High Purity Germanium Detector (HPGe)

HPGe is the best detector for gamma-ray spectroscopy, with a resolution <0.3%.

HPGe detector is similar to a PN diode, where a bias voltage (several kV) creates a depleted region. This region is sensitive to ionizing radiations, especially X-ray and gamma ray. When a high energy photon from such radiations interact with the material in the depleted region, a current pulse is produced due to the creation of electron-hole pairs. This current is integrated by a charge sensitive preamplifier and turned into a voltage pulse signal. The height of the signal indicates the energy of the radiation.

Drawing an energy spectrum using the amount of count vs. signal height (channel), we can qualitatively and quantitatively know the component of the radiation from a radioactive source.

For HPGe detector, the area of the depleted region is a key, since a bigger area allows a more completed charge collection process and an increased efficiency. To broaden the depleted region, the material should be ultra pure. For a common HPGe detector, the purity of germanium is higher than 13N (99.99999999999%).

Moreover, HPGe detector must be operated under liquid nitrogen temperature to avoid the production of electron-hole pairs due to the thermal energy, which will significantly deteriorate the resolution. To ensure this isolation low temperature, the detector is insulated in a high vacuum environment.

I have found two HPGe from Ebay—one from Princeton Gamma Tech, model IGC18 (+2000V, 18% efficiency, P-type coaxial crystal) for median to high energy, and the other from Canberra, model 711007E (-1500V, low energy detector with a beryllium window).

When I get this two detectors, both of them are defective. The IGC18 cannot produce proper spectrum as it only accepts high voltage up to 1000V. When I try to increase high voltage, the test point voltage (indicating the leak current from the HV line through vacuum) also increases. This is normally because of the bad vacuum condition due to long storage time. Therefore, I first try to reevaluate the chamber using a turbo pump and a customized tool for opening the valve.

The vacuum port of the detector has a special cap with many holes, and it requires a special tool called manual pull valve. This manual pull valve is designed with a special fork shaped end to fit into the cap.

This is the valve I used for the Canberra detector. The one I used for this PGT detector has a same structure but just a different ending (the Canberra valve has threads but the PGT one is a fork-shaped thing). The valve is connected to the port before applying a vacuum. And when the vacuum pump is connected, the head of the valve can be sued to connect to the cap either though threads or the fork. The cap can be taken off to evacuate the chamber and reinstalled after that in the vacuum condition throughout.

The chamber is evacuated using a turbo pump, till approximately 5E-6mbar. Then the detector is baked using a heater of 120ºC for outgassing for 24h.

After that, the detector is put into liquid nitrogen for a night, and it is fully operational. It was lucky that I got a fully functional set of NIM module from Canberra including spectrum amplifier and HV supply, but due to some unknown issue, I cannot get a proper spectrum using the amplifier (perhaps due to wrong settings). Therefore, I currently use my Redpitaya STEMLab14 to directly collect signals from the preamplifier, whose result is surprisingly satisfying.

The negative output from the preamplifier and the positive output from the main amplifier. Unfortunate I forgot to record the signals with a trigger at that time.

Nevertheless, I still cannot increase the HV to the rated +2000V. So I guess that the FET in the detector is fault and needed to be replaced as this is the most common problem for the old detectors. This also convinced me to explore the inner structure of the detector.

Opening the big cap of the detector, one can see that the structure is not sophisticated—the thick copper rod conducts heat and helps cool down the crystal, and a few electrical parts such as the FET and high voltage capacitor are located to form a integrator. The signal is outputted to the preamplifier outside through a vacuum connection in the lower part not visible.

FET: 2N4393. This is the most commonly used FET due to its outstanding performance under ultra low temperature. You can find them on second hand websites. But make sure to buy the ones with golden legs. I have tried to get many 4393 with steel legs and they all failed.

After the replacement, the detector can now accept +1500V without an increase in the test point voltage. The spectrum is extraordinary in comparison with the ones produced by normal detectors like NaI.

Spectrum of a radium paint:

Spectrum of a piece of depleted uranium:

For the other detector from Canberra, since its preamplifier is defective and the low energy range is not useful for me, I sold it. However, I dismantle it before shipping it out.

Removing the cap of this detector is very annoying, since it applies an indium seal. The indium sticks to the stainless steel every hard so it is difficult to remove it. Also, making a new indium seal is both expensive and unfriendly for any novice. It took me hours on forming a working seal.

Beryllium window on the output cap. Due to the low atomic number, beryllium is almost transparent to X-rays, so it is normally used in nuclear detectors. However, it is extremely toxic, so make sure not to touch it and wash your hand after any operation.

Opening the inner cap:

An edge of the crystal can be seen (the shiny part around the white central circle).