The VAMOS spectrometer

The VAMOS spectrometer is a variable mode, high acceptance magnetic spectrometer designed specifically for radioactive beam experiments including experiments performed with Exogam. It is the subject of a Memorandum of Understanding between CEA and CNRS/IN2P3 (France), GSI Darmstadt (Germany) and the EPSRC (UK) and is funded within the Exogam project. The Surrey personnel in the TIARA team have been involved with the design and planning of VAMOS from its inception. The magnetic design is QQ-Wien-D. The main features of VAMOS are summarized in Table 2.

 

Table 2: Main properties of the VAMOS spectrometer, being built at GANIL as part of the Exogam project.

Property	Description
Solid angle	130 msr
Angular acceptance	$\pm$200 mr
Dispersion $(x \mid \delta )$	2 cm/%
First order resolution $(x \mid x)x_0 \,/\, (x \mid \delta )$	1/1000
Mass resolution $\Delta M/M$	1-4 % (limited by energy measurement)
Target-focal plane distance	4 m
Dipole radius	1 m
Dipole bending angle	up to 57 degrees
Momentum acceptance	$\pm 5$%
Beam rejection	Wien filter $E\times B$, length 1 m
Rigidity $B\rho _{{\rm max}}$	1.3 T.m (>2.3 T.m for $\Delta \Omega < 42$ msr)

In the case of transfer reactions with TIARA, the potential advantages of using VAMOS in some experiments are the mass measurement it offers (in addition to Z-identification by $\Delta$E.E techniques) and the ability to reject the beam from the detectors, thus enabling a higher incident beam intensity to be employed in principle. These potential benefits are sufficiently important that compatibility with VAMOS has been explicitly taken into account in the design of TIARA. The main additional constraints that this adds are to do with the support of the Exogam gamma-ray detectors and the tight limits space availability at forward angles. In its full solid angle configuration, the entrance flange of the VAMOS vacuum vessel is very close to the target since the entrance quadrupole itself is only 40 cm from the target. TIARA has been designed to fit onto the VAMOS entrance flange to permit use of the full solid angle of the spectrometer. When used in stand-alone mode, an extra top-hat part of the vacuum vessel is simply used to extend the VAMOS flange and provide the mounting for the zero degree telescope at an extended distance from the target.