**5. Dark matter direct detection and related studies**

The last two decades have seen an important effort devoted to search for dark matter, using underground direct detection apparatus [36, 37, 43–47]. The first stage was to design a detector that is able to discriminate between particles in order to identify the so-called weakly interacting massive particles that are thought to be a constituent of dark matter. The developed detectors include two channels: a "thermal" channel which is based on the thermalization of phonons and a ionization channel which is proportional to the number of carriers collected. In reality, the so-called Luke-Neganov effect [48, 49] affects the properties of the detector. The Luke-Neganov effect consists in the amplification of the phonon signal by the electron (carrier) drift [50].

The charge signal may be expressed as:

$$\mathbf{s}\_{\mathbf{c}} = \frac{\mathbf{E}}{\mathcal{E}} \mathbf{q},\tag{5}$$

where E is the average energy for electron-hole pair creation, and

$$\mathbf{s}\_{\rm T} = \frac{\mathbf{E}}{\mathbf{e}} \mathbf{q} \mathbf{V} + \mathbf{E} \tag{6}$$

The ST term is proportional to the charge multiplied by the voltage drop, so we can write that if t is the time necessary for the charge to be collected,

$$\mathbf{I} = \mathbf{S} \mathbf{c} / \mathbf{t} \tag{7}$$

**77**

**Figure 2.**

*DM cryogenic experiments such as EDELWEISS.*

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

entire energy deposited by the recoils, except for the fraction needed for vacancyinterstitial (defect) creation. If a neutron is fully absorbed in the bolometer following multiple scattering, the total energy of all recoils matches the initial energy of the impinging neutron, and therefore the heat channel signal reflects

The calibration runs yielded the ratio of the ionization signal to the phonon energy equal to 30%. This is very close to the value of 25%/73% = 0.35 determined in a SRIM simulation. When the particle interacts with matter purely via ionization, as it is the case for photons, the ratio of ionizing energy to the total energy is close to one, if no Luke-Neganov amplifying occurs. Therefore, this configuration can be used for particle discrimination [46]. For the WIMPS experiments, if we consider that these particles interact with the nuclei of the detecting media, this provides a way of discriminating the photons from other events. At this cryogenic energy (a few mK), the shallow and deep levels may disturb the charge transport through the detector (**Figure 2**). This is the reason why the detector is saturated with photogenerated carriers prior to data acquisition. In spite of this, more detailed defect studies on the starting material should be made [52]. In particular, as very shallow levels may have an impact, the nonthermal carrier emission or capture should be studied. Additionally, a method using alternately biased electrodes on each surface of the detectors, including the sides, has enabled the measurement of volume-only events, eliminating near-surface events related to low-energy particles strongly interacting with the medium [46]. Other techniques have been developed [53] to

*Diagram showing the energy of the trapping level in Ge band gap versus temperature, using a simple Boltzmann factor. This is valid at high temperature for thermal emission with a time constant Tau. The region investigated by DLTS is delimited by the upper square on the right. The purple arrow shows the operation of* 

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

the total energy deposited.

solve this problem.

First term: ItV, which is a Joule-like term. Energy = power × time = current × voltage × time = charge × voltage.

These relations indicate the need to operate at weak field to achieve a reduction in the contribution of the phonon signal. At high fields, the phonon signal tends to grow linearly with the applied voltage. Of course, the heat signal can only be detected if the calorific capacity is low enough, and so the operating temperature should be very low. Germanium detectors are also good cryogenic (mK range) bolometers. Another reason for the choice germanium is that its nucleus is nucleon rich (for instance compared with silicon). This should enhance interaction cross sections of WIMPS with the detecting medium. If we consider the interaction of fast neutrons with germanium (in the MeV range) as an example, the elastic-scattering cross section is much higher than the inelastic cross section [31] and is of the order of 3 barns [5]. Hence, the mean free path of these fast neutrons is close to a few centimeters. The recoil nucleus with an energy of a few tens of keV dissipates 25% of its energy into ionization, which amounts to a few keV (5 keV). The ionization channel monitors this fraction of energy. A similar phenomenon was observed, but not published, in planar HPGe 77 K detectors, exposed to neutrons, in the MeV range [51]. The heat channel monitors almost

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

entire energy deposited by the recoils, except for the fraction needed for vacancyinterstitial (defect) creation. If a neutron is fully absorbed in the bolometer following multiple scattering, the total energy of all recoils matches the initial energy of the impinging neutron, and therefore the heat channel signal reflects the total energy deposited.

The calibration runs yielded the ratio of the ionization signal to the phonon energy equal to 30%. This is very close to the value of 25%/73% = 0.35 determined in a SRIM simulation. When the particle interacts with matter purely via ionization, as it is the case for photons, the ratio of ionizing energy to the total energy is close to one, if no Luke-Neganov amplifying occurs. Therefore, this configuration can be used for particle discrimination [46]. For the WIMPS experiments, if we consider that these particles interact with the nuclei of the detecting media, this provides a way of discriminating the photons from other events. At this cryogenic energy (a few mK), the shallow and deep levels may disturb the charge transport through the detector (**Figure 2**). This is the reason why the detector is saturated with photogenerated carriers prior to data acquisition. In spite of this, more detailed defect studies on the starting material should be made [52]. In particular, as very shallow levels may have an impact, the nonthermal carrier emission or capture should be studied. Additionally, a method using alternately biased electrodes on each surface of the detectors, including the sides, has enabled the measurement of volume-only events, eliminating near-surface events related to low-energy particles strongly interacting with the medium [46]. Other techniques have been developed [53] to solve this problem.

#### **Figure 2.**

*Use of Gamma Radiation Techniques in Peaceful Applications*

recoil velocities that may exceed 30% of the speed of light [42].

**5. Dark matter direct detection and related studies**

electron (carrier) drift [50].

The charge signal may be expressed as:

sc = \_\_

sT = \_\_

voltage × time = charge × voltage.

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

The last two decades have seen an important effort devoted to search for dark matter, using underground direct detection apparatus [36, 37, 43–47]. The first stage was to design a detector that is able to discriminate between particles in order to identify the so-called weakly interacting massive particles that are thought to be a constituent of dark matter. The developed detectors include two channels: a "thermal" channel which is based on the thermalization of phonons and a ionization channel which is proportional to the number of carriers collected. In reality, the so-called Luke-Neganov effect [48, 49] affects the properties of the detector. The Luke-Neganov effect consists in the amplification of the phonon signal by the

E

E

The ST term is proportional to the charge multiplied by the voltage drop, so we

I = Sc/t (7)

First term: ItV, which is a Joule-like term. Energy = power × time = current ×

These relations indicate the need to operate at weak field to achieve a reduction in the contribution of the phonon signal. At high fields, the phonon signal tends to grow linearly with the applied voltage. Of course, the heat signal can only be detected if the calorific capacity is low enough, and so the operating temperature should be very low. Germanium detectors are also good cryogenic (mK range) bolometers. Another reason for the choice germanium is that its nucleus is nucleon rich (for instance compared with silicon). This should enhance interaction cross sections of WIMPS with the detecting medium. If we consider the interaction of fast neutrons with germanium (in the MeV range) as an example, the elastic-scattering cross section is much higher than the inelastic cross section [31] and is of the order of 3 barns [5]. Hence, the mean free path of these fast neutrons is close to a few centimeters. The recoil nucleus with an energy of a few tens of keV dissipates 25% of its energy into ionization, which amounts to a few keV (5 keV). The ionization channel monitors this fraction of energy. A similar phenomenon was observed, but not published, in planar HPGe 77 K detectors, exposed to neutrons, in the MeV range [51]. The heat channel monitors almost

where E is the average energy for electron-hole pair creation, and

can write that if t is the time necessary for the charge to be collected,

<sup>ε</sup> q, (5)

<sup>ε</sup> qV + E (6)

**76**

*Diagram showing the energy of the trapping level in Ge band gap versus temperature, using a simple Boltzmann factor. This is valid at high temperature for thermal emission with a time constant Tau. The region investigated by DLTS is delimited by the upper square on the right. The purple arrow shows the operation of DM cryogenic experiments such as EDELWEISS.*
