State purity test1/5/2024 ![]() Depending on the size and design, the holding time of vacuum flasks ranges from a few hours to a few weeks.Ĭooling with liquid nitrogen is inconvenient, as the detector requires hours to cool down to operating temperature before it can be used, and cannot be allowed to warm up during use. The temperature of the liquid nitrogen is held constant at 77 K (-195.8☌ -320☏) by slow boiling of the liquid, resulting in the evolution of nitrogen gas. The immersion of the cold finger into the liquid nitrogen maintains the HPGe crystal at a constant low temperature. The cold finger extends past the vacuum boundary of the cryostat into a Dewar flask that is filled with liquid nitrogen. The input stages of the preamp are also cooled. ![]() Since the preamp should be located as close as possible so that the overall capacitance can be minimized, the preamp is installed together. The germanium detector preamplifier is normally included as part of the cryostat package. ![]() The combination of the vacuum metal container, the cold finger and the Dewar flask for the liquid nitrogen cryogen is called the cryostat. The cold finger transfers heat from the detector assembly to the liquid nitrogen (LN 2) reservoir. The HPGe crystal inside the holder is in thermal contact with a metal rod called a cold finger. The end-cap, is also generally made of aluminum. The holder is generally made of aluminum and is typically 1 mm thick. The detector holder as well as the “end-cap” are thin to avoid attenuation of low energy photons. Germanium crystals are maintained within an evacuated metal container referred to as the detector holder. Therefore, HPGe detectors are usually equipped with a cryostat. Cooling to liquid nitrogen temperature (-195.8☌ -320☏) reduces thermal excitations of valence electrons so that only a gamma ray interaction can give an electron the energy necessary to cross the band gap and reach the conduction band. Recall, the band gap (a distance between valence and conduction band) is very low for germanium (Egap= 0.67 eV). Otherwise, leakage current induced noise destroys the energy resolution of the detector. Because germanium has relatively low band gap, these detectors must be cooled in order to reduce the thermal generation of charge carriers to an acceptable level. The major drawback of germanium detectors is that they must be cooled to liquid nitrogen temperatures. Since HPGe detectors produce the highest resolution commonly available today, they are used to measure radiation in a variety of applications including personnel and environmental monitoring for radioactive contamination, medical applications, radiometric assay, nuclear security and nuclear plant safety. In order to achieve maximum efficiency the HPGe detectors must operate at the very low temperatures of liquid nitrogen (-196☌), because at room temperatures the noise caused by thermal excitation is very high. Consequently, germanium crystals were doped with lithium ions (Ge(Li)), in order to produce an intrinsic region in which the electrons and holes would be able to reach the contacts and produce a signal. Impurities in the crystals trap electrons and holes, ruining the performance of the detectors. Interstitial atoms caused by radiation damage.Interstitial atoms and vacancies within the lattice due to structural defects.Impurities within the semiconductor lattice.Moreover there must be no traps which can prevent them reaching the collecting contacts. The electron-hole pair collection within the detector must be done within a reasonably short time. Purity of a detector material is of the highest importance. Moreover silicon detectors cannot be thicker than a few millimeters, while germanium can have a depleted, sensitive thickness of centimeters, and therefore can be used as a total absorption detector for gamma rays up to few MeV.īefore current purification techniques were refined, germanium crystals could not be produced with purity sufficient to enable their use as spectroscopy detectors. Due to its higher atomic number, Ge has a much lager linear attenuation coefficient, which leads to a shorter mean free path. In comparison to silicon detectors, germanium is much more efficient than silicon for radiation detection due to its atomic number being much higher than silicon and due to lower average energy necessary to create an electron-hole pair, which is 3.6 eV for silicon and 2.9 eV for germanium. High-purity germanium detectors ( HPGe detectors) are the best solution for precise gamma and x-ray spectroscopy.
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