Present:
Chris Allgower, Andy Bacher, Hermann Nann, Tom Rinckel,
and Ed Stephenson
ANALYSIS:
TARGET TRACEBACK
John discussed using MWPCs XY-1 and XY-2 to trace rays back to their point of origin in the target. In order to do this, you have to compensate for the focussing in the septum magnet. John scaled the slope in the X2-X1 and Y2-Y1 correlations to its expected value near 1.5, having found that it was about 1.2 for Y and 1.8 for X. This produced a tight spot for the 3-He events in the commissioning run.
[This has been run later on alpha+pi0 candidates and looks good, see below.]
TIMING
Pb-glass timing has been added to the mix. The calibration of the TFC's on the Pb-glass arrays appears to be about 17.3 channels/ns. The problem is that for alpha+pi0 events, this appears at the wrong channel. Some discussion led to the suggestion that this is an artifact of the change in trigger timing. [This was later checked and found to be correct.]
SELECTING A FEW GOOD EVENTS
Based on having surveyed some 3-He events, Ed presented energy spectra for the Pb-glass based on summing the highest energy Pb-glass counter and its 20 nearest neighbors. These were grouped into four subclasses, and it was found that each had contributions to make for some events. The classes are (1) 4 neighbors on each of 4 sides, (2) 4 neighbors at the corners, (3) 4 neighbors out form the sides but with one detector in between, and (4) the 8 neighbors next to class 3 (knight's move away in chess).
The spectra show lots of no-hit events, and exponential tail, and a broad distribution that extends above channel 1000. The broad distribution is not well separated from the exponential tail. In order not to be confused with some stray events, cuts were placed just above the exponential noise. These are at channels 320 on the left and 470 on the right, where channel 500 is the place to which cosmic ray peaks are adjusted.
Ed presented replays based on having all of the latest cuts in place, including (1) pulse heights in the channel scintillators, (2) correct range of time of flight, as observed in the reconstructed missing mass, (3) Pb-glass timing, and (4) Pb-glass pulse heights. The Pb-glass information was divided into two classes, one that contained one hit on each side for a total of two hits, and one that was based on seeing only one hit (with the two hit as a subset). There were 12 events in the first class and 22 in the second. Ed showed a series of spectra. When the spectrum was used for a gate, that gate was taken out so as to see what other events might be included. For all but missing mass in the first (2-hit) class, no other events were introduced. This suggests that we now have a rather robust set of conditions and that there is a high probability that events which pass these cuts are valid alpha+gamma+gamma events. The single-hit condition was less robust, but still all cuts seemed to give a valid set of events when working together.
In the end, the missing mass of these small event sets are distributed through the allowed range, suggesting that they all belong to double radiative capture. There is no enhancement at the pion mass.
OTHER
PUMPS
Jack has now replaced the lower stage-2 cryopump with a new one.
WHERE ARE THE PB-GLASS?
Chris Allgower has started a project to use cardboard cutouts fitted to the front and back of the Pb-glass arrays to map out the detector locations for each layer. Once the cutouts are made, they are attached to full scale pieces of paper at the right location. Then lines are drawn to mark the boundaries between detectors. From this we should later be able to locate each detector for the purpose of including it in a GEANT model of the Pb-glass and making a table for on-line sorting.