S13ST_Apparatus
Experimental Apparatus
The experimental apparatus was designed to count the number of times a particle (muon) passed through a certain angular range. It did this by using pairs of scintillator panels set up in coincidence.
The apparatus consists of four plastic scintillator panels connected to photomultiplier tubes (PMTs), arranged as shown here. The top panel was held up using a stand built out of bolted-together metal beams.
The measurements are:l=70 cm, w=19 cm, and h=123.5cm, all with uncertainty
. The distance was varied for each detector to change the centerline angle and the resulting angular acceptance range.
The PMTs emit a very short (~ ns) electrical pulse each time they detect a photon from the scintillator. The PMTs are connected through discriminators (to clean up the pulses and eliminate background thermal noise), then to coincidence units to determine when a particle has passed through both the top panel and one of the lower panels. The resultant individual coincidences are then counted by separate counters. A schematic of the electrical connections of the panels is below:
(The discriminators are labeled with DSC, the coincidence units with COINC. The top PMT and discriminator belong to the top, elevated panel, the other three to the bottom three panels.)
Technical details
Discriminator module: <nop>LeCroy model 821 (4-input)
Settings: Pulse width 100 ns, threshold -30 mV.
Coincidence module: <nop>LeCroy model 465 (4 x 4-input coincidence gates)
Counters: Initially, Ortec model 994 and Joerger Visual Scale model VS
Automated counting: NI DAQ 6602E with <nop>LabVIEW software.
Shielding of Electrons
In order to ensure that the particles passing through the detectors are muons and no electron events were contaminating the data, it was necessary to ensure that all cosmic-ray electrons would be shielded in some way. Since the apparatus was in the basement of the Tate labs, it was surmised that the surrounding concrete would serve this taks adequately. A quick check of NIST's ESTAR tool confirms that the range of electrons at 81 <nop>MeV, the energy at which cosmic-ray electrons are most numerous [Particle Data Group 2012], in concrete is 12.3 cm. This indicates that even one good-sized concrete wall will shield out most cosmic-ray electrons. Since the lab was located in a basement with at least two concrete walls between the apparatus and the outdoors in any direction, no additional shielding (lead or otherwise) was used.
Monte Carlo Simulation
The finite angular acceptance range of each detector pair necessitated that a Monte Carlo simulation be used to compute the expected count-rate distribution as a function of lower-panel distance
(or, relatedly, centerline angle ) given an expected angular flux distribution
. See this page for the details of the simulation.
Setup Notes
Beware, the counter in the NI DAQ 6602E card (with an internal 80 MHz clock) has a finite capacity and will roll over at some point. The way it was used here, the clock pulses were counted to measure a time interval in which counts would be automatically recorded, so it was effectively counting 40 MHz clock cycles. The 32-bit counter rolled over at 107 seconds, so no individual collection period could be longer than one minute. The solution was to set the DAQ card to record multiple one-minute samples in order to collect a full day's worth of data. However, we had some very strange-looking data and it took us a few days (and wasted data runs) before we figured this out.