Characteristics of SPEG
Description
The SPEG spectrometer is situated in the hall G3. The complete device consists in an analyser (line with quadrupoles and a large dipole DA up to the target point) and in the spectrometer proper, situated after the target point. The target point is the image point of the analyser and the object point of the spectrometer.
Different target chambers can be mounted, according to the desired target-position detection setup. A very large, multi-purpose chamber is used to install charged-particle detection arrays (such as MUST2) in vacuum. A much smaller chamber is used in combination with gamma detectors (such as the BaF2 scintillators of the Chateau de Cristal, or the Ge detectors of EXOGAM) and neutron detectors.
To measure angular distributions, the spectrometer can rotate between -10 and +105 degrees.
Principle
[main reference: L. Bianchi et al., Nucl. Instrum. Methods Phys. Res. A 276 (1989) 509]
The ensemble of the spectrometer is achromatic, i.e. all particles at the object point are re-focused on the focal plane irrespective of the energy or the angle of the particles.
If however a material (target) is inserted at the object point, the interacting particles changing their momentum by a quantity Δp are separated at the focal plane by the quantity:
Δx = D × Δp/p
where D is the dispersion of the spectrometer (8.1 m). In this sense, SPEG is an energy-loss spectrometer. A resolution at the focal plane of about 1 mm allows to reach 10-4 in momentum resolution, thus overcoming the intrinsic momentum dispersion of the incident beam which is 10-3. For incident heavy-ion beams at an energy of ≈100 MeV/nucleon, the energy of populated states can be measured with a resolution of 100 keV.
Characteristics of the analyser
| Dispersion | 10 m |
| Horizontal magnification | 0.175 |
| Vertical magnification | 1 |
| Mean radius of trajectory in the dipole | 3 m |
| Dipole deviation angle | 75 degrees |
| Angle entrance face dipole | 23.5 degrees |
| Dipole maximum induction | 1 tesla |
| Weight | 33 ton |
| Dipole structure | "Window-frame" |
Characteristics of the spectrometer
| Nominal dispersion (adjustable) | 8.1 m |
| Horzontal magnification | 0.8 |
| Vertical magnifcation | 4.7 |
| Maximum aperture, horizontal | ±2 degrees |
| Maximum aperture, vertical | ±2 degrees |
| Maximum solid angle | 4.9 msr |
| Mean radius of trajectories in dipoles | 2.4 m |
| Deviation angle of each dipole | 42.5 degrees |
| Angle of exit face of second dipole | 24 degrees |
| Momentum acceptance | 7% |
| Length of focal plane | 60 cm |
| Angle of focal plane | 8 degrees |
| Dipole maximum induction | 1.2 tesla |
| Weight | 200 ton |
| Structure of dipoles, entrance quadrupole and sextupole | "C" |
| Exit quadrupole | Collins type |
Detection ensemble in the spectrometer
The detectors, placed after the spectrometer dipoles, allow identifying the products of the reactions, but also the reconstruction of the momentum distribution (parallel and transverse) and the associated diffusion angles.
The setup is composed of 1) two drift chambers (CD1 and CD2 in the figure) for the measurement of the position (in x and y); 2) an ionisation chamber (CHIO) for the measurement of the energy loss ΔE ; 3) a plastic detector for the residual energy E and the time-of-flight (with respect to a beam detector, usually placed immediately before the target).
The focal plane is situated between the two drift chambers.
