A clover detector is a
gamma-ray detector that consists of 4 coaxial
N-type high purity
germanium
Germanium is a chemical element; it has Symbol (chemistry), symbol Ge and atomic number 32. It is lustrous, hard-brittle, grayish-white and similar in appearance to silicon. It is a metalloid or a nonmetal in the carbon group that is chemically ...
(Ge) crystals each machined to shape and mounted in a common
cryostat
A cryostat (from ''cryo'' meaning cold and ''stat'' meaning stable) is a device used to maintain low cryogenic temperatures of samples or devices mounted within the cryostat. Low temperatures may be maintained within a cryostat by using various ...
to form a structure resembling a
four-leaf clover
The four-leaf clover is a rare mutation of the common three-leaf clover that has four Leaflet (botany), leaflets instead of three. According to traditional sayings, such clovers bring good luck, a belief that dates back to at least the 17th cent ...
.
The clover is the first composite Ge detector. It remains widely used in the detectors of particle accelerators, where multiple clover modules form an array all around the target to capture the rays. More complex composite detectors, such as the 7-element hexagonal cluster detector used on the Euroball, offer even better data.
Operation
A gamma ray may interact with a single Ge crystal and deposit its full energy. The resulting charge collected will then be proportional to this energy. However, through the process of
Compton scattering
Compton scattering (or the Compton effect) is the quantum theory of high frequency photons scattering following an interaction with a charged particle, usually an electron. Specifically, when the photon hits electrons, it releases loosely bound e ...
, a gamma ray may interact with two (or possibly more) crystals resulting in the energy (and thus the liberated charge) being shared by the crystals. In this case, a process known as add-back, where the charge collected by each of the crystals is summed, can be used to determine the energy of the incident gamma ray.
In addition to add-back, the Clover also uses
Compton suppression using a fence of BGO detectors. A gamma ray can escape the Clover array via Compton scattering, causing incorrectly low charge values. A BGO detector can detect the escaped ray and have the computer ignore the wrong reading from the Clover.
Advantages
There are a number of advantages offered by using clover detectors as opposed to the more conventional single crystal
germanium detectors. Large volume high purity single crystals of Ge can be expensive. By mounting four smaller crystals in a common cryostat a detector of a given volume can be created at a reduced cost. In addition, the individual smaller Ge crystals present a smaller solid angle than a large volume Ge detector thus significantly reducing the effects of
Doppler broadening
In atomic physics, Doppler broadening is broadening of spectral lines due to the Doppler effect caused by a distribution of velocities of atoms or molecules. Different velocities of the emitting (or absorbing) particles result in different Doppl ...
on the resulting spectra.
Doppler broadening is further reduced using techniques that exploit the individual readings from the smaller crystals.
A clover detector can also be used to determine the electric or magnetic nature of the incident photons (e.g. if the gamma ray is an electric quadrupole or a magnetic dipole) as the Compton scattering process for these two types of radiation is different. This is called
Compton polarimetry.
References
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External links
CLover Array for Radioactive ION beams
Spectrometers
Particle detectors