Everything about High Purity Germanium

High purity germanium (HPGe) is a semiconductor material with extremely high purity, which is usually used to make detectors. Such detectors are widely used in particle physics, astrophysics and nuclear physics experiments, especially in dark matter detection and neutrinoless double beta decay research.

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High purity germanium is an important semiconductor material with many unique physical and chemical properties. Here are some of the main properties of high purity germanium:

1. Physical properties: The density of high purity germanium is 5.32g/cm³. Its melting point is 937.2℃ and its boiling point is 2830℃. The hardness of high purity germanium is 6.0 Mohs. High purity germanium crystals are silver-gray. The resistivity of high purity germanium is very high, which can reach 800 Ω·cm.

2. Chemical properties: High purity germanium is insoluble in water, hydrochloric acid and dilutes caustic soda solution, but it can be dissolved in aqua regia, concentrated nitric acid or sulfuric acid. At high temperatures, germanium oxides (such as GeO and GeO₂) are formed, which are easily volatilized or dissolved in the air.

3. Electrical properties: High purity germanium has good semiconductor properties, and can form P-type germanium semiconductors after being doped with trivalent elements (such as indium, gallium, and boron). In the intrinsic state, the electron concentration of germanium is 1.7 x 10¹³ cm^-3, while that of silicon is 4.6 x 10¹⁴ cm^-3. High purity germanium detectors have extremely high energy resolution and low energy threshold, and are suitable for detecting X-rays and gamma rays.

4. Application areas: High purity germanium plays an important role in scientific research and technological applications due to its unique physical and chemical properties. High purity germanium detectors are considered the gold standard radiation identification equipment due to their excellent energy resolution and high detection efficiency. They have higher energy resolution than other types of detectors (such as NaI, LaBr, CeBr, or SrI), about 35 times that of NaI and 15 times that of LaBr, CeBr, or SrI. This advantage makes HPGe detectors excel in radiation identification tasks, providing better performance, including longer detection range, lower false alarm rate and higher sensitivity to threat materials.

The manufacture of HPGe detectors involves the use of high-purity germanium single crystals. These single crystals are usually prepared by zone melting purification and single crystal growth technology, and can reach a purity of 12N (that is, only one impurity atom per 10¹² germanium atoms). This high-purity germanium single crystal needs to be grown in a hydrogen atmosphere during the growth process to prevent the introduction of oxygen and other electrically active impurities.

In addition, the working principle of HPGe detectors is based on the semiconductor properties of germanium. In the detector, gamma rays or X-rays interact with free electrons in germanium to produce electron-hole pairs. These electron-hole pairs are separated and collected under the action of an electric field, resulting in a current signal, which is then amplified and analyzed to determine the energy and intensity of the rays.

HPGe detectors are also used in fields such as environmental monitoring because they can accurately identify and quantify radionuclides in samples. For example, during the decay of Co-60, the HPGe detector can detect two gamma rays of specific energies (1.17 MeV and 1.33 MeV), while during the decay of Argon-41, it can detect a gamma ray with an energy of 1.29 MeV.

In short, high-purity germanium detectors play an important role in scientific research and daily applications with their excellent energy resolution and high detection efficiency. With the advancement of technology, the application of HPGe detectors in low-background rare event detection has also been further developed.

5. Preparation process: High-purity germanium single crystals are usually prepared by methods such as zone melting and crystal growth, and their purity can reach 12N (i.e. 99.999999999%). The impurity concentration needs to be strictly controlled during the preparation process to ensure the high purity and excellent performance of the material.

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