ELSEVIER

Nuclear Instruments and Methods in Physics Research B 103 (1995) 94-98

Beam Interactions with Materials & Atoms

Construction of a charged particle irradiation chamber for the use with plastic detectors

M.M. E1-Hawary, A. Hussein ~, A. E1-Rahmany, A.A. Ammar, A.R. E1-Sersy Department of Physics, Faculty of Science, Monoufia University, Shebin El-koam, Egypt

Received 9 January 1995; revised form received 6 April 1995

Abstract A simple sensitive charged particle irradiation chamber was designed and constructed in our laboratory. A circular

rotating table with six detector holders, is contained in the chamber. The rotation is controlled electronically via a stepper motor with a precision of 1.8 °. The beam-to-detector direction is changeable with an angle less than 5 °. An alpha source is allowed to move vertically using another stepper motor. The source-to-detecior distance is determined with an accuracy better than 1.0 mm. An energy monitoring system, composed of a silicon surface barrier detector and a signal electronic system attached to a PC is used for energy calibration. Chamber characterization and its application in nuclear track registration are studied under different conditions. The energy resolution of CR-39 plastic detector has been obtained at various alpha energies and etching durations. Results showed that good detector resolution could be achieved at high etching duration.

1. Introduction

An increasing number of papers have dealt with the use of solid state materials [1-4] in radiation measurements. The induced effects in such materials are the principle tool for such investigations. These effects are strongly depen- dent on the internal structure of the absorber (such as glasses, polymers and crystals) as well as on the kind and energy of the incident radiation (such as gamma, neutrons, and charged particles).

Different techniques of radiation measurements [5-9] have been employed and the track development in plastic foils has been widely applicable in the field of radiation dosimetry. Plastic detectors attain many advantages over the others; they have been successively used in heavy charged particle identification and detection with a wide range of energies. They have been used [10-14] in person- nel neutron dosimetry, radon level determination, uranium and thorium estimation in rocks, water and plants, etc.

The interaction of charged particles with plastic detec- tors results in track formation inside the materials. Several investigators [15-17] have paid considerable importance to the study of track parameters such as etching speeds, profiles and track range. This would lead to a precise determination of particle identification and detection. In

order to make such measurements more accurate, particle energy and incident angle should be precisely determined.

The aim of the present work is to design and construct an irradiation chamber to be used in a variety of dosimetric applications. It is designed in such a way that projectile energy and incident angle are precisely controlled and accurately measured.

2. Experimental

Plastic sheets of CR-39 polycarbonate 500 ~m each thick (TAS-TRACK, supplied by Bristol U.K.) were used. For alpha irradiation facility an 241Am thin source was used. All plastic foils were etched chemically in 6.25 N NaOH at 70°C. The bulk etch rate, V B, was measured using the mass decrement method [15]. Track diameters were measured using an optical Olympus microscope at- tached to an eye piece micrometer with each division equal to 0.22 ~xm.

3. Results and discussion

3.1. Construction of the irradiation chamber

* Corresponding author.

The design of the irradiation chamber depends on the purpose of its use. Our field of interest is specialized with radiation dosimetry studies using track-storing materials.

0168-583X/95/$09.50 © 1995 EIsevier Science B.V. All rights reserved SSDI 0168-583X(95)00565-X

M.M. El-Hawary et a l . / Nucl. Instr, and Meth. in Phys. Res. B 103 (1995) 94-98 95

So, the chamber was really designed to cover all our needs as well as to be used in future plans of studies.

Fig. 1 shows a schematic diagram of the designed irradiation chamber. It is essentially a stainless steel bell jar of diameter and height 30 and 40 cm, respectively. This jar is tightly mounted on a 50 × 50 cm square base, made of 2.5 cm thick plexiglas, through an O-ring in order to assure good vacuum inside the chamber. On one side of the jar there is a window (W) which can be used as a view to observe what is going in just before irradiation starts. Inside the chamber there is a circular rotating table (T) of diameter 15 cm made of bakelite fibre materials and supported to the chamber base through a metal rod. It is allowed to rotate electronically using an underneath pully connected to a stepper motor (M1) through rubber belt (see Fig. 1). M1 is connected to a drive so that speed and direction of rotation can precisely be controlled from outside. The stepper motor drive circuit was also designed using IC-SAA 1027. There are five plastic detector holders (H) and a sixth one (D) is used for the surface barrier detector. All holders are mounted on the table (T) and simultaneously rotate with it. Each holder (H) is attached to an angle scale with a precision better than 2 °, so one can precisely determine the beam incident angle. Fig. 2a shows

a schematic diagram of one of the plastic detector holders (H). The surface barrier detector is connected to a radiation analyzer system compacted to a PC multichannel computer for energy monitoring and calibration.

At the upper part of the chamber there is a source holder (SH) (see Fig. 1) which is attached to a bakelite fibre roof. Fig. 2b shows the holder (SH) in more explicit way. The SH holder easily moves up and down using a second stepper motor (M2) and consequently be controlled from outside. An alpha source (S) is supported on the lower end of the SH holder so the source is facing the table (T). The source-to-detector distance can be deter- mined with an accuracy better than 1.0 mm. There is a magnetic shutter (MS), attached to the axial metallic rod, which was constructed in order to accurately control the irradiation duration. The chamber is connected to a two- stage rotary pump (P) and the vacuum is monitored and controlled using a needle valve and a digital pirany gauge (G). Through the chamber base there are four cables labeled 1, 2, 3, and 4 which are connected to the surface barrier detector, motor (M1), motor (M2) and the magnetic shutter (MS), respectively. In this way, the movement of everything inside the chamber is now electronically con- trolled from outside.

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Fig. 1. Block diagram of the constructed irradiation chamber.

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M.M. El-Hawary et aI. / NucI. Instr. and Meth. in Phys. Res. B 103 (1995) 94-98

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Fig. 2. A design of the detector holder, H, (a) and source holder, SH, (b).

3.2. Particle energy measurement

In order to facilitate the irradiation procedure, an en- ergy calibration curve should first be determined. The energy calibration was performed using an alpha spectrom- eter model TC 256 and a TC pulser compacted to a 4k multichannel analyzer and PC computer. Then the irradia- tion chamber was evacuated to the various values of pressure and source-to-detector distance was varied from 5 to 11 cm. The table (T) (see Fig. 1) was aligned at a position where the surface barrier detector was facing the alpha source.

Fig. 3 shows a group of curves which represent the relation between alpha energy, E, and the pressure inside the chamber (P) . It is obvious from this figure that E shows a decrease with increasing P at all the studied distances, d, between the source and the detector. An empirical relationship of the form

E = E o + A P + B P 2

Pro=~=;uro t T o r t ]

Fig. 3. Variation of alpha particle energy (E) with pressure (P) at different source-to-detector distances, D, where curves a, b, c, d, e and f represent D equals 11, 10, 8, 7, 6 and 5 em respectively.

" I I I 4 ],4,T

I I 1 I I I I

S 8 10 12

D | = ~ , ~ L n ¢ O [ C m ]

Fig. 4. Variation of alpha panicle energy (E) with distance (D) at different pressure values.

M.M. El-Hawary et al. / Nucl. Instr. and Meth. in Phys. Res. B 103 (1995) 94-98 97

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diameter were then obtained at various etching durations of 4, 6, 8 and 10 h using an optical microscope.

The dependence of the track diameter, dr, on alpha energies (E) was extensively studied under various condi- tions, where d r showed an increase with either decreasing E from 5.0 to 2.0 MeV or increasing the etching duration from 4 to 10 h. The resolution of CR-39 detectors has been determined at alpha energies between 2.0 and 5.0 MeV and etching duration from 4.0 to 10.0 h. The observed full width at half maximum, hdT, due to the diameter forma- tion statistics within the detector, has been calculated at each alpha energy and etching duration. From each A d T value the corresponding energy spread, AE, was obtained from the dr-E relationship. The energy resolution, R, was given by the relation

zXE R = - - × 100%.

E

R was obtained at each value of energy and etching time. Fig. 5 shows the relation between the energy resolu- tion, R, of CR-39 plastic detectors as a function of alpha

E n ~ r ~ I Me¥ 1

Fig. 5. Dependence of the resolution (R) on alpha energy (E) at different etching times (te).

was found to fit the data of Fig. 3 quite well with a correlation coefficient of the order of 99%, where E 0 is the initial alpha energy, A and B are fitting parameters depending on the d-value; they are given by:

A = ( 1 . 6 9 - 1.26d) × 10 -~ and

B = - [0.75 + 0.2d(1 + d)] × 10 -6.

E, d and P given in MeV, cm, and Torr, respectively. Fig. 4 shows the variation of alpha energy, E, with d at

various values of P, where E shows a decrease with increasing d. Therefore, for energy determination, with the aid of the calibration curve given in Fig. 3, one has to make precise adjustment of both the pressure value and the distance d which leads to the desired value of energy.

3.3. Energy resolution of plastic detectors

The energy resolution of any detector is an essential test before its usage in radiation measurements. In this part, the detector energy resolution has been determined via the track diameter measurements. Sheets of 500 g.m thick CR-39 plastic samples were pre-chemically etched for 10 h in 6.25 N NaOH for background reduction. Samples were then exposed to different alpha energies from 2.0 to 5.0 MeV in steps of 0.5 MeV using our constructed irradiation chamber. Measurements of track

Fig. 6. Photomicrographs of alpha tracks in 500 Izm thick CR-39 exposed to different energies and etched at different etching times in 6.25 N NaOH at 70°C where (a) and (b) represent a 4 MeV alpha track etched for 4 and 6 h, respectively. (c), (d) and (e) represent 2, 3.5, 4.5 MeV alpha tracks, respectively etched for 10 h.

98 M.M. El-Hawary et aL / NucL Instr. and Meth, in Phys. Res. B I03 (1995) 94-98

1 |

a

1

~4

Fig. 6 (continued).

energy and etching duration. It is clear from this figure that R is strongly dependent on both energy and etching time; where R shows an improvement as the energy or etching duration increases. A resolution of about 1.0% was ob- tained at an alpha energy of 5.0 MeV and etching duration from 8 to 10 h. Fig. 5 shows some photomicrographs of alpha tracks formed in CR-39 plastic detector exposed to different energies and etched in 6.25 N NaOH at 70°C for various etching durations using our constructed irradiation chamber. The importance of these photographs lies in some features such as: almost diminished background,

obvious track contrast and overall tracks of different ener- gies are easily distinguished through their diameter mea- surements with high degree of accuracy.

4. Conclusion

The present constructed irradiation chamber operates quite well where alpha energies are now being precisely determined via a fine adjustment of pressure and source- to-detector distance. An empirical energy-pressure rela- tionship is obtained and applicable within a wide range of energies with a correlation coefficient of about 99%. Five plastic detectors of different orientations to the beam direc- tion can be adjusted in the chamber at once. The energy resolution is seriously affected by alpha energy and etch- ing conditions and a value of 1.0% is obtained under optimal conditions. Finally we can say that our system can be successfully used in many dosimetric applications using the plastic track detectors with high reliability output.

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