Two-parameter analysis of the temporal behaviour of resistive detectors

Two-parameter analysis of the temporal behaviour of resistive detectors

Tiago P. Peixoto^I; Paulo R. Pascholati^I; Vito R. Vanin^I; Carmen C. Bueno^II; Josemary A.C. Gonçalves^II

^ILaboratório do Acelerador Linear, Instituto de Física da Universidade de São Paulo, CP. 66318, 05315-970, São Paulo, SP, Brazil

^IIInstituto de Pesquisas Energéticas e Nucleares, IPEN-CNEN/SP, Caixa Postal 11049, 05422-970, São Paulo, SP, Brazil and Departamento de Física, Pontifícia Universidade Católica de São Paulo, PUC/SP, Rua Marques de Paranaguá 111, 01303-050, São Paulo, SP, Brazil

ABSTRACT

The biggest constraint in the RPCs operation is the drop of their efficiency with the counting rate, consequence of charge gain decrease. This effect is normally attributed to the voltage drop on the dielectrics, although not supported by quantitative measurement. In this work we present the first results of a two-parameter analysis of the charge pulse height time variation from a cylindrical resistive detector, operating in proportional regime, under high irradiation rates. The dynamic behaviour of this detector was investigated through the determination of the time decay constants related to its stationary condition, where the charge gain becomes constant. To perform this study, a data acquisition system, which allows the users to observe in real-time the temporal variation of the energy spectrum, was specially designed. The fitting of the peak centroid position as a function of time, obtained for rates range from 220Hz up to 1230Hz, showed it can be described by a sum of two exponentials plus a constant term, what is in accordance to the dielectric delayed polarization processes.

1 Introduction

Detectors with highly resistive electrodes, namely the Resistive Plate Chambers (RPCs), have been studied extensively in recent years envisaging their use as a muon trigger in the Large Hadron Collider at CERN. However, the biggest constraint in the RPCs operation is the drop of their efficiency with the counting rate, consequence of charge gain decrease [1, 2, 3, 4, 5]. The origin of this problem has been investigated assuming a stationary regime, where the charge gain becomes constant. Since some authors attributes this effect to the polarization phenomena of the dielectrics, we decided to investigate the charge gain variation of a cylindrical resistive detector during the occurrence of these processes (dynamic behaviour), which are particularly important because a small area around the discharge spot remains inactive during a given period of time, thus limiting the rate capability of the detector.

In order to improve the results that some of us previously obtained about the dynamic behaviour of resistive detectors[6, 7, 8], mainly from what concerns the charge pulse amplitude reduction with the increase of current across the gas gap, it was developed a data acquisition system[], which allows the users to observe in real-time the temporal variation of the energy spectrum and exports the data as text or binary file to be used with an external analysis package. In this work, we present the first successfully results on the two-parameter fitting of the peak centroid position, using specific analysis functions trying to describe the transient response of resistive detectors.

2 Experimental Setup

The relaxation mechanisms of resistive detectors was investigated using a glass cylindrical proporcional counter, described in previous papers[6, 7, 8], operating with Ar + 10% CH₄ gas mixture at atmospheric pressure and irradiated with different counting rates by 22 keV X-rays from a ¹⁰⁹Cd source. The charge gains of this detector were measured using a conventional charge amplifier electronic system and a NIM ADC (Nuclear Data - ND582), whose digital output is retrieved with an IO board, as can be seen in Fig. 1. The computer program gets the data from the device driver and then builds and displays a two-dimensional matrix (energy × time histogram) representing the data. Fig. 2 shows a edited display window of the program. In the upper part of it appears the data matrix and in the lower one the one-dimensional histograms corresponding to two different time intervals of it.

3 Data Analysis

Taking into account that the peak behaves as a Gaussian, according to the Eq. , its position as a function of time can be described by a sum of two exponentials plus a constant term, due to the complexitiy of the glass delayed polarization processes[7, 8, 10]:

where

is the centroid of the gaussian peak. The standard deviation was modeled by a sum of a exponential plus a constant term, as:

where a₁, b₁, a₂, b₂, s₀, s₁ and s₂ are parameters; C is a constant related to a possible flat background and A corresponds to counts.

4 Results

Figures 3 and 4 present the charge pulse height as a function of time for 270Hz and 1230Hz counting rates, respectively.

The fitted function and the corresponding residues are shown in Figures 5 and 6. The residues being defined by

where y(x, t) is the counts in ''channel'' (x, t).

The resulting fitting parameters (adjusted by least squares minimum method) obtained for rates range from 219 Hz up to 1 230 Hz are summarized in Table 1 (assuming A = constant). The time decay constants, t₁ and t₂, extracted from these data are plotted in Figs. 7 and 8 as a function of counting rate. The analysis of these curves evidences the decrease of time constants with the increasing rates, what is in accordance with previous results[8].

5 Conclusion

Even though these results are preliminary, they have been shown that is possible to apply the fitting procedure described in this work to study the transient behaviour of resistive detectors. It is important to stress that in our method the running time for the fitting is lower than that spent in conventional one (as detailed in refs. [6, 7, 8]), since it treats all the data simultaneously, giving all parameters needed.

Acknowledgments

We would like to thank F. Camargo and P. Gouffon for their very special collaboration. We wish to acknowledge the support of Conselho Nacional de Desenvolvimento Científico e Tecnológico - CNPq (number 520448/98-7) and Fundação de Amparo à Pesquisa do Estado de São Paulo (numbers 99/12687-1 and 00/05856-0).

Received on 29 October, 2003

• [1] I. Crotty et al., Nucl. Instr. and Meth. A346, 107 (1994).
• [2] I. Crotty et al., Nucl. Instr. and Meth. A337, 370 (1994).
• [3] E. Cerron Zeballos, et al., Nucl. Instr. and Meth. A367, 388 (1995).
• [4] P. Fonte et al., Nucl. Instr. and Meth. A431, 154 (1999).
• [5] W. Riegler et al., Nucl. Instr. and Meth. A518, 86 (2004).
• [6] M. M. Fraga, et al., IEEE Trans. Nucl. Sci. 45, 263 (1998).
• [7] M. M. Fraga, et al., Scientifica Acta, XIII(2), 1 (1998).
• [8] M. M. Fraga, et al., Nucl. Instr. Meth. in Phys. Res. A419, 485 (1998).
• [9] T. P. Peixoto et al., in Proceedings VI ENAN National Meeting on Nuclear Applications, Outubro 2002, Rio de Janeiro, Brazil, CD-ROM.
• [10] D.G. Holloway, Physical Properties of Glass, London, 1973.

Publication Dates

• Publication in this collection
  26 Oct 2004
• Date of issue
  Sept 2004

History

• Received
  29 Oct 2003