**1. Introduction**

This chapter provides a technical overview of HPGe detectors and their applications, both in science and day-to-day life. This overview covers the applications of HPGe as a material for γ-ray detection and its other more recent use in particle physics. It represents an introduction rather than a complete and exhaustive description of possible detector applications of HPGe.

Since the 1970s, photon detectors (γ and X) have been developed from high purity germanium [1]. The reason why HPGe has remained in use for such a long time as high-resolution γ- and X-ray detector material is mainly because it contains a very low concentration of electrically active defects [2, 3] which may be lower than 109 cm<sup>−</sup><sup>3</sup> . Such value is difficult to reach in compound semiconductors. Even for detector-grade silicon, the doping level is slightly higher. High electron density (Z = 32), together with low average energy, is needed for e-h pair generation. Above 1 keV of initial particle energy, germanium is a good choice for the detection of photon or ionizing particles. See **Table 1** and Refs. [4–10] for studies, both


#### **Table 1.**

*Semiconductors that may be used for the direct detection of γ rays and their average energy for electron hole creation Ehn and other quantities, the linear energy transfer is indicated for high-energy charged-particle detection (at Minimum Ionization).*

theoretical and experimental, on ionization in semiconductors. In the early days of germanium γ-ray detection, less purified material was used. The compensation method for net doping density reduction was based on the lithium drifting technique [11]. With an applied voltage bias, the drift of the interstitial Li+ ions through the detector is made possible leading to the neutralization of acceptors by the creation of Li+ acceptor pairs. However, progress in Ge purification led to the obsolescence of compensation techniques. Lithium is still used as a doping ion for the creation of n+ contacts, even though ion-implantation is now a mainstream technique for this purpose. Room-temperature γ-ray detection is still a challenge as the best candidate material exhibits higher defect concentrations and/or is difficult to grow into large crystals. Cadmium telluride is a material of choice for photodetection, and gallium arsenide is another option. However, considerable progress would be necessary to match the easiness of use of HPGe in its present-day applications, despite the need for cryogenic apparatus.

The main contributions to the development of high-purity germanium have been made already in the 1970s by the LBNL [1] and other US laboratories as well as industrial companies. There are presently a few corporations in Europe which supply such materials, without mentioning those from Eastern Europe. High-purity single crystals are usually grown in hydrogen atmosphere in a silica crucible [9, 12]. This prevents the introduction of oxygen and other electrically active impurities. However, this process introduces substitutional silicon, which is electrically inactive, similar to carbon. These crystals exhibit high levels of interstitial hydrogen, which may form complexes with some metallic impurities such as copper [13]. Good quality crystals (despite being small) may be fabricated in University labs [11, 14] which show that nowadays the availability of such crystals is better than ever. Gamma-ray detector manufacturers usually use commercially available crystals, which are subsequently processed, or in some cases fabricate the material itself. It should be noted that except for isotopically enriched crystals (for physics experiments), most crystals are made of natural germanium with a proportion of isotopes indicated in **Table 2**.

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*High Purity Germanium: From Gamma-Ray Detection to Dark Matter Subterranean Detectors*

In this chapter, we introduce the principle of γ-ray detection and spectroscopy. Most high-energy photons (100 keV–10 MeV) interact with the electrons in the HPGe material [15]. As the density of electrons is proportional to Z, Ge is a material close to the optimum among well-characterized semiconductors, together with CdTe and GaAs. As the absorption coefficient increases along with Z, this results in a good detection efficiency for a given detector volume compared for instance with silicon. Here are the three main processes by which gamma rays lose energy in the detecting media: the photoelectric effect, Compton scattering, and e+e− pair creation. The photoelectric process [16] is dominant at low energies (<100 keV approximately) and is related to emission of electrons from the atomic shells. This process depends on the energy of the photons and the atomic number of the detecting media. The energy of a photon is absorbed by an inner-shell electron and leads to its emission from the atom. Subsequently, the photoelectrons lose their kinetic energy in the semiconductor by electron-hole pair generation. This first term is the photoelectron energy (Ephotoelectron), and the second is the difference between the

gamma energy (Eγ) and the electron binding energy (Ebindingenergy).

Ephotoelectron = E<sup>γ</sup> − Ebindingenergy, (1)

<sup>E</sup><sup>3</sup> (2)

The electron binding energy for K-shell valence electrons is of the order of 11 keV in Ge, compared to around 1.8 keV in silicon. Other, shallower shells may also be excited, contributing to the signal, which means that as the photon energy increases, inner shells can be excited gradually and the absorption will exhibit abrupt increase. This does not have a direct influence on the average number of electron-hole pairs generated per unit of energy deposited; we will discuss this in

This means that the contribution of the photoelectric effect vanishes at high energies (E), when the other two processes of interaction of gamma rays with matter

the following paragraph [8]. The absorption coefficient is proportional to:

<sup>Z</sup><sup>4</sup> \_\_\_

*DOI: http://dx.doi.org/10.5772/intechopen.82864*

**2. Gamma-ray detection and spectroscopy**

*Stable isotopes found in natural germanium with the nuclear moments and abundance.*

**Table 2.**

*High Purity Germanium: From Gamma-Ray Detection to Dark Matter Subterranean Detectors DOI: http://dx.doi.org/10.5772/intechopen.82864*


**Table 2.**

*Use of Gamma Radiation Techniques in Peaceful Applications*

tions, despite the need for cryogenic apparatus.

theoretical and experimental, on ionization in semiconductors. In the early days of germanium γ-ray detection, less purified material was used. The compensation method for net doping density reduction was based on the lithium drifting technique [11]. With an applied voltage bias, the drift of the interstitial Li+ ions through the detector is made possible leading to the neutralization of acceptors by the creation of Li+ acceptor pairs. However, progress in Ge purification led to the obsolescence of compensation techniques. Lithium is still used as a doping ion for the creation of n+ contacts, even though ion-implantation is now a mainstream technique for this purpose. Room-temperature γ-ray detection is still a challenge as the best candidate material exhibits higher defect concentrations and/or is difficult to grow into large crystals. Cadmium telluride is a material of choice for photodetection, and gallium arsenide is another option. However, considerable progress would be necessary to match the easiness of use of HPGe in its present-day applica-

*Semiconductors that may be used for the direct detection of γ rays and their average energy for electron hole creation Ehn and other quantities, the linear energy transfer is indicated for high-energy charged-particle* 

The main contributions to the development of high-purity germanium have been made already in the 1970s by the LBNL [1] and other US laboratories as well as industrial companies. There are presently a few corporations in Europe which supply such materials, without mentioning those from Eastern Europe. High-purity single crystals are usually grown in hydrogen atmosphere in a silica crucible [9, 12]. This prevents the introduction of oxygen and other electrically active impurities. However, this process introduces substitutional silicon, which is electrically inactive, similar to carbon. These crystals exhibit high levels of interstitial hydrogen, which may form complexes with some metallic impurities such as copper [13]. Good

quality crystals (despite being small) may be fabricated in University labs [11, 14] which show that nowadays the availability of such crystals is better than ever. Gamma-ray detector manufacturers usually use commercially available crystals, which are subsequently processed, or in some cases fabricate the material itself. It should be noted that except for isotopically enriched crystals (for physics experiments), most crystals are made of natural germanium with a proportion of

**68**

**Table 1.**

*detection (at Minimum Ionization).*

isotopes indicated in **Table 2**.

*Stable isotopes found in natural germanium with the nuclear moments and abundance.*
