S17plasticscintillator

A MEASUREMENT OF SCINTILLATOR TRIGGER EFFICIENCY FOR LDMX

Introduction

LDMX Background:

The Light Dark Matter Experiment (LDMX) is a proposed search for sub-GeV dark matter, which will use a fixed-target missing-momentum technique. LDMX will shoot 10 GeV electrons at a tungsten target, ideally hoping to observe a large change in electron momentum from before the target to after the target that cannot be accounted for by any known standard model interaction. The detector requires a mechanism to indicate when an electron has passed through the target. This can be done by placing a piece of thin scintillator directly after the target. While an efficiency as close to 100% as possible is desirable due to the limitations of beam time, the scintillator’s thickness must be on the order of millimeters to reduce instances of rare, hard-to-detect background events.

Project Goals:

1.To determine the detection efficiency of a thin scintillator

2.To determine the (x,y) location of an event

Theory

The efficiency of a thin scintillator can be calculated using:

However, this equation is deceptively simple as there is no concrete definition for a muon detection. While the Bethe-Bloch equation gives a mean energy deposition of 0.4 MeV in 3mm thick scintillator, muon energy deposition is probabilistic, particularly in thin materials. Thus, to define whether or not a muon was detected, the number of photoelectrons detected by the SiPM array must be above a threshold set by the background noise present.

M Methods

A muon telescope, as shown in Figure 1 was constructed. Cosmic muons largely incident from above deposit energy in all 3 scintillators via electromagnetic interactions, which is converted into optical photons by the fluors in the scintillator. A silicon photomultiplier, or SiPM, was used to measure optical photons in the scintillator. An array of four SiPMs was used to read out of the ”target scintillator.”

Figure 1: A muon passes through the scintillators, where top and bottom “control scintillators” are used to indicate when

a muon event has occurred. Four SiPMs were mounted to a circuit board to read out from the target scintillator.

Signals from the PMTs attached to the control scintillators were passed through discriminators and a coincidence unit. This coincidence was used to trigger a Lecroy oscilloscope, which read out signals from the SiPMs attached to the target scintillator. The scope was controlled using a LabView program.

Results and Analysis

For a given data set, the pulse area for each SiPM for a given muon event was made into a histogram using ROOT, shown in Figure 2. A sum of Gaussians was fit to the discernable photoelectron peaks to characterize the response of each SiPM in terms of photoelectrons detected.

Figure 2: The first six photoelectron peaks were fit using

f(x)=∑_(i=1)^6▒〖A_i exp[-1/2 ((x-〖[μ〗_1+(i-1)d])/σ)^2 ],

where A_i is the amplitude of a Gaussian, μ_1 is the mean of the first photoelectron peak,d is the fixed distance between each

peak, σ is the fixed width of each peak, i is the peak number, and x is the pulse area in nVs.

The number of photoelectron detected by the entire SiPM array for a given muon event was determined by summing photoelectrons across all four detectors, the results of which are shown in Figures 3 and 4.

Figure 3: Shows photoelectron counts summed over all SiPMs for the 3mm scintillator

Figure 4: Shows photoelectron counts summed over all SiPMs for the 22mm scintillator

A measurement of background noise was made and fitted with a Gaussian distribution. The mean, μ, of the distribution was determined to be (0.184 ± 0.008) photoelectrons and its standard deviation, σ, was (0.76 ± 0.01) photoelectrons. A muon event was defined as having been detected if the number of photoelectrons summed across the entire SiPM array was more than μ+3σ, as represented by the blue dotted lines shown in Figures 4 and 5. Using (1), the efficiency for the 3mm scintillator was determined to be 0.88 ± 0.01 and that of the 2mm scintillator was 0.77 ± 0.02. These error are rather large as only 700 events were collected to determine efficiency. The muon cross section was moved around the target scintillator in order to see if the planar location of an event could be determined by examining the relative number of photons detected by each SiPM for a given event. Plots comparing each combination of SiPMs as in Figure 5 were made.

Figure 5: To visualize any trends, the relationship between two SiPMs as a function of muon target location were examined. When plotting, a condition was requited that the sum of the two SiPMs must be more than 3 to eliminate non-events

One can see some apparent trends in the figure above, namely the shifts in photoelectron means and amplitudes relative to incident location. Future experiments should seek to lower the background threshold to improve the efficiency measurement and perform a thorough, quantitative analysis to localize incident muons.

Conclusions

We measured the efficiency of 3 mm and 2 mm thick scintillators for minimum ionizing particles using a muon telescope. A positive muon event was determined by a 3σ cutoff of the background noise, giving a 99.7\% confidence in our measuring an event verse known noise sources, assuming a Gaussian distribution. We measured efficiencies of 0.88±0.01 and 0.78±0.02 for the 3 mm and 2 mm scintillators respectively. Additionally, we were able to show that event localization was feasible using the relative number of photoelectrons in each SiPM.