Energy loss distribution in argon based gas detectors

ENERGY LOSS DISTRIBUTION IN ARGON BASED GAS DETECTORS

Nilgun DEMİR and Ilhan TAPAN

Department o f Physics, Uludağ University, 16059 , Bursa - TURKEY e-m ail: ilhanffxdudag. edu. tr

This paper reports a Monte Carlo simulation of energy loss distribution for charged particles in a gas volume under different temperatures. The calculation has been performed in Argon based mixtures which are common in detector applications in particle physics experiments.

INTRODUCTION

The energy loss mechanism for a charged particle passing through a gaseous medium is mainly by interactions with atomic electrons i.e. ionization loss. The ionization energy loss of charged particles is fundemantal to most particle detectors.The multiwire proportional counter (MWPC) can be given as an example of such gas detectors. These are widely used whenever measurement of energy loss of radiation is required. These are also sensitive to the position at which an incident charged particle interacts. The basic MWPC (figure 1.) usually consist of a set of thin, parallel and equally spaced anode wires, symmetrically sandwiched between two cathode planes. An ionizing particle passing through the chamber creates electron-ion pairs depending on its energy. The generated electrons cause an avalanche multiplication in the vicinity of one of the wires that behave as cylindirical counter. The incident particle can thus be located by observing signals from the wires that separetely amplified and displayed [1].

Fig. 1. - A schematic view of MWPC

Energy Loss Distribution

The Bethe-Bloch formula [2] describes the average energy loss of charged particles. The energy loss is dependent on the particle velocity, but not its mass. Gaussian distribution for the energy loss is in agreement for non-relativistic particles. At high energies, the fluctuation of the energy loss around the average energy is described by an asymmetric distribution, the Landau distribution [3]:

1 1 X

f (X) = = exp{- - (X + e - )} (1)

2n 2

where the reduced energy variable X represents the normalized deviation from the most probable energy loss A Emp:

X =

A E - A E_mp

4

where A E is the actual energy loss and 4 is the average energy loss. Figure 2 shows a Landau distribution for the ionization energy loss which has been measured for 3 GeV electrons at DESY in a test beam [4].

Fig. 2. - Energy loss distribution of 3 GeV electrons in a multiwire drift chamber [4] The curve is significantly skewed toward higher energy losses because of the production of delta rays, ionization where the electron is kicked out with high enough energy to itself form ionizations along its path [5].

SIMULATION AND RESULTS

Landau distribution for the ionization energy loss of minimum ionizing particles ( fiy = 4) in the Argon based gas filled chamber of 1 cm thickness has been simulated by using a Monte Carlo technique. This was a rejection method to choose numbers associated distribution function according to equation (1). In this method, one simply chooses points randomly and uniformly in the space, using the function value at each point (divided by the maximum function value) as

the probability of accepting the point. The point is then accepted only if f/fmax is greater than a uniform random number choosen between zero and one [1].

Figure 3 shows the simulated energy loss distributions, for 10000 incident minimum ionizing particles, in pure Argon at three different temperatures, 0 0C, 27 0C and 50 0C respectively

1200 1000 800 > 0 o 600 0 E 400 200 0

energy loss (keV) energy loss (keV)

Fig. 3. - Energy loss distributions at the different temperatures

At the constant gas pressure, increasing temperature decreases the density of gas. Thus the value of mean energy ionization losses decreases with increasing temperatures.

Simulation has also been performed at different ratio Ar-CH4 mixes (Figure 4). Increasing CH4 ratio in Argon increases the value of mean energy ionization losses and signal value with larger fluctuation. 1000 800 600 400 200 0 % 80 Ar-% 20 Methane 0 1 2 3 4 energy loss (kev)

0 E 5 1000 800 600 400 200 0 % 60 Ar-% 40 Methane

energy loss (keV)

Fig. 4. - Energy loss distributions at the different mixes

0 1 2 3 4 5 energy loss (keV)

It is clear from the distributions that, the energy loss is a strong function of temperature and gas mixtures. Mean energy losses increase by decreasing temperature and increasing CH4 ratio in Argon. As the value of mean energy loss increases, the relative fluctuations in the distribution get worse.

REFERENCES

1. Thomas Ferbel, Experimental techniques in high energy physics, Addison Wesley Publishing Company, Inc., 1985.

2. Fano, U., Ann. Rev. Nucl. Sci., 13:1., 1963

3. Landau, L., On the energy loss of fast particles by ionization, J. Exp. Phys. (USSR) 8 1944,201

4. Affholderbach, K. et all., Nucl. Instr. Meth. A 410, 1998, 166

5. Sheaff, M., VIII ICFA Instrumentation in elementary particle physics school, Turkey, AIP conference proceedings 536, 2000