Light simulation in plastic scintillator strip with embedded wavelength shifting fiber
LLight simulation in plastic scintillator strip with embedded wavelength shifting fiber
Usubov Z.U.
Joint Institute for Nuclear Research, Dubna, RussiaJuly 9, 2020
Abstract
The simulation study of the light yield and attenuation in the plastic scintillatorwas performed. The wavelength shifting fiber readout was embedded in the groovesmachined along the entire strip surface. The scintillator strips was irradiated witha radiation source Sr or cosmic muons along and across the strip. Cosmic muons are the important contributors to background processeswhen search for the conversion of a muon to an electron[1, 2]. Cosmic rayveto geometry surrounding the detectors and stopping target should becarefully eliminated this background. Passive and active shielding shouldprovide background of ∼ a r X i v : . [ phy s i c s . i n s - d e t ] J u l ven for this purpose, a Monte Carlo simulation can adequately predictthe experimental results only if the detector parameters are sufficientlyclose to their true values. Some of parameter, e.g. surface boundaries de-scriptions, can be tuned by using measurements for particular scintillatorstrip configurations. Figure 1: The β -particle spectrum of Sr provided by Geant4 simulation. The Monte Carlo simulation with all possible processes play a crucialrole in the feasibility study of the proposed detector module and in iden-tifying detector parameter values. Low-energy optical photons (photonswith a wavelength much greater than the typical atomic spacing) undergothe following processes: bulk absorption, Rayleigh scattering, reflectionand refraction at medium boundaries, and wavelength shifting.The boundary processes on all scintillator play an important role in trac-ing photons in strips. Compared to them, photon self-absorption in scintil-lator is less significant[6]. In Geant4.10.06 [7] simulation we combined the polished scintillator surface finishes with the backpainted wrapping optionwhich represents diffuse (Lambertian) reflection. In this simulation we usethe UNIFIED model for the processes between two dielectric materials.
The peak of emission light of a plastic scintillator (e.g., Saint GobainBC400 series) does not matches the peak sensitivity of used photodetec-tors. To solve this problem it is necessary to use the WLS fiber to transferlight to the photodetector. In this simulation the fiber are multiclad con-sisting of a scintillating core surrounded by an acrylic inner cladding andan outer cladding which made of a fluor-acrylic material (similar to theKuraray double clad fibers of type Y11(200)[8]). It was assumed that ina scintillator strip a mean value of 10000 scintillation photons per MeV ofdeposited energy were emitted. For this scintillator, the maximum emis-sion is at a wavelength of 431 nm and refractive index is 1.58.For WLS fiber attenuation length of 500 cm for its own radiation, andfor plastic scintillator attenuation length of 300 cm are assumed. The totaldiameter of fiber is 1.2 mm. The total thickness of cladding structure is cm and Sr radiation source (see the text).Figure 4: Light intensity distribution in the fiber cross-section at the photodetector sideof the scintillator strip. 4 % of the diameter of a fiber. In this simulation the strip contains one ortwo co-extruded grooves with 3 mm depth and 1.3 mm width for insertionof the WLS fiber. The selected strip and fiber parameters are close tothose used in the test-beam measurements at JINR (Dubna, Russia). Figure 5: The distribution of the number of photons at the photodetector side when themiddle of the strip irradiated by Sr source. This simulation was performed using Geant4 for plastic scintillator withthe dimension 4*1*300 cm and co-extruded TiO white diffuse reflective(R=98%) coating. The strip contain one at the center or two grooves at adistance of 2 cm from each other along the entire length of scintillator strip.We collect photons from a WLS fiber at one of the strip ends (hereinafterreferred to as photodetector side). On the photodetector side at the fiberend the photons are fully absorbed. The opposite ends of the fibers areblackened.The Sr source was simulated in the Geant4 framework. The sourceprovides an electron flux in a wide energy range up to 2.3 MeV (see Fig.1).The radiation source was enclosed in a shell with a lead collimator. Thediameter of the collimator outlet was 1 mm. The source was located at adistance of 2 mm above or below the scintillator strip.Cosmic muons were generated according to[9] in the range 0.3-5000 GeV. Sr sourcelocated over and under the strip. The zenith angle and energy distribution for simulated cosmic muons aredisplayed in Figure 2.In Figure 3, we show the distribution of the energy deposited in thescintillation strip when the middle of the side 4*300 cm is irradiated witha Sr source.Figure 4 shows the light intensity distribution in the end of a fiber asseen by the photodetector side. This simulation study show that the lightintensity increases towards the edge of the fiber core. The mean wave-length of light collected by the photodetector is 535 nm.The distribution of the photon number at the photodetector side whenthe middle of the strip side 4*300 cm is irradiated with a Sr is shownin Figure 5.Figure 6 show the light yield when strip with one fiber irradiated with a Sr source which located over or under the strip at Z=0.0 cm. The stripis located at X= ± ± ± Sr source is located over thescintillator strip.Figure 8: A comparison of light yield in two fibers when Sr source is located under thescintillator strip. 7 n Figures 7 and 8, the same thing is shown as in Figure 6 but for thecase with two fibers in the strip. Note that, in both cases (strip with oneand two fibers), the behaviors of the light yield when a radiation sourceis above and below the strip differ from each other. But this difference isnot significant.Figure 9 shows the relation between the mean number of optical pho-tons detected at the photodetector side and the distance between pointof impact of electrons from the radiation source and photodetector side.This graph is fitted by a function N phot ( z ) = A ∗ e − z/λ + B ∗ e − z/λ . Note that this formula was proposed by Kaiser et al.[10] for the case whenlight is collected from the ends of the scintillator using a photomultipliers.The first term is the transmission behavior for photons that travel directlyto the photodetector side. The second term is the transmission behavior
Figure 9: The simulated light attenuation in a scintillator strip with two fiber with Sr source irradiation. The first point is excluded from fit. for photons that hit the detector after a series of reflection on a scintillatorsurface. The first point in the Figure is 75 mm away from the photode- ector side and each step is 75 mm. The curve in the figure correspondsto the parameters λ =43.5 m and λ =1.75 m with almost 100% errors.To study the light attenuation when the strip surface is irradiated bycosmic muons we retreated on each side of the surface by 1 mm and dividedit into 40 equal parts. Each sector has been uniformly irradiated by 500muons with energies, azimuth and zenith angles modeled accordingly to[9].In Figure 10, we demonstrate the light attenuation for this case. Thepoints in the Figure are located in the center of each of the 40 sections.For the given points, the results of one exponential and double exponentialfit are the same (blue curve in the Figure), λ = λ =5.88 m. The greencurve in Figure corresponds to the fit by the formula N phot ( z ) = A ∗ e − z/λ + B, where λ =2.32 m. Figure 10: The simulated light attenuation in a scintillator strip with cosmic muonirradiation. The first point is excluded from fit.9
Conclusion
In this note, we modelled the light output and attenuation in a scin-tilltion strip with dimensions of 4*1*300 cm . The simulated radiationsource Sr and cosmic muons were used as beam particles. The scin-tillation strip was irradiated both from the side of the embedded fibersand from the opposite side along and across the strip. Optical photonswas collected from one and two fibers embedded in the strip along theentire length. It was shown that the attenuation of light depending onthe distance to the photodetector is described by a double exponentialfunction.We are sincerely grateful to Z. Tsamalaidze and Yu. Davydov for initi-ating this work. References