Measurement of Decay Rate of Cosmic Ray Muons in Various Materials V. Dixit, V. Singh and V.S. Subrahmanyam Department of Physics, Banaras Hindu University, Varanasi -221005, India *Email:
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In this article, we are reporting cosmic ray muon decay rate using two and three coincidence method and using plastic scintillation detectors. We obtained that the decay rate of muons is higher for the materials having high density or having large atomic number. The slope of fitting curve which represents ratio of Muon decay rate per meter square per minute to the density of various materials is found to be - 0.98±0.04.
Introduction
electromagnetic forces, so they pass through different materials without any decay or capture. However, if they pass through a sufficiently higher density or higher Z value material, they can decay into electrons and neutrinos. In the present work we used three coincidence method using three plastic scintillators (PS) at a certain separation (12 cm) between second and third PS for placing different materials. Muon decay rate is calculated by taking ratio of three to two coincidence count rate and subtracted from unity [3-4].
Cosmic rays with energies ranging from 108 to 1020 eV continually bombarding the Earth's upper atmosphere, where they interact with different atoms and molecules of atmosphere to produce secondary cosmic ray particles, including pions, Kaons, etc. which subsequently decay to muons and electrons [1]. At sea level the composition of cosmic ray is such that about 80% are muons, the rest are electrons and protons. The flux is on the order of 10-2 cm−2.sec−1.sr−1. Since Muons are unstable particles having lifetime 2.19 µs, they decay into electrons, anti-neutrino and neutrino. However, being relativistic particles, they travel a very large distance with a speed close to the speed of light in different materials. The muons can also be captured by the nucleus. The probability of capture depends on the atomic number (Z) of the absorbing material and goes Fig.1: Schematic Diagram of the experimental set like Z4 for smaller Z values [2]. up for determination of muon decay rate in various materials. Experimental Details The main component of the experimental set up is three plastic scintillators kept parallel to one another at a certain separation with their faces horizontally. The scintillator material emits a light when an ionizing particle deposits energy in it. The light pulses are converted into electrical pulses by (the 29 mm diameter and 11pins) photomultiplier tubes (PMTs). The signals from the three PMTs are fed as an input to the discriminators and out put of this fed onto the coincidence unit. Coincidence unit and a gate generator form the trigger logic that defines when the events should be captured by the digital storage oscilloscope (Tektronix 500 MHz). The whole experimental set up is shown in Figure 1. Since muon does not interact with matter via the strong force but only through the weak and
Results and Discussions Figure 2 represents the variation of decay of muons at ground level per square meter per min. with respect to the variation of thickness of the Aluminum material and decay rate starts to saturate above 32 cm. Figure 3 represents the variation of decay of muons at ground level per square meter per min. with the variation of thickness of different materials such as Clay, Glass, and Lead etc. showing different thickness of decay rate saturation. The Table 1 represents the essential fit parameters for Al, Clay, Marble, Granite and Lead. From the Figures we can conclude that the
decay rate of cosmic ray muons is greater in the materials. Data points are fitted by 2ndorder materials having higher density or larger Z value. polynomials.
Table 1: Shows the essential fit parameters for Al, Clay, Marble, Granite and Pb. Fig. 4: Shows the muon decay rate saturation depth in to the material as a function of material density. Summary & Outlook We made a systematic study of decay of muons in different materials using triple coincidence method using plastic scintillators. It is observed that the decay rate of muons is higher for the materials having high density or having high atomic number (Z). Figure 4 shows that the interaction length of cosmic ray muons in various materials is inversely proportional to its density. The slope representing the decrease in interaction Fig. 2: Shows the decay rate (Y-axis) of muons lengths in lead, glass, granite, marble, aluminum with thickness (X-axis) for Aluminum. The data and clay with their densities and slope is found to points are fitted by 2nd order polynomial. be -0.98 ± 0.044. Acknowledgement VD acknowledges the help of Mr. P. K. Khandai and other colleagues in the experimental arrangement.
Reference [1] Cosmic Rays on Earth Physics data, O. C. Allkofer and P. K. F. Greider, Enegie Physik Mathematik GmbH, Karlsruhe (1984). [2] R. C. Hovis and H. Kragh, Am J Phys 59, 779 (1991). [3] V. Cirigliano, R. Kitano, Y. Okada, P. Tuzon, Phys. Rev. D 80, 013002 (2009). [4] A Conceptual Introduction to Muon Physics, Fig. 3: Shows the decay rate (Y-axis) of muons David Van Baak (2010) with respect to the thickness (X-axis) of different
