental effects of the PTC from mechanical vibration on spectrometric performance and detector signal noise were tested. In order to evaluate CVIK performance, an isolation performance test on a Kistler 9257B table was executed. Figure 4 shows the compressor isolation performance test configuration on the Kistler 9257B. The compressor with CVIK was mounted on the Kistler 9257B, and a cooler finger was rigidly fixed on other base plate on a sponge mat. The \"S\" shape copper transfer line may have affected the transmissibility of the cooler finger. The compressor vibration level was tested under the rigid connection and vibration isolation connection of the compressor. To provide a lightweight PTC system for the HPGe detector, the compressor and cold finger inevitably needed to be fixed to the same base plate. Thus, the vibration performance of the compressor with CVIK and cooler finger mounted in the same plate needed to be evaluated. Figure 5 illustrates the vibration transmissibility of the CVIK. The compressor was driven by 64 Hz, 50 W AC. The percentages in the figure indicate the ratio of compressor vibration force with and without the CVIK. The vibration peak value of the rigid condition was less than 1 N, and the peak value of the isolation condition was less than 0.1 N. The figure indicates the vibration level without the CVIK was higher in harmonic frequency because the higher frequency modes were excited. CVIK transmissibility was less than 10% in higher-frequency modes. The main frequency vibration transmissibility was higher than theoretical calculation for some reason, such as the transfer line stiffness affecting CVIK nature frequency and gas reservoir vibration. However, the main frequency vibration force was smaller, which met the requirement of less than 0.2 N. Figure 6 shows the isolation performance of the PTC when the compressor and cold finger were mounted on the same plate. The figure indicates that the vibration force of the PTC without an isolation system was up to 4 N, and the isolation performance was basically the same as the compressor. Figure 4: Test configuration of the compressor isolation performance on the Kistler9257B. Figure 5: Comparison of compressor vibration level with and without an isolation system. Figure 6: Comparison of PTC vibration level with and without a compressor isolation system. In this section, we focus on the evaluation of detector mounting interface vibration attenuation by introducing heat sinks. Detector mounting interface vibration came from heat links and the FPR-supporting plate. Since the cold tip and detector mounting interface were hard to mount on the Kistler 9257B, a PCB three-dimensional piezoelectric accelerometer was chosen for evaluation of heat link vibration isolation performance. Figure 7 shows an assembly drawing of the heat links and accelerometer mounting positions. Two measurement cases were performed under the CVIK: for case 1, the base plate, cold head flange and cold tip vibration acceleration were tested; for case 2, the accelerometer was moved to the detector mounting interface from the cold tip and the other two accelerometers stayed the same. The ratio of vibration acceleration of the detector mounting interface to vibration acceleration of the cold tip was used to evaluate the vibration isolation performance of the heat links. Base plate and cold head flange acceleration were used to monitor case 1 and case 2 in consistent test conditions. Figure 8 shows the vibration isolation performance of heat links; the percentages were the ratio of detector mounting interface vibration to cold tip vibration.