Experimental solutions

A novel solution to the relatively low intensities for radioactive beams is to use a gas-filled time-projection chamber, in which the detector also acts as the target. Elastic scattering has been studied in this way with the detector IKAR [16]. With more conventional detectors, solid targets are most useful since they help to define the scattering angle more precisely, and (CH₂) _n and (CD₂) _n targets have been used successully. The experimental options for such studies of transfer reactions, detecting charged particles from reactions in inverse kinematics, can be grouped into three categories:

1.

Rely on detecting the beam-like ejectile in a magnetic spectrometer,

2.

Rely on detecting the target-like ejectile in a position sensitive detector,

3.

Detect decay $\gamma$-rays in addition to charged particles.

The resolution considerations for methods 1 and 2 have been analysed in the case of neutron transfer [17]. Method 1, employed for p(11Be,10Be)d [4], has favourable kinematical focussing, unless the beam mass is so great that the angular resolution limits the resolution. Also, any spread in beam energy translates directly into the excitation energy E _</I>x and hence must be corrected by either particle-by-particle energy tagging, or a dispersion matched spectrometer. A further inherent limitation on E _</I>x resolution is imposed by broadening from $\gamma$-ray emission in flight. Coincident light particles reduce the background [4] but span a large angular range. Method 2 was used to study d(56Ni,57Ni)p [11]. The effect on E _</I>x of any spread in the beam energy is minimized by the inverse kinematics (cf. fig.1a). The E _</I>x resolution is limited mainly by the target thickness, via the energy loss of the light ejectile, imposing a maximum thickness of around 0.5-1.0 mg/cm² for (CH₂) _n. This implies that experiments are feasible with beam intensities $\sim 10^4$ pps. Method 3 is attractive because it offers improved energy resolution, in the case of bound final states. Current Ge $\gamma$-ray arrays can achieve absolute efficiencies of $\sim 25$% whilst also retaining sufficient angular resolution to limit Doppler broadening. The $\gamma$-ray energy information then allows a relaxing of the target thickness limits, so targets $\geq 4$ times thicker can be used and still give improved E _</I>x resolution ($\sim 40$ keV) and a higher net counting rate. The angular distributions of unresolved particle groups can then be extracted by careful analysis, using the $\gamma$-ray energy. A detector array (TIARA) optimised for this type of experiment is being developed in the UK, to be used with EXOGAM and VAMOS at GANIL.