previous next Up Title Contents Index

3.2.5 Calibration of the Light Yield

The statistical term in the expression for a detector's energy resolution (Eq. 3.a) is directly proportional to the light yield of the scintillating crystal. The relevant quantity that accounts for possible inefficiencies of the scintillator-phototube system is Nph, the overall number of photoelectrons per MeV energy deposition. Nph was measured in the RASTA-apparatus described above. The light yield of each CsI crystal was determined by a light emitting diode(LED)-based system, since LED light generates a fast 20 ns PMT anode signal. The LED was mounted on the back of the crystal to probe. It was pulsed at 1 kHz rate with adjustable driving voltage and the split output from the driver generated a 100 ns wide ADC gate. (In a separate measurement the ADC pedestal values were established.) The integrated number of photoelectrons from the LED corresponds to energy depositions between 10 and 100 MeV, depending on the LED voltage. From simulation one knows the average energy deposition of cosmic muons in the different CsI crystal shapes which lies between 39 and 51 MeV. Hence, an absolute energy scale was established and the LED light output was precisely cross-calibrated against the cosmic muon events in the crystals.

A total of 9 different LED amplitudes were used in data collection with each CsI crystal, since the variance ( s T) of the photodiode peak depends on the number of photoelectrons created for unit energy deposition. Hence

where E is the LED spectrum peak position, Nph represents number of created photoelectrons per unit deposited energy, and s i are assorted variances, such as instabilities of the LED driving voltage, temporal pedestal variations, etc[16]. The measured points are fitted with a linear function and such the number of photoelectrons per MeV for each CsI crystal was established. That number varies from 40 to110 photoelectrons/MeV.

Figure 3-13 Obtaining the number of photoelectrons by the slope of LED amplitude variances. The offset from zero is caused by noise sources different from Poisson statistics. Photomultiplier and Optical Coupling

The use of quartz window PMTs is mandatory due to the low emission wavelength of the scintillator crystals. In order to accomplish linearity and gain-stability 3 inch EMI 9822QKB and 2 inch EMI 9211QKA phototubes are used for the readout of the CsI crystals. They offer good quantum efficiency and linearity over the full dynamic range of the calorimeter using a properly designed voltage divider[17]. SbCs dynodes have been chosen to reduce rate dependent effects of secondary electronemission. These effects can be caused by ion migration or direct impact heating [Smi95].

We used Dow Corning's Sylgard 184 for the permanent attachment of the PMT to the crystal. Sylgard 184 is an Polydimethylsiloxane (PDMS) elastomer that is cured by an organometallic crosslinking reaction. After curing it provides a hydrophobe rubber film with good transmission of UV light. The siloxane base oligomers contain vinyl groups. The cross-linking oligomers contain at least 3 silicon hydride bonds each. The curing agent contains a proprietary platinum-based catalyst that catalyses the addition of the SiH bond across the vinyl groups, forming Si-CH2-CH2-Si linkages. The multiple reaction sites on both the base and crosslinking oligomers allow for three-dimensional crosslinking [Cam97]. One advantage of this type of addition reaction is that no waste products, such as water, are generated. Curing agent and Sylgard were mixed for the application in a weight ratio 1:8 and pre-processed in vacuum. The yet bubble-free viscoseus mixture was poured on the PMT. Then in a dedicated frame the PMT was slightly pressed onto the rear face of the crystals. Due to adhesion it couples the quartz-glass to CsI; hence the phototube can be removed by applying strong shear forces. After ca. 24 hours of hardening the film offers mechanical and optical properties that are stable over time.

[16] The presence of the so-called excess noise would add a linear term to this formula (the `Fano factor' F). For a LED the assumption F-1=0 is sufficiently accurate.

[17] The voltage dividers are designed and built by B. Stephens from UVA

previous next Up Title Contents Index