Reasons for a partial detector

Already a few months after the July '96 renewal of the PSI -- AMCRYS-H contract for the PIBETA CsI crystals it became clear that the rate of deliveries was not fast enough for a timely completion of the calorimeter.

The status on 23 October 1996 was as follows:

Crystals in hand at PSI: 94

Delivery rate of crystals passing our specs: 9 crystals/6 weeks = 1.5 cr/wk

Foreseen completion of CsI deliveries: (240 - 94)/1.5 = 97 weeks, i.e., allowing for surface treatment and calibration measurements, the detector could not be completed before Oct. 1998 and measurements could start no sooner than 1999.

In order to ensure rational use of the resources at our disposal, as well as timely advancement of the graduate student collaborators and continued vitality of the collaboration, it is imperative to have an operational device earlier.

Plans for a partial calorimeter

Several options exist for a spatially symmetric partial calorimeter:
  1. an "equatorial" ring,
  2. two "polar" caps,
  3. a "polar" (meridional) ring, where the calorimeter "poles" are determined by the beam entry axis.

    The third option, a polar ring, provides the best combination of detector acceptance and speed of completion given the mix of modules already in stock at PSI. The proposed design consists of 124 active modules. The balance of the missing modules are expected to be delivered at PSI by April/May 1997.

    For details of the partial calorimeter layout see the Mercator projection drawing of the polar circle detector. The shaded modules are active, while the unshaded modules would be blanks. Of course, any additional detector modules available at buildup time would be used instead of blanks. The acceptance of this detector expressed as a fraction of the acceptance of the full sphere is 52.9 +/- 0.7% for the pi -> e nu events, and 51.3 +/- 0.7% for the pion beta decay events (the quoted uncertainties reflect the Monte Carlo statistics.)

    The main benefit of a partial calorimeter is reflected in running one year earlier with ~50% acceptance. Additionally, this would provide us with valuable experience prior to the buildup of the entire detector.

    Additional costs

    The main additional cost associated with an early implementation of a partial PIBETA calorimeter is in personnel effort involved in the buildup and disassembly of the setup. The assembly is estimated to take at least 3 months with ~4 persons involved at any time. Physical restrictions of the spherical housing (Kugelgehaeuse) do not permit more than a few people working at the same time. The costs of machining the blanks would not be prohibitive, about 10 kSF or less, because the PSI shop is already set up to make them using computer controlled machines.


    Jan. - May 1997: Surface treatment and calibration measurements of the remaining CsI modules.

    June/July 1997: Acceptance calibration in beam, using the 1/4 sphere setup as described in the Nov. '95 Update.

    Aug. - Dec. 1997: Build up the half-sphere detector, cable it and commence cosmic muon runs.

    1998: Start in-beam shakedown and running. Collect a statistically significant data set of a few thousand pion beta decay events.

    P.S. (16 December 1996)

    The rate of deliveries of CsI has picked up in the last two months, as has the quality of the crystals, both in terms of mechanical tolerances and fast/total light component ratios. The current delivery rate is 2 cr/wk , and we have now 104 accepted crystals at PSI. If this trend should hold, we may have all of the crystals in hand before May 1998 . We clearly need to reevaluate the CsI inventory status and our plans in a few months' time.

    D. Pocanic, Nov/Dec 1996