A1900 Momenum Acceptance: Slits and Momentum Correction at I2

The A1900 has a full momentum acceptance of 5% (i.e. ±2.5%). However, with the standard particle identification strategy described elsewhere, it is usually not possible to use the full momentum acceptance without losing the resolution among the different isotopes reaching the focal plane. Since the A1900 optics are set up so that the momentum of a fragment maps into its horizontal position at I2, it is possible to use horizontal slits at I2 to cut the momentum acceptance. The I2 slits, therefore, provide a means for maintaining the PID resolution at the focal plane. The cost of this strategy, however, is a loss in particle yield. This loss is roughly proportional to the momentum acceptance being cut by the slits.

The I2 PPACs provide a way to recover the particle identification resolution at the focal plane while making use of the full A1900 momentum acceptance. The large size of the I2 PPACs – 400 mm (horizontal) by 100 mm (vertical) – accommodates the broad fragment distribution at I2. The strategy here is to introduce a correction to the time-of-flight signal based on the observed horizontal fragment position (hence, momentum) at I2. This approach can be used for both A1900-only experiments and for experiments performed downstream.

An alternative way to recover the particle identification resolution at the focal plane while making use of the full A1900 momentum acceptance is to use the I2 thin timing scintillator. In this detector a position signal can be derived from the time difference in the light from a fragment as observed by the two photomultiplier tubes – one at each end of the scintillator in the horizontal direction. The advantage of using this detector rather than the I2 PPACs is that it can operate at a higher readout rate. The disadvantage is that it has a lower position resolution and, therefore provides a less precise correction based on the momentum. This approach also can be used for both A1900-only experiments and experiments performed downstream.

Currrent information on the performance characteristics and acceptable operating environments for the I2 PPACs and the I2 scintillator can be found in the A1900 Service Level Description (PDF).