Registration e�ciency of some SSNTDs with source area included

H. El-Samman a, M. Mansy b, A. Hussein a,c,*, M. El-Hawary a, A. El Sersy b

a Faculty of Science, Department of Physics, Meno®a University, Shibin El-Koom, Egyptb National Institute for standards, El-Haram, Giza, Egypt

c Faculty of Science, Department of Physics, Qatar University, POB 2713, El-Doha, Qatar

Received 16 February 1999; received in revised form 18 May 1999

Abstract

In this work, registration e�ciency dependence on alpha particle energy and incident angle of some SSNTDs has

been studied where the source area (241Am thin source) was included in the calculations. Ratio of the number of re-

corded track density rate (q/t) to the incident ¯ux (/) has been determined using LR-115, CN-85 and CR-39 track

detectors etched in the most recommended optimum etching conditions.

It was found that the detector registration e�ciency is independent on the source-to-detector distance (d) if the ¯ux is

accurately calculated and the point source approximation is worked only for d greater than six times the source di-

ameter when irradiation is performed in air. Results of the present work are discussed within the framework of alpha

particle interaction with track detectors where the geometrical con®guration of the source and detector arrangement is

taken into consideration. Ó 1999 Elsevier Science B.V. All rights reserved.

1. Introduction

Nuclear track etching methodology is still beingone of the most widely used techniques in manyapplications. Track detectors are successfully used[1±8] in 222Rn concentration measurements, de-termination of U, Th contents in natural samples,personal neutron dosimeter, etc. They have excel-lent properties such as high registration e�ciency,low background noise and can be used for longterm exposure period without any additional cost.From the experimental point of view, the deter-mination of detector registration e�ciency should

be the main goal before its use in the measure-ments. Detector e�ciency depends not only on thedetector material used and radiation energy butalso on the geometric arrangement between thesource area and the detector surface. E�ciency isessentially a strong function of the measured ra-diation-induced e�ect in the detector, i.e. the re-sultant tracks.

In this study, the determination of the detectore�ciency of some SSNTDs is carried out via theratio of the number of recorded track density perunit exposure time (q/t) to the number of the in-cident alpha particle ¯ux (/). This is performedusing an 241Am alpha source of known activity (A)and of circular area with radius Rs. In orderto evaluate /, let us consider a solid angle X

Nuclear Instruments and Methods in Physics Research B 155 (1999) 426±430

www.elsevier.nl/locate/nimb

* Corresponding author.

0168-583X/99/$ ± see front matter Ó 1999 Elsevier Science B.V. All rights reserved.

PII: S 0 1 6 8 - 5 8 3 X ( 9 9 ) 0 0 4 8 5 - 1

subtended by the source parallel to SSNTD ofcircular aperture of radius Rd. X is de®ned as theratio of the number of particles per second emittedinside the space con®ned between the source areaand detector surface to the total activity of thesource.

Fig. 1 shows a representation of X in case of aplane source parallel to a plane detector. One can®nd an expression for X in terms of an integralform of Bessel function [9] which leads to the fol-lowing formalism

X � x2

41

�ÿ 3

4x2ÿ � w2

�� 15

8

x4 � w4

3

�� w2x2

�ÿ 38

15

x6 � w6

4

� �� 3

2w2x2 w2x2

ÿ ��; �1�

where w�Rs/d, x�Rd/d and d is the source-to-detector distance.

The expected number of alphas that reaches thedetector surface per second is then given by

/1 � AX; �2�

where A is the activity of the radiation source. Incase of d � Rs and for 1 cm2 detector aperture, /tends to the usual approach of point source ap-proximation, i.e.

/p � A=4pd2: �3�

Formula for ¯ux calculation [10,11] were also in-troduced by other investigators and take the fol-lowing forms

/2 �A

4pR2s

ln R2s

��� d2 ÿ R2

d

����������������������������������R2

s ÿ d2 ÿ R2d�2

q� 4R2

dd2

�4d2� �

�4�

and

/3 �A

pR2s

ln

�����R2

s

p � dd

( ); �5�

where symbols are de®ned as before.In the case of inclined incidence of radiation,

i.e. the plane of the detector makes an angle h withthat of the source. Eq. (2) takes the following formfor the inclined incidence case

/h �Ax2

41

�ÿ 3

4w2� � x2 1

ÿ � sin2h��

� 15

8

w4 � x4

3

�� w2

�� x2 1

�� 1

4x2 � 2

3w2

�sin2h

�: �6�

2. Experimental

2.1. Detectors and etching conditions

Plastic sheets of three di�erent track detectorswere used in this study and etched at the mostrecommended optimum etching conditions [4,5] ofetching concentration and temperature. They wereCR-39 detector of thickness 250 lm (TASTRAKsupplied by Bristol, UK) etched in 6.25N NaOH at

Fig. 1. Description of solid angle for disk source and circular

detector.

H. El-Samman et al. / Nucl. Instr. and Meth. in Phys. Res. B 155 (1999) 426±430 427

70°C, LR-115 detector (Kodak, France) etched in2.5N NaOH at 50°C, 60°C. CR-39 detectors wereetched for di�erent etching duration, while LR-115and CN-85 detector were etched for 1.5 and 6 h,respectively.

2.2. Irradiation facility

Detectors were exposed to a thin electroplated241Am disk alpha source of activity 34.4 kBq and asurface area of 19.63 mm2. Irradiation was carriedout in air and in irradiation chamber [8] designedfor such purpose where alpha energy is varied byvarying the pressure inside the chamber. The di-mensions of the developed track were then mea-sured under an optical microscope attached to aneyepiece micrometer with each division equals to0.22 lm. Background tracks originated in an un-exposed detectors were ®rst measured and subtr-acted, if it were found, before any track densityevaluation.

3. Results and discussion

Alpha particle ¯ux (/) based on expressionsrepresented by Eqs. (1)±(5) are calculated atdi�erent values of d and shown in Fig. 2. From in-spection of this ®gure, it is clear that the variationsof /2, /3 and /p with d are almost the same; theyshown a rapid decrease with increasing d within therange from 0.5 cm to about 1.5 cm. Beyond thisregion these functions continue to decrease slowlywith d up to 6.0 cm. On the other hand, although /1

varies with d in the same manner as /2, /3 and /p,but the /1±d curve lies lower.

For values of d > 4 cm, all /, curves coincide,i.e. values of alpha ¯uxes calculated from all thestudied approaches (/1, /2, /3 and /p) are almostthe same at each value of d. As a result we maysuggest that the point source approximation (/p) issu�ciently enough to be applied in the region ofd > 4 cm where this approximation works well withhigh degree of accuracy.

The dependence of registration e�ciency ofLR-115 detector on alpha energy is represented inFig. 3. Detectors were irradiated in an irradiationchamber, etched and counted under the micro-

scope. From the inspection of this ®gure, it is clearthat, in alpha energy range from 1.5 to about 4.5MeV, the detector e�ciency register a value ofabout 90% for etching times from 2 to 6 h. Beyond4.5 MeV, the e�ciency drops to zero. The data

Fig. 2. Variation of ¯ux as a function of source-to-detector

distance.

Fig. 3. Variation of registration e�ciency with alpha energy of

LR-115 detectors etched in 2.5N NaOH at 50°C for di�erent

etching duration.

428 H. El-Samman et al. / Nucl. Instr. and Meth. in Phys. Res. B 155 (1999) 426±430

represented in Fig. 3 provides an energy windowfor possible track development in such detectorunder study.

A comparison between the registration e�-ciency of LR-115 detector irradiated in air and inthe irradiation chamber is given in Fig. 4. It is clearthat irradiation in air increases the registratione�ciency of the detector by a factor of about 15%which re¯ects the importance of oxygen concen-tration in medium surrounding the detector.

Fig. 5 shows the dependence of registration ef-®ciency of alpha particle from 1.0 to 5.48 MeV ofLR-l15, CN-85 and CR-39 detectors. It is obviousfrom this ®gure that the e�ciency of LR-115 andCN-85 detectors is about 80% in the energy rangefrom 1.0±3.5 MeV and then shows a decrease withfurther increase in energy reaching 5% at 5.48 MeV.On the other hand the e�ciency of CR-39 varieswith energy in di�erent manners. It slightly in-creases with energy reaching a value of about 100%at 2.5 MeV and then behaves almost steadily withfurther increase in energy up to about 5.48 MeV.

From the comparison between the e�ciencycurves displayed in Fig. 5, one can notice that theCR-39 detector represents the best e�ciency ap-plicable detector in the entire range of the studied

alpha energies. This re¯ects the feasibility of suc-cessfully using the CR-39 in many dosimetric ap-plications.

Fig. 6 shows the variation of the inclined inci-dent ¯ux (as calculated from Eq. (5)) with incidentangle at constant source-to-detector distance (5cm) in vacuum. It also shows the variation of track

Fig. 4. Comparison between registration e�ciency of LR-115

track detectors irradiated in air and in an irradiation chamber.

Fig. 5. Comparison of registration e�ciencies versus alpha

energy for CN-85, LR-115 and CR-39 track detectors, where all

detectors were irradiated in air and etched for a duration of 1.5

h for both LR115 and CN-85 while CR-39 was etched for 5 h.

Fig. 6. Inclined ¯ux and alpha track density as a function of

incident angle.

H. El-Samman et al. / Nucl. Instr. and Meth. in Phys. Res. B 155 (1999) 426±430 429

density with incident angle. From this ®gure it iseasy to notice that both of ¯ux and track densityshow a decrease as the angle-to-beam directiondecreases.

Fig. 7 represents the variation of e�ciency withincident angle using LR-115 track detectors. It isobvious from this ®gure that the inclined regis-tration e�ciency has constant value at about 80%for energies from 1.0 up to 3.5 MeV with anglefrom 90° to 40° while at 4.5 MeV, the e�ciencydecreases from 80% to 20% as the angle variesfrom 90° to 30°.

4. Conclusion

Alpha ¯ux of 241Am disk source was calculatedat normal and inclined incidence using di�erent

theoretical approaches. Absolute registration e�-ciency of di�erent types of track detectors viz.,CR-39, CN-85 and LR-115 was determined underdi�erent conditions. It was found that both of LR-l15 and CN-85 have a well de®ned energy windowof registration e�ciency while CR-39 showed thesame value of e�ciency (within the experimentaluncertainties) for all studied alpha energies. It wasalso found that the registration e�ciency of LR-115 track detector exposed in air increases by afactor of about 15% than those irradiated underlow air pressure.

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Fig. 7. Dependence of registration e�ciency of LR-115 track

detectors on incident angle at di�erent alpha energies.

430 H. El-Samman et al. / Nucl. Instr. and Meth. in Phys. Res. B 155 (1999) 426±430