The primary goal of T-438 was to study the response of a prototype shower-maximum detector (SMD), constructed from triangular cross section scintillating strips with the apex-to-base height of 0.5 cm. The strips employed in the two planes of the prototype SMD (60 vertical strips for x plane and 100 horizontal strips for y plane) were produced by the plastic extrusion company RDN for the D0 collaboration. Alan Bross (FNAL) graciously loaned us material for these tests.
One test of the response of the SMD was to expose the prototype detector to the FFTB beam, suitably defocused so that some of the two-electron events incident on the calorimeter would be spatially separated by as much as 1 cm. Below is a figure showing two individual two-electron events for a run at 10 GeV where the beam was defocused. The selection of two-electron events was made by demanding that the observed tower 7 pulse height corresponded to the energy deposited by two 10-GeV electron showers. In the picture below the SMD x- and y-plane response for one event is shown in the top row, and is shown for a different event in the bottom row.
The plots in the left-most column correspond to the response of the SMD x-plane, and those in the right-most column correspond to the response of the SMD y-plane. In all cases, what is displayed is the raw ADC count (uncorrected for known pixel-to-pixel gain variations in the on-line analysis) for individual pixels from the Hamamatsu H6568, a 16-anode multi-anode photomultiplier tube (MAPMT). Light from the SMD scintillator strips is collected by wave-length shifting optical fibers, and then transported via clear optical fibers to one of 16 pixels on one of 10 MAPMT.
In the event shown in the top row there is good evidence for a separation of the electromagnetic showers produced by the two incident electrons. It is unlikely that this separation is due to gain variations from these MAPMT pixels, since the SMD x-plane response for the event shown in the bottom row shows a reasonably symmetric peak involving the same strips. Two 10-GeV electrons produced showers in the calorimeter similar to a 40 GeV neutral pion, decaying into two equal-energy photons.
In the offline analysis of the T-438 data, an application of the ‘shower-shape analysis’ algorithm will be applied to events of this type to quantitatively establish if the simulations of the performance of the STAR endcap electromagnetic calorimeter SMD are accurate. Similar data were obtained at both 10 and 20 GeV incident energy, scanning the defocused FFTB beam both horizontally and vertically across the prototype.
L.C. Bland (IUCF), for the T-438 collaboration
Last revised: 31 October 1999