Clover (detector)

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, 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 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

References

1. (1999). "The Clover: a new generation of composite Ge detectors". Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment.
2. (2008). "From Ge(Li) detectors to gamma-ray tracking arrays–50 years of gamma spectroscopy with germanium detectors". Progress in Particle and Nuclear Physics.
3. Jones, P.M.. (August 1985). "Calibration of the new composite "clover" detector as a Compton polarimeter for the EUROGAM array". Nucl. Instrum. Methods Phys. Res. A.