Detector charge collection

Charge collection. Electric fields within the photosensitive material collect charge into pixels. The detector can be designed to collect either electrons or holes. 

The short penetration depth of UV/blue photons is the reason that frontside CCD detectors have very poor QE at the blue end of the spectrum. The frontside of a CCD is the side upon which the polysilicon wires that control charge collection and transfer are deposited. These wires are 0.25 to 0.5 /xm thick and will absorb all UV/blue photons before these photons reach the photosensitive volume of the CCD. For good UV/blue sensitivity, a silicon detector must allow the direct penetration of photons into the photosensitive volume. This is achieved by turning the CCD over and thinning the backside until the photosensitive region (the epitaxial layer) is exposed to incoming radiation. 

Thorium. Multiple-collector measurement protocols by TIMS for thorium isotopic analysis typically involve the simultaneous measurement of Th and °Th (for silicate rocks), or Th and °Th, then Th and Th (for low- Th samples), using an axial ion counter and off-axis Faraday collector (Table 1). Various methods are used to correct for the relative gain between the low-level and Faraday detectors and 2a-uncertainties of l-5%o are typically obtained (Palacz et al. 1992 Cohen et al. 1992 McDermott et al. 1993 Rubin 2001). Charge-collection TIMS protocols enable Th, °Th and Th to be monitored simultaneously on a multiple-Faraday array and can achieve measurement uncertainties at the sub-permil level (Esat et al. 1995 Stirling et al. 1995). 

A new generation of detectors has been developed with NbSi thin-film sensors (instead of the NTD sensors for present detectors). Each detector consists of a Ge crystal with two NbSi sensors acting also as electrodes for charge collection. These thin-film sensors are sensitive to the athermal component of the phonon signal, acting as near-surface interaction tag [49],... 

Another kind of linear solid state position sensitive detectors are the Photo-Diode-Arrays (PDA s), which are different from CCD s. A PDA consists of an array of separate photodiodes, each with an associated capacitance and a multiplexing read-out system (see Fig. 21). The charges collected in each cell are simply switched to the output, one by one. Unlike in the case of CCD s, the photosensitive elements are separated completely from the transfer circuity. 

It is always the objective in either an ionizatin or a semiconductor detector to collect all the charges produced by the incident particle. This is achieved by establishing an electric field in the detector such that there is zero recombination of electrons and ions (or holes) before they are collected. In a semiconductor detector, even if recombination is zero, some charge carriers may be lost in trapping centers of the crystal, such as lattice imperfections, vacancies and dislocations. The incident radiation creates crystal defects that cause deterioration of the detector performance and, thus, reduce its lifetime (see Sec. 7.6). 

Capability of supporting strong electric fields. This property is related to property (1). Its importance stems from the fact that the stronger the field, the better and faster the charge collection becomes. Also, as the electric field increases, so does the depth of the sensitive region (Eq. 7.11a) for certain detectors. 

The measurement of particle energy with any type of detector is based on the assumption that the charge collected at the output of the detector is proportional to the energy of the incident particle. The assumption is valid if all the particle energy is lost in ionizing collisions and all the charge produced is collected, i.e., no recombination takes place. 

Most of the position-sensitive semiconductor detectors determine the position of the incident particle by employing the method of resistive-charge division To illustrate the method, consider the detector in Fig. 13.25. The detector is a reverse-biased p-n junction with electrodes on both front and back. The front electrode with considerable resistivity has two electrical contacts a distance L apart. The back electrode has low resistivity and provides a good electrical contact to the base material. When a particle enters the detector, electrons and holes are created that move under the influence of the electric field. If the resistivity of the front electrode is homogeneous, and charge-sensitive low-impedance amplifiers are used, the charge collected at one of the two contacts of the front electrode is proportional to the distance between the point of impact... 

Under the influence of an applied voltage, the electrons move toward the positive end and the holes toward the negative end of the detector. The total charge collected at the positive contact is ... 

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