Elastically scattered radioactive beam particles

The TIARA array is designed primarily to enable experiments with the weakest radioactive beams that can be used for transfer studies, which means beam intensities of typically 10⁴ to 10⁶ pps. In these cases, there are likely to be few problems due to the radioactivity of scattered beam which is stopped inside the compact particle array. For more intense beams, however, the radioactivity of the scattered particles may cause an unacceptably high counting rate in the surrounding gamma-ray detectors (depending also on the probability of beta-delayed gamma-ray emission). In such cases, it will be necessary to allow the scattered beam to be removed from the region around the target so that it is further from the gamma-ray detectors. The division of the array into a barrel and separate annular detectors provides a natural division of scattering angles. The precise value of the division should take into account the kinematic focussing typically found in transfer reactions. For such reactions, when the forward laboratory angles contain the main part of the yield, a coverage of 30 to 40 degrees in the laboratory frame generally includes the most structured parts of the angular distributions. This angular range is approximately the maximum that can be spanned by a single silicon detector at a reasonable distance from the target. It is also sufficient to allow a suitable amount of scattered beam to escape to a greater distance (recalling that the Exogam array is designed to ensure only that the forward $22.5^{\circ }$ escapes). For experiments that employ relatively intense radioactive beams (say, 10⁸ to 10⁹ pps), it could well prove necessary to remove the forward annular detector so as to avoid trapping radioactive beam particles too close to the gamma-ray array around the target. These experiments might be fusion-evaporation experiments or Coulomb excitation experiments, in which cases the barrel detectors are most important in terms of yield and the loss of the forward annulus can perhaps be tolerated. Alternatively, for a transfer reaction in these circumstances, the forward annulus could be retained if the primary beam were high enough in energy to punch through the thickness of silicon that substantially stops the light reaction products. Clearly, a number of possibilities present themselves when the forward annulus needs to be removed, including the provision of a larger area detector such as a PPAC at a greater distance from the target. The design of the TIARA array will allow for the forward annulus to be moved along rails to larger distances from the target. There is also provision for a PPAC detector to be employed as an alternative. Thus, whilst the array is intended primarily for use with weaker beams, there are solutions built into the design to deal with radioactivity problems from more intense beams, should these problems arise for particular experiments.