Alpha particle absorption and inclined incidence trackparameters evaluation in plastic detectors

M.M. El-Hawary a, M. Mansyb, *, A. Hussein c, A.A. Ammara,A.R. El-Sersyb

aFaculty of Science, Menou®a University, Shebin El-koam, EgyptbNational Institute for Standards, NIS Cairo, Egypt

cFaculty of Science, Physics Department, Qatar University, Doha, Qatar

Received 1 November 1997; received in revised form 13 August 1998; accepted 7 September 1998

Abstract

In this work the use of our recently constructed irradiation chamber was involved in the current experiments. The

absorption of alpha particle in air has been studied through a set of experiments in which the stopping power hasbeen measured. A comparison between the calculated values and the present experimental results is given and agood agreement has been found. Critical angle (yc) determination has been carried out using two di�erenttechniques, via indirect and direct measurements, under di�erent etching conditions and at various alpha energies.

An empirical ®t of ycÿh (h is the removal thickness layer) dependence has been calculated and found to work well inthe studied h ranges. Also, the inclined alpha tracks parameters of energies between 1.0 and 5.0 MeV have beenstudied. Results can be successfully applicable in alpha autoradiography studies and detector e�ciency

determination for track registration in plastic recorders. # 1999 Elsevier Science Ltd. All rights reserved.

1. Introduction

Solid state nuclear track detectors (SSNTDs) have

been successfully employed in di�erent applications

in science and technology. They have been used in

alpha auto-radiography, radiation dosimetery and

particle identi®cation (Hussein et al., 1989; El-

Hawary et al., 1995; Ellis, 1986; Singh et al.,

1988a,b; Suzuki and Tomura, 1990). They have

many advantages as compared with other detectors.

They are relatively cheap and provide permanent

records of events (tracks), as well as proven to oper-

ate successfully under various environmental con-

ditions and have almost no fading under normal

storage of temperature and pressure (Todd, 1992;

Abou El-Khier et al., 1993a; Hussein, 1994; Andom,

1992; Abou El-Kheir, 1993b).

The critical angle of etching (yc) represents the mini-mum angle to the detector surface that a track canmake in order to be revealed by etching

sin yc � 1=V � VB=VT �1�

where V is the etch rate ratio, VB is the bulk etch rateand VT is the track etch rate (Durrani and Bull, 1987).The etching e�ciency (Z) of a detector is de®ned as the

proportion of tracks etched out expressed as a fractionof particles actually incident on the detector surface.From the measurements of yc, the etching e�ciency (Z)can be determined using the formula

Z � 1ÿ sin yc �2�

The purpose of the present work was two-fold: (i) tostudy the alpha particle absorption through a set of ex-

periments in which the stopping power is measured asa function of pressure values inside our constructed ir-radiation chamber; (ii) to obtain the critical angle (yc)

Radiation Physics and Chemistry 54 (1999) 547±550

0969-806X/99/$ - see front matter # 1999 Elsevier Science Ltd. All rights reserved.

PII: S0969-806X(98 )00292-8

Radiation PhysicsandChemistry

PERGAMON

* Corresponding author.

via the VB and VT measurements (indirect method)and also from a direct way using the irradiation

chamber under di�erent conditions and at di�erent aenergies. The etching e�ciency (Z) was then determinedby the mean of yc extracted values which re¯ects the

best conditions of the usability of the studied trackdetectors.

2. Experimental

Plastic sheets of CR-39 (TASTRACK), each 500 mmthick, were used. All samples were irradiated in ourrecently constructed irradiation chamber (El-Hawary

et al., 1995) and etched in 6.25 N NaOH at 708C. Thebulk etch rate (VB) was measured using the massdecrement method (Hussein, 1994). Track parameters

were measured using an optical Olympus microscopeattached to an eye piece micrometer with each divi-sion = 0.22 mm.

3. Results and discussion

3.1. Rate of energy loss determination

The air molecules were used as a stopping medium for

the incident projectiles alphas using our irradiationchamber (El-Hawary et al., 1995). Inside the chamber,alpha energy could be varied via either the adjustment of

the source-to-detector distance (x) at ®xed pressure (P)or by varying the air pressure at ®xed distance (see Fig. 1).The dominant interaction mechanisms between

alpha particles and the molecules of the mediumthrough which they pass are mainly excitation and ion-isation processes caused along their paths.Accordingly, energy loss rate of alpha depends mainly

on the density of the stopping medium or the pressureinside the chamber as long as the incident energy is®xed. The dependency of the energy loss rate (dE/dx)

on the pressure has been evaluated from the data ofFig. 1. The corresponding theoretical values of (dE/dx)are calculated using the TRIM computer program

(Zigler, 1985) where the density±pressure relationshipof air was used. Fig. 2 shows the variation of themeasured (dots) and calculated (solid curve) values of(dE/dx) with P, where a good agreement is obtained.

This procedure can also be used in alpha range evalu-ation in the stopping medium as a function of pressureinside the chamber which is aimed to be done in

another article using di�erent gases.

3.2. Critical angle determination

In the following experiments, sheets of CR-39 detec-tors were exposed to alpha particle of di�erent energies

incident at an angle 608 to the surface of the detector.

Both minor and major axes were measured. Figs 3 and

4 show the variation of minor and major axes (d and

D) respectively with alpha energies (E) at di�erentetching durations. The dependence of d and D on

energy is more obvious for t>5 h. Using values of d,

D, incident angle yc and etching time t one can calcu-late both of the bulk etch rate VB and the track etch

rate VT.

Fig. 1. The relation between alpha energy (E) and the press-

ure (P) inside the irradiation chamber at di�erent distances

(x).

Fig. 2. The variation of the measured (point) and calculated

(solid curve) values of (dE/dx) with the pressure (P).

M.M. El-Hawary et al. / Radiation Physics and Chemistry 54 (1999) 547±550548

In this part the critical angle of etching (yc) wasobtained by two techniques, i.e. indirect and direct

methods. In the ®rst method plastic detectors wereexposed to normally incident alpha energies (from 1 to5.5 MeV) and then etched to removal thickness layer

of 5 mm. From the track diameter measurements andEq. (1), yc was determined and represented in Fig. 5(curve a) as a function of energy. In the second

method, sheets of CR-39 detectors were irradiated witha certain value of alpha energy at angles y varyingfrom yc+5 to ycÿ5 in steps of 28. The choice of yc wasdetermined from the normal incidence case (indirectmethod) at the same alpha energy. Successive etchingprocesses were performed until the track could be seenunder the microscope. In this way yc was determined

as a function of removal thickness layer (h). By consid-ering di�erent alpha energies and going through theprevious procedures, one can get a complete set of ycdependence on both E and h. A comparison betweenyc values obtained from both methods is shown inFig. 5 where curve (b) represents the data obtained

from direct method at removal thickness layer ofh= 5 mm. Examples of the variation of the criticalangle with alpha energy at di�erent removal thickness

layer are shown in Fig. 6 using the direct method. Anempirical ycÿh ®t with uncertainty of about 3% wasfound to work well in the h-range from 4±10 mm whichcan be written in the form

E sin yc � A exp E=B� � ÿ C � h �3�where A, B and C are ®tting parameters of values0.256, 2.1 and 0.055 respectively. In a range outside 4±

10 mm the ®t was not easy. A ycÿE chart obtained

from Fig. 6, is very useful and greatly needed in anyfuture investigation concerning alpha radiography stu-dies and detector e�ciency determination for track

registration in plastic recorders.

3.3. Etching e�ciency (Z) of SSNTDs

The etching e�ciency of SSNTDs has been deter-mined via the critical angle measurements obtained

from both cases (direct and indirect methods). Fig. 7shows a comparison between the e�ciency values of

Fig. 4. The dependence of major axis (D) with alpha energy

(E) at di�erent etching times(t).

Fig. 5. The variation of the critical angle (yc) as a function of

the incident alpha energy (E).

Fig. 3. The dependence of minor axis (d) on alpha energy (E)

at di�erent etching times (t).

M.M. El-Hawary et al. / Radiation Physics and Chemistry 54 (1999) 547±550 549

the detector based on both methods as a function of

energy and at the same removal thickness layer (5 mm).This comparison indicated that there is a good agree-ment between both results, where yc shows a maximum

at about 2.0 MeV.

4. Conclusion

The energy loss rate values were obtained andshowed a good agreement with the theoretical data.

The minor and major axes in case of inclined incidencewere determined for di�erent alpha energies. Anempirical ®t of ycÿh dependence has been calculated

and found to work well in the studied h ranges Thedetector etching e�ciency showed a strong dependenceon both removal thickness layer and alpha energy

where a value of 85% has been resulted under the opti-mal conditions of h>13 mm.

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Fig. 6. The variation of the critical angle (yc) with alpha

energy (E) at di�erent removal thickness layers (h) (direct

method).

Fig. 7. Comparison between the e�ciency values of the detec-

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and at the same removal thickness layer (5 mm).

M.M. El-Hawary et al. / Radiation Physics and Chemistry 54 (1999) 547±550550