**3.4 Alternatives to HPGe**

For a long time, materials alternative to HPGe have been proposed, and corresponding detectors were fabricated. GaAs is one example (mostly for X-ray detection), and, more importantly, CdTe. The goal was to eliminate the need for cryogenic operation by the use of large band gap semi-insulating material (diamond is also considered). Many difficulties still exist, mostly related to the defect density that is much higher in binary materials [9]. The other drawback is the possibility to grow large crystals, which proves to be more difficult for alternative materials. Large diamond single crystals with a low nitrogen content are difficult to grow as they need high-pressure processing. However, some CdTe photon detectors using segmented crystals for photon identification have been successfully implemented on space missions (INTEGRAL) [24]. Segmentation allows reducing drift length and therefore trapping, so that the resolution can be maintained at an acceptable level.


**75**

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

Particle identification through pulse shape discrimination is one of the developments that are used mainly in nuclear physics [35]. These techniques can be adequately used to determine the region of the detector where the interaction took place. There have been early reports for methods of particle identification in germanium and silicon detectors [36, 37]. The detection of photons is basically through ionization, and there is no important interaction with the nucleus, which could transfer momentum to the nucleus. If we consider other particles with a significant mass, their interaction with the nucleus may induce the recoil, which in turn is slowed down in the detector material. The slowing-down process results in ionization (electron-hole pair generation), defect creation (vacancies and interstitials), and phonon creation. One should note that according to SRIM simulations, in the typical energy range for the recoil (20–30 keV), the most important contribution (73%) is from phonon emission. The energy used for vacancy creation is of the order of 3%, which indicates that defect monitoring [38] could provide an alternative way to estimate the total integrated flux of interacting particles, since when stable defects are created, their concentration is proportional to the total number of interacting particles in a given time interval, which can be very long. The rest of the energy is used for ionization (25%). This result is close to the experimental result obtained for cryogenic detectors, as it will be discussed in the paragraph on dark

Simulations of the interaction of photons with matter have been given a certain attention. Monte Carlo codes have been developed. Recently, the NWEGRIM code [18] from Pacific Northwest laboratories has been used to simulate the interaction of photons in silicon, but these simulations could also be applied to germanium. GEANT4 has been used for simulation of charged particles interacting with

High-purity germanium γ-ray tracking arrays, such as AGATA (Advanced GAmma Tracking Array) [40] and GRETINA/GRETA (Gamma Ray Energy Tracking Array) [41] represent the state-of-the-art in high-resolution γ-ray spectroscopy for nuclear physics experiments. These spectrometers are composed of highly segmented large-volume HPGe crystals. Pulse-shape analysis, applied to the recorded signals from the segments, yields three-dimensional interaction positions with a typical precision of about 2 mm [40, 41]. Subsequently, a γ-ray tracking algorithm is applied to the determined interaction points in order to group and order them in sequences corresponding to individual γ-rays. In this procedure, the geometrical criteria and the Compton scattering formula are used, and the full energy of a γ-ray is determined as the sum of the energies of the interactions ascribed to the

The γ-ray tracking arrays provide improved energy resolution for in-beam nuclear physics studies, thanks to much reduced Doppler broadening as compared to standard γ-ray spectrometers. The use of γ-ray tracking also eliminates the need for Compton-suppression shields, commonly used with HPGe crystals in order to improve the peak-to-total ratio. Consequently, the entire 4π solid angle can be surrounded with Ge crystals, which leads to significantly increased detection

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

**3.5 Particle identification**

matter detection.

**3.6 Simulation**

germanium [39].

same trajectory.

**4. Gamma-ray tracking arrays**

#### **Figure 1.**

*SRIM simulation showing the vacancy distribution for a Ge recoil of 30 keV.*

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

## **3.5 Particle identification**

*Use of Gamma Radiation Techniques in Peaceful Applications*

and other detector studies.

**3.4 Alternatives to HPGe**

acceptable level.

analytical model has been developed that clearly explains this effect using simple assumptions [33]. This mitigates the direct role of disordered regions as being the sole origin of carrier capture at 77 K. The sizes of these disordered regions areof the order of the range of primary recoil atoms (**Figure 1**). Isolated defects should contribute greatly to the trapping process. The recoil of an atom is induced by the collision with the impinging particle. SRIM simulations show that its range is of the order of 10 nm at 10–30 keV, with around a few hundred vacancies being created on its trajectory. The recoil energy has been computed for neutrons in the MeV energy range, see **Figure 1**. In most cases, a thermal treatment above room temperature is used to remove radiation damage. In Ref. [31], recombination-enhanced annealing using minority-carrier injection was applied but with no significant results, at least at room temperature. At low temperature, when the defects are not stable, no improvement could be observed with this method. However, a strong dependence of the annealing stages on the material type (p or n) was observed in Refs. [3, 25]

For a long time, materials alternative to HPGe have been proposed, and corresponding detectors were fabricated. GaAs is one example (mostly for X-ray detection), and, more importantly, CdTe. The goal was to eliminate the need for cryogenic operation by the use of large band gap semi-insulating material (diamond is also considered). Many difficulties still exist, mostly related to the defect density that is much higher in binary materials [9]. The other drawback is the possibility to grow large crystals, which proves to be more difficult for alternative materials. Large diamond single crystals with a low nitrogen content are difficult to grow as they need high-pressure processing. However, some CdTe photon detectors using segmented crystals for photon identification have been successfully implemented on space missions (INTEGRAL) [24]. Segmentation allows reducing drift length and therefore trapping, so that the resolution can be maintained at an

**74**

**Figure 1.**

*SRIM simulation showing the vacancy distribution for a Ge recoil of 30 keV.*

Particle identification through pulse shape discrimination is one of the developments that are used mainly in nuclear physics [35]. These techniques can be adequately used to determine the region of the detector where the interaction took place. There have been early reports for methods of particle identification in germanium and silicon detectors [36, 37]. The detection of photons is basically through ionization, and there is no important interaction with the nucleus, which could transfer momentum to the nucleus. If we consider other particles with a significant mass, their interaction with the nucleus may induce the recoil, which in turn is slowed down in the detector material. The slowing-down process results in ionization (electron-hole pair generation), defect creation (vacancies and interstitials), and phonon creation. One should note that according to SRIM simulations, in the typical energy range for the recoil (20–30 keV), the most important contribution (73%) is from phonon emission. The energy used for vacancy creation is of the order of 3%, which indicates that defect monitoring [38] could provide an alternative way to estimate the total integrated flux of interacting particles, since when stable defects are created, their concentration is proportional to the total number of interacting particles in a given time interval, which can be very long. The rest of the energy is used for ionization (25%). This result is close to the experimental result obtained for cryogenic detectors, as it will be discussed in the paragraph on dark matter detection.

### **3.6 Simulation**

Simulations of the interaction of photons with matter have been given a certain attention. Monte Carlo codes have been developed. Recently, the NWEGRIM code [18] from Pacific Northwest laboratories has been used to simulate the interaction of photons in silicon, but these simulations could also be applied to germanium. GEANT4 has been used for simulation of charged particles interacting with germanium [39].

### **4. Gamma-ray tracking arrays**

High-purity germanium γ-ray tracking arrays, such as AGATA (Advanced GAmma Tracking Array) [40] and GRETINA/GRETA (Gamma Ray Energy Tracking Array) [41] represent the state-of-the-art in high-resolution γ-ray spectroscopy for nuclear physics experiments. These spectrometers are composed of highly segmented large-volume HPGe crystals. Pulse-shape analysis, applied to the recorded signals from the segments, yields three-dimensional interaction positions with a typical precision of about 2 mm [40, 41]. Subsequently, a γ-ray tracking algorithm is applied to the determined interaction points in order to group and order them in sequences corresponding to individual γ-rays. In this procedure, the geometrical criteria and the Compton scattering formula are used, and the full energy of a γ-ray is determined as the sum of the energies of the interactions ascribed to the same trajectory.

The γ-ray tracking arrays provide improved energy resolution for in-beam nuclear physics studies, thanks to much reduced Doppler broadening as compared to standard γ-ray spectrometers. The use of γ-ray tracking also eliminates the need for Compton-suppression shields, commonly used with HPGe crystals in order to improve the peak-to-total ratio. Consequently, the entire 4π solid angle can be surrounded with Ge crystals, which leads to significantly increased detection

efficiency. The resolving power of a 4π γ-ray tracking array is estimated to be up to two orders of magnitude better than that of the existing conventional γ-ray spectrometers, depending on the physics case [41]. This is particularly important for studies of very exotic nuclei far from stability, employing weak radioactive-ion beams at intermediate energies (up to several hundred MeV/u), which leads to recoil velocities that may exceed 30% of the speed of light [42].
