Detectors
There are several types of detector at GANIL, depending on the nature of the radiation to be studied. The EXOGAM multi-detector, for example, is dedicated to the observation of gamma ray radiation (see below). At GANIL, there are other detectors used for detecting neutrinos or charged particles.
The example of EXOGAM
EXOGAM is a high-efficiency gamma radiation detector, dedicated to nuclear spectroscopy of the exotic nuclei produced at GANIL.
Why was EXOGAM built?
It is dedicated to the study of the structure of atomic nuclei, one of the fields of fundamental research at GANIL. A wide European collaboration including Germany, Spain, Denmark, Finland, France, Poland and UK initiated this project. The name EXOGAM comes from the words Exotic and Gamma.
The main characteristics of EXOGAM
|
| For example, how can it be shown that the nucleons of a nucleus are located at discrete energy orbitals, as predicted by quantum mechanics? One method is to detect the gamma photons and then check whether they have well-defined energies corresponding to the differences in energy between the orbitals that the nucleons are likely to occupy. Similarly, detecting gamma photons may provide information on the way protons and neutrons vibrate when the nucleus is perturbed, or on the temperature of a nucleus resulting from the fusion of two smaller nuclei. |
The specificities of EXOGAM
EXOGAM is used to detect and measure the energy of gamma photons. Although its basic principle, as well as its appearance, are entirely different, EXOGAM could be compared with a photographic camera with an extremely fast exposure time, allowing it to record a very small light flash emitted when a nucleus is de-excited.
► The beam measurement principle
EXOGAM is comprised of 16 detectors using a high-purity germanium crystal, and are assembled according to a highly compact geometry. When gamma photons penetrate the crystal, some of them release all or part of their energy. This energy is transformed into a displacement of the germanium atom's electrons, thereby generating a very brief electric signal (0.00000001 second) as an output. This signal is then amplified by a series of electronic modules. The amplitude of the signal eventually obtained is indicative of the photons' energy.
► High precision measurements
Until 2001, the accelerated ions were derived from stable atoms such as those found on Earth. Now that SPIRAL is in operation, the GANIL is capable of accelerating beams said to be "exotic".
The methods used to produce, and then accelerate, exotic nuclei, is not the same as for stable nuclei. The number of nuclei accelerated per second by SPIRAL is 100 to 1000 times smaller than that achieved until now for the accelerated beams of stable nuclei. To compensate for this deficiency, and observe phenomena having a very small probability, EXOGAM was designed to detect approximately twice as many gamma photons as could be achieved with the previous detectors of a similar type.
| Example: If 100 photons are generated during nucleus de-excitation, EXOGAM is capable of measuring the energy of 20 of these. The other 80 will unfortunately pass through EXOGAM without being detected. |
► Just another tool
The GANIL possesses many detectors, which may be coupled to EXOGAM in order to measure complementary parameters. Since EXOGAM was designed for this purpose, it is modular with a variable geometry, and may be placed in other GANIL experiment chambers. It may also be coupled to a spectrometer, in order to provide measurements with a higher precision.
Coupling EXOGAM with the VAMOS spectrometer.
