The aim of this student project was to construct a portable cryogenic experimental apparatus for laboratory works and to prepare software tools together with manuals. The apparatus is based on a helium cryostat (Janis Research Company, Inc.) equipped by a chamber, a helium compressor, and an electronic unit controlling temperature from 11 to 400 K. The helium compressor of this apparatus is cooled by a water-cooling system (JDK, s.r.o.). The entire setup is designed as a portable experimental system easy to transport even to large distances. The software tools are based on plug-in modules. The tools serve for communication between PC and other electronic devices (e.g., high-voltage sources, electrometer, control units, etc.).

The portable cryogenic experimental apparatus was built and preliminary tested. The tests include (i) determination of limits in temperature controlling, (ii) tryout of material tests, and (iii) verification of spectroscopy-signal quality.

Temperature is lowered by expansion of helium gas in cryogenic chamber. It can be increased either naturally using non perfect thermal isolation or constrainedly using a heater. The time which is need for decreasing of temperature from room temperature to a minimum (11 K) is approximately 2 hours. The natural increase of temperature from the minimum to room temperature takes roughly 20 hours. Rates of the constrainedly temperature increase is determined using power settings of the heater. The tests have proved that the active heater has not measured influence on leakage-current values.

A measurement of thermally stimulated current (TSC) was used for the tryout of material tests. The TSC was done with a GaAs pad detector deteriorated using fast neutrons. The GaAs pad detector was made from a 250 μm thick Semi-Insulating Liquid Encapsulated Czochralski (SI-LEC, produced by CMK Ltd., Zarnovica, Slovakia) GaAs substrate. The detector was irradiated in the neutron field provided by the cyclotron-based fast neutron source of the Nuclear Physics Institute in Řež near Prague. The (p,n) reaction of 37 MeV protons on a thick beryllium target was used as neutron source reaction. The neutron spectral flux data were taken from the work of Lone et al. Within the energy range (2–30) MeV the spectrum of neutrons arising in the beryllium target is an approximately continuous plateau, which is followed by an exponential decrease up to 37 MeV. The fast neutrons interact with Ga and As atom nuclei via predominantly (n,p) and (n,α) reactions. The recoil nuclei, protons, and alpha particles produce defects in the crystallographic lattice of the GaAs substrate. These defects can cause sharp variations of the TSC. Such variations were measured with the GaAs pad detector. The TSC measurements with radiation detectors are still underway.

A test of the spectroscopy-signal quality was performed with the same GaAs pad detector. The detector was placed inside the cryostat chamber. The spectroscopy signals from this detector were taken out the chamber using coaxial cables. These cables were connected with a charge sensitive preamplifier followed by a linear amplifier with semi-gaussian shaping. The resultant spectroscopy signals were monitored by an analog oscilloscope with band width of 200 MHz. The monitored signals were intensely destroyed during processing of the helium compressor (expansion of the helium gas make mechanical shocks on which the spectroscopy system is very sensitive). The disturbances disappear when the helium compressor is turned off. Therefore, any spectroscopy measurement has to be made only in regime of the naturally temperature escalation.

The portable cryogenic experimental apparatus has great potential for the investigation of semiconductor radiation detectors and other scientific applications as well. Future work will be predominantly oriented on material characterizations and on preparation of special spectroscopy systems with cryogenic radiation detectors. The material characterization will contain (i) Thermally Stimulated Current spectroscopy (TSC), (ii) Deep Level Transient Spectroscopy (DLTS), (iii) Photo-Induced Charge Transient Spectroscopy (PICTS), etc. Furthermore, the portable cryogenic experimental apparatus will be potentially used also in material research oriented on temperature-dependent studies of crystallographic lattices using X-rays (e.g., Debbie-Scherer method) and electric- and magnetic-field distributions using electron-positron annihilation.

An example of the special spectroscopy systems is the utilization of the portable cryogenic experimental apparatus at small particle accelerators. In such case, the apparatus can cool a radiation detector which is situated in particle beam with the aim to measure the spectroscopy response under different temperatures. Another example is a spectroscopy experimental setup of a cooled radiation detector registering photons of X-rays under extreme conditions (e.g., under conditions reign during plasma explosion induced by a short high-intensively laser pulse inside a vacuum chamber of the PALS system).

The portable cryogenic experimental apparatus can be also used in wide variety of additional experiments and applications. Use of the cryogenic system and collaboration with the Institute of Experimental and Applied Physics of the Czech Technical University in Prague are kindly welcome.