The wide variety of products emitted by hot nuclei, when they become de-excited, requires a detector combining high detection efficiency with a geometrical coverage close to 4Pi, so that an extended range of energies can be measured, from approximately 1 Mev to 1 GeV, in order to obtain a comprehensive characterization of collision kinematics, on an event-by-event basis.
In the early 90s, four French laboratories (DAPNIA, IPNO, Caen's LPC and GANIL) thus initiated a collaborative effort, to build a detector that would meet the following requirements:
- Wide angular coverage: ~90% of 4Pi
- High granularity
- High dynamic range in energy, with small detection thresholds of ~1 MeV/A
- High charge resolution, of up to Z ~ 50
- Isotopic identification of light charged particles
Building on the experience gained with the first 4Pi detectors installed in France, namely NAUTILUS at GANIL and AMPHORA at S.A.R.A., INDRA (an acronym for "Identification of Nuclei and Detection with Increased Resolutions") is specifically designed for the study of the decay properties of hot nuclei, formed during heavy ion collisions with energies ranging between 30 and 100 MeV/nucleon.
Based on simulations, it was possible to assess the average fragment multiplicities (~10), as well as the multiplicities of light particles (~ 35), which can be expected with central collisions for intermediate-mass systems. To maintain the multiple detection rate below 5%, it is desirable to have an optimum number of detectors, namely about 300 cells for light particle identification and 80 cells for fragment identification. These orders of magnitude were the starting point for the modular design of the 4Pi devices, and an INDRA geometry adapted to the study of multi-fragmentation.
INDRA is composed of 17 detection rings covering 90% of the geometrical space surrounding the target. Each ring is subdivided into a variable number of sectors (8, 12, 16, or 24) to account for focusing effects at small angles within the angular distributions of the particles, and to maintain an approximately constant granularity at the reaction's centre of mass. These rings comprise 336 modules combining different types of detector, each covering the same angular range. A schematic diagram of this device shows that there are three detection areas:
- from 2° to 3°, ring 1 is made up of 12 plastic scintillators capable of supporting the high counting rates that result from small-angle elastic scattering
- from 3° to 45°, because of the high energy dynamics of the reaction products, rings 2-9 comprise 3 detection stages including one ionization chamber, followed by a wafer with 3 or 4 silicon detectors, behind each of which is placed a cesium iodide (CsI) scintillator
- from 45° to 176°, since the expected energy range for fragments at the back angles is reduced, rings 10-17 include just 2 detection stages: an ionization chamber coupled to 2, 3 or 5 CsI scintillators. Each ring, however, is provided with a calibration telescope comprising a 80-µm thin silicon detector, followed by a 2-mm Si(Li) detector, for the energy calibration of the CsI scintillators.
The holes, including that which permits the beam to be transmitted (from 0° to 2° and from 176° to 180°), the target (from 88° to 92°), and the mechanical structures supporting the detectors, represent INDRA's dead zones. These lead to a decrease in the available solid angle, by 0.2%, 3.5% and 6.3% of 4Pi, respectively. Under operating conditions, the experimental set-up, which is approximately 2 m in length and 0.8 m in diameter, is placed inside a vacuum chamber. INDRA thus has a total of 628 detectors:
- 96 ionization chamber (ChIo).
- 180 x 300 µm thick silicon detectors
- 324 thallium-doped, cesium iodide scintillators (CsI)
- 16 calibration detectors.
- 12 NE102-NE115 plastic scintillators (phoswiches).
The combination of several types of detector makes it possible to identify, by means of the conventional DeltaE-E method, reaction products having different charge and energy characteristics. For particle identification, the first detection stage must be traversed. The choice of a gas detector for this stage is a unique characteristic of INDRA. This stage has been designed and built by a group of physicists from SPhN's technical departments.
To learn more about the INDRA multi-detector and its electronics, refer to the articles published by J. Pouthas et al., which describe the detector's technical characteristics, as well as the numerous enhancements achieved in its electronics.
|1||Pouthas (J.), Borderie (B.), Dayras (R.), Plagnol (E.), Rivet (M.F.), Saint-Laurent (F.), Steckmeyer (J.C.) et al. - INDRA, a 4 charged product detection array at GANIL. - Nucl. Inst. Meth. A, vol. 357, 1995, p. 418.|
|2||Pouthas (J.), Bertaut (A.), Borderie (B.), Bourgault (P.), Cahan (B.), Carles (G.), Charlet (D.), Cussol (D.), Dayras (R.), Engrand (M.), Jouniaux (O.), Le Botlan (P.), Leconte (A.), Lelong (P.), Martina (L.), Mosrin (P.), Olivier (L.), Passerieux (J.P.), Piquet (B.), Pagnol (E.), Plaige (E.), Raine (B.), Richard (A.), Saint-Laurent (F.), Spitaels (C.), Tillier (J.), Tripon (M.), Vallerand (P.), Volkov (P.) et Wittwer (G.). - The electronics of the INDRA 4 detection array. - Nucl. Inst. Meth. A, vol. 369, 1996, p. 222.|