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Studies of the response of CR-39 track detectors to protons from a 3 MeV Van de Graaff accelerator
Nuclear Instruments North-Holland
and Methods
in Physics
Research
B53 (1991) 61-66
61
Studies of the response of CR-39 track detectors to protons from a 3 MeV Van de Graaff accelerator L. Bernardi
‘) , A. Cecchi
2,, C. Gori 2,3), F . Lucarelli
2,4) and R. Renzi ‘)
” Dipartimento di Fisica Medica, Universitd di Firenze, Iialy Ii Istituto Nazionale di Fisica Nucleare, L. go E. Fermi 2, 50125 Firenze, Italy -?,Servizio di Fisica Sanitaria, U.S.L. IO/D, Firenze, Ita& 4i Dipartimento di Fisica deN’lJniversitd, L. go E. Fermi 2, 50125 Firenre, Italy Received
8 June 1990 and in revised form 6 August
1990
A Van de Graaff proton accelerator was tested for its application to the study of CR-39 SSNTD response to accelerated protons. The energies of the proton beams ranged from 0.2 to 2.3 MeV. Two different beam angles of incidence were taken into consideration for each proton energy (i.e. normal incidence and 45O). The complete procedure for the irradiation of a CR-39 sample required relatively little time and the results obtained - in agreement with others as reported in the literature - demonstrated that the Van de Graaff proton accelerator can find a useful application to the problem of neutron dosimetry using SSNTDs.
1. Introduction
incidence: their responses using both chemical trochemical etching were then observed.
At present solid state nuclear track detectors, or SSNTDs for short, are widely used in the field of neutron detection. Particularly good results have been attained using a plastic solid state nuclear track detector, CR-39, the organic compound polyallyl-diglycolcarbonate. In spite of some unexplained phenomena, these detectors are already in practical use as neutron dosimeters. However much work is still in progress for a full characterization of the CR-39 response to various neutron irradiation conditions (i.e. energy and angle of incidence) [l]. Several researchers dealing with that problem have recently started using proton beams instead of neutron beams because of their more accurate energy and precise angle of incidence [2-41. This approach is useful as regards the problem of SSNTD application to neutron dosimetry: within the energy range from 0.1 to 10 MeV the contribution of recoil protons to track formation is by far more important than that of carbon and oxygen recoils or alphas originating from the 0i6(n, cy)Ci3 reaction [5,6]. The aim of this work was to test the efficacy of a 3 MeV Van de Graaff proton accelerator to study the response characteristics of plastic SSNTDs. An additional interest in utilizing this accelerator was its ability to produce proton beams of energy below 0.5 MeV, the energy range which requires the greatest attention when dosimetric applications are considered. We therefore arranged an experimental setup to irradiate the plastic detectors at different energies and angles of 0168-583X/91/$03.50
0 1991 - Elsevier Science Publishers
and elec-
2. Experimental setup 2.1. Irradiation The irradiation of the CR-39 detectors was performed using proton beams with an energy range from 0.3 to 3.0 MeV obtained using the KN 3000 Van de Graaff accelerator installed at the Department of Physics of the University of Florence. Since the required optimum track density over the detector surface must not be higher than 200 tracks/mm2, even the minimum beam current (0.1 nA) obtainable using this accelerator, was too high. A special arrangement was thus introduced into the irradiation chamber as depicted in figs. la and lb. A thin carbon target (60 pg/cm2) was positioned perpendicularly to the beam direction. The detector was therefore indirectly hit only by those protons undergoing a 135O angle backscatter from the carbon target. Since the probability of multiple interaction in the target was very low, the scattering process did not introduce any appreciable degradation of the original proton energy which was, however, reduced by a factor 0.75. Additionally, this allowed for the irradiation of detector areas larger than the actual beam section (= 1 mm2) with virtually parallel protons as long as the diameter of the irradiated area was much smaller (= 5
B.V. (North-Holland)
L. Bernardi et al. / Response of CR-39 track detectors to 3 Mel/protons
62
relation to that of the CR-39 detector. This permitted the control, in real time, of both the energy spectrum of the 135” angle scattered protons and the counting rate over its known area (which was collimated to 32 mm2). The energies used to irradiate the CR-39 detectors were 0.24, 0.40, 0.79, 1.54, and 2.30 MeV. Other samples were irradiated using protons with reduced energy (0.15 MeV) by superimposition of a 1.5 pm thick Mylar foil over the CR-39 surface. In this latter condition the energy degradation was approximatively 15% FWHM.
Si detector
2.2. Chemical
a
CR 39
b Fig. 1. Experimental setup. a) Horizontal section, b) vertical section.
mm) than the distance between the detector and the carbon target. Under these conditions, the time required to obtain the optimum track density was about 30 s. This amount of time was adequate to allow for our manual control of the irradiation process. A silicon surface barrier detector was positioned within the vacuum chamber in a symmetrical position in
Fig. 2. Examples
of tracks
from
normally
incident
and electrochemical
etching procedure
After being irradiated the samples were separated into two groups. One group underwent chemical etching under those conditions (KOH 6M, 60 a C) which provide the best ratio between the track and the bulk etch velocity [7]. The second group underwent electrochemical etching with a short chemical pre-etching. The electrochemical etching consisted of a chemical etching in the presence of an oscillating electrical field (30 kV/cm (RMS) at 2 kHz), according to methods already reported in the literature [8,9] as being the best empirically determined. The two groups were observed step by step in order to follow the track evolution as the etching process was taking place. At regular time intervals, (see table 1) the samples were extracted from the thermostatic bath, immediately washed and then observed by means of the “System III Image Analyzer Computer” connected to an optical microscope. This arrangement allowed for the measurement of several parameters of the track shape (i.e. area, maximum and minimum diameter, perimeter length, etc.), in addition to the calculation of the track density. Figs. 2a and 2b show the tracks obtained by chemical etching. Figs. 3a-3d show a sequence of the track
protons: a) 0.79 MeV protons. (19 h them. etch.).
(19 h them.
etch.),
b) 2.30 MeV protons.
63
L. Bernardi et al. / Response of CR-39 track detectors to 3 MeVprotons
Fig. 3. Electrochemically
etched tracks from 0.40 MeV normally
Table 1 Mean diameter (pm) of normally standard error of the mean. Etching time (hours) 0.5 1 3 5 7 9 11 13 15 17 19
incident
protons
vs chemical
incident
etching
protons.
Etching
time at different
time: a) 1 h. b) 2 h. c) 3 h. d) 5 h.
energies.
Values
are reported
Energy (MeV) 0.15
0.24
0.40
0.79
1.54
_ _ _
_ _ _ 3.9 +0.5 6.0f0.5 6.7+0.8 8.5 + 1 _
4.5 +0.5 6.4kO.5 7.6kO.8 8.9*1
_
2.30
_ 5.4*0.1 7.5 + 0.2 9.650.3 1o.s+o.4 13.1 &OS 15.1+ 0.6 16.8 f 0.7
3.2kO.l 5.6 f 0.2 7x+0.3 9.5 + 0.4 11.8F0.4 13.9*0.4 16.750.5
_ 4.4kO.l 6.0+0.3 7.8kO.3 9.2 i 0.4 10.9 f 0.5 13.7*0.5
4.4 f 0.1 5.2 f 0.2 6.6kO.4 7.7kO.4 8.9*0.4 11.1*0.5
with their
L. Bernardi et al. / Response of CR-39 track detectors to 3 MeVprotons
64
Table 3 Calculated V,/V,
4
3
zo-
$
E”
.;
15-
q
0.79 MeV
o
1.54
n
2.30 MeV
MeV
values for three different proton energies.
Energy (MeV)
v,/vb
0.79 1.54 2.30
1.45 1.24 1.18
2 f
10 -
S-
0
1;
:
1; Etching
2b time
(hours)
Fig. 4. Mean diameter of tracks from normally incident protons vs chemical etching time for three different energies. (0.79, 1.54,2.30 MeV).
formation etching.
obtained
by means
of the electrochemical
In fig. 4 an almost linear dependence of the mean track diameter vs etching time can be seen. This allows for the calculation of the ratio between the etch velocity (V,) along the track and the bulk etch velocity (V,) (table 3) according to the simple model published by Somogyi [ll] which considers the etch velocity as being constant along the entire track.Our results demonstrate that this model is applicable because of the small variation (about 20%) of the calculated V,/V,, values within the energy range 0X-2.3 MeV. Furthermore, the experimental results on track detectability reported in table 2 for protons incident at an angle of 45 o agree with the theoretical detection condition: (V,/V,)
3. Results The results obtained after the chemical etching are summarized in tables 1 and 2. In table 1, the track diameter vs. etching time is reported for different energies of normally incident protons. In table 2 the track area vs etching time and proton energy is reported for protons incident at an angle of 45O. The data show that protons with energy above 0.8 MeV are no longer detected if the angle of incidence is 45 O, This result agrees with experimental data already reported in the literature [3,10].
cos e > 1,
(0 being the angle of incidence) if in this inequality the actual V/V, values of table 3 (normally incident protons) are introduced. The tracks detectability obtained by application of the electrochemical etching does not differ from that resulting from tables 1 and 2, as obtained by chemical etching. In particular also in this case it was impossible to detect protons incident at an angle of 45 o at energies above 0.8 MeV. This is in accordance with the assumption that pits obtained by chemical etching are necessary before the electrical field is allowed to enhance the material damage through the breakdown phenomena.
Table 2 Mean area (urn*) of tracks from protons incident at an angle of 45” vs chemical etching time for different proton energies. Values are reported with their standard error of the mean. Etching time (h) 0.5 1 3 5 1 9 11 13 15 17 19
Energy (MeV) 0.15
- a)
0.24
11+2 16+5 _ _
0.40
_ 10*0.5 23kl 5052 7453 90_+3 96+5 100+5
0.79
_ 11 f0.6 19&l 31+2 45+3 60+5 78+5
‘I Tracks are recognizable but too much blurred for a significant assessment of their area.
1.54
2.30
_
_
_ _ _ _
L. Bernardi et a/. / Responseof CR-39 track detectorsto 3 Me Yprotons
1500
lOB0
500
2000
500
AREA Pm2
1000
1500
60"
0
1000
500
2000
AREA I*m2
1500
2000
(4
0
500
1000
1500
2000
AREA pm2
AREA pm* Fig. 5. Comparision of track size distribution obtained with 0.40 MeV protons and electrochemical etching: a) 45 ’ incidence: tracks counted without any selection on the track shape. b) 45 ’ incidence: tracks counted according to the condition FF > 0.8. c) Normal incidence: tracks counted without any selection on the track shape. d) Normal incidence: tracks counted according to the condition FF 7 0.8.
The strong dependence of proton detectability on the angle of incidence is a serious drawback for the use of CR-39 as a neutron dosimetric tool. Actually, recoil protons generated from neutron interaction are scattered in any direction (from 0 o to 90 “). A possible approach to this drawback might be the restriction of track count-
ing to only those tracks which are produced by recoil protons which almost normally hit the detector surface. The spots of tracks from normally incident protons seem to be, in fact, characterized by a rather identifying circular shape when compared to others. This characteristic is also present, though to a lower extent, in the tracks produced by electrochemical etching, which, on the other hand, are best suited for automatic counting. One of the features of the System III Image Analyzer is the possibility of calculating the so called “shape factor” FF of each detected track, defined as: FF=
4n x AREA (PERIMETERI
.
Therefore it is possible to select the counting of only
the tracks with an almost circular shape, according to the condition that the shape factor FF is preset above a certain value. The effect of selective counting in accordance with the track spot shape is depicted in fig. 5. It can be seen that a remarkable reduction of the number of counted tracks produced by protons incident at an angle of 45 o occurred after the condition FF > 0.8 was introduced. On the other hand, the number of counted tracks remained almost unaltered for the normally incident protons, whether the shape factor condition FF > 0.8 was applied or not. Finally, our results reported in tables 1 and 2 show a maximum of detectability (tracks detectable over different angles of incidence) at an energy level of about 0.5 MeV. Since the mean energy transferred from a neutron to a proton in an elastic scattering is one half of the initial neutron energy, we expect a m~mum of neutron detectability at an energy level of about 1 MeV. Actually, data on neutron detectability vs neutron energy as evaluated from the experimental data of Azimi-Carakani
L. Bernardi et al. / Response of CR-39 track detectors to 3 MeYprotons
66
et al. [12,13] corrected for the fluence to dose equivalent conversion function [14] seem to confirm this consideration.
4, Conclusions The complete irradiation
The energy spread was 20 keV FWHM. Finally, due to the Iittle energy spread of the beams produced by a Van de Graaff accelerator, its use can be relevant in studies on the physico-chemical processes involved during the chemical or electrochemical etching of the detector surface.
procedure using a 3 MeV
Van de Graaff accelerator was rather simple. Each sample was irradiated and changed for another within a few minutes. Simple arrangements could be introduced within the vacuum chamber to change the samples in a
shorter time than has been previously described. Nevertheless the i~adiation procedure used in this study is suitable for some practical applications, such as the quality control of new batches of plastic detectors, where irradiation with protons of very few different energies and angles of incidence are required. The vacuum chamber used in this study was designed to allow for the verification of the spread of proton energy before - as we11as during - each irradiation by means of the silicon detector. This was additionally useful to verify, after the irradiation, whether or not the desired amount of protons per mm’ actually reached the target. Theoretical calculations by Sadaka and others [15] have been extensively developed with the aim of establishing an accurate correlation between the absorbed dose and the number of tracks in a plastic detector of appropriate geometry. However these calculations require precise knowledge of the detection limit angle as a function of the energy of the recoil protons, Up to now only limited information is available on the subject and additional experimental work is necessary. The Van de Graaff accelerator could be useful for research appiications in this still limited field. A 3 MeV Van de Graaff accelerator allows for an accurate study of SSNTD response to protons from a maximum energy of 2-3 MeV to a ~nimum of 0.2 MeV. However further decreases in the proton energy are possible by superimposing a thin foil over the plastic detector. During some preliminary studies, protons of 70 keV have been obtained with a thin Mylar foil.
The authors wish to thank Dr. Luigi Tommasino and Dr. Giancarlo Torri of ENFA-DISP for their theoretical advice and assistance during the etching and counting procedure. The authors wish also to thank Dr. Augustine Kyere for the helpful discussions during the preliminary studies for .this work. References [I] L. Tommasino, Nucl. Instr. and Meth. A255 (1987) 293. 121 W.Cr.Cross, Nuel. Tracks 12 (1986) 533. [3] W.C. Cross, A. Arneja and H. Ing, Nucl. Tracks 12 (1986) 649. 141 A.K. Kyere, A. Cecchi and C. Gori, Report Univ. of Plorence DFF 80 (1988). 151 K.N. Beckurts and EK. Wirtz, Neutron Physics (Springer Verlag Berlin, 1964). 161 K. James, Matiull~ and S.A. Durrani, Rad. Prot. DOS. 19 (1987) 5. [7] S.A. Durrani and R.K. Bull, Solid State Nuclear Track Detection (Pergamon Press, Oxford, 1987). [8) L. Tommasino and KG. Harrison, Rad. Prot. DOS 10 (1985) 219. 191 I.. Tommasino, Rad. Fret. DOS. 17 (1986) 219. [lo] Matiuil~ and S.A. Durrani, Rad. Prot. DOS.19 (1987) 15. 1111 G. Somogyi, and S.A. Szalay, Nucl. Ins@. and Meth. 109 (1973) 211. [El D. Azimi-Can&am, B. Flares, I_. Tommasino and G. Toni, I&ados Cendos Report 1987-01, KfK-4305 (1987). 1131 L. Bernardi, L. Tommasino, G. Torri and D. AzimiGarakani, Eurados Cendos Report in press (1989). 1141 ICRP Publication No. 21 (Pergamon, Oxford-LondonNew York, 1976). [15] J.L. Descossas, S. Sadaka, J.C. Vareille, L. Makovicka and J.L. Teyssier, Nucl. Tracks 12 (1986) 543.
and Methods
in Physics
Research
B53 (1991) 61-66
61
Studies of the response of CR-39 track detectors to protons from a 3 MeV Van de Graaff accelerator L. Bernardi
‘) , A. Cecchi
2,, C. Gori 2,3), F . Lucarelli
2,4) and R. Renzi ‘)
” Dipartimento di Fisica Medica, Universitd di Firenze, Iialy Ii Istituto Nazionale di Fisica Nucleare, L. go E. Fermi 2, 50125 Firenze, Italy -?,Servizio di Fisica Sanitaria, U.S.L. IO/D, Firenze, Ita& 4i Dipartimento di Fisica deN’lJniversitd, L. go E. Fermi 2, 50125 Firenre, Italy Received
8 June 1990 and in revised form 6 August
1990
A Van de Graaff proton accelerator was tested for its application to the study of CR-39 SSNTD response to accelerated protons. The energies of the proton beams ranged from 0.2 to 2.3 MeV. Two different beam angles of incidence were taken into consideration for each proton energy (i.e. normal incidence and 45O). The complete procedure for the irradiation of a CR-39 sample required relatively little time and the results obtained - in agreement with others as reported in the literature - demonstrated that the Van de Graaff proton accelerator can find a useful application to the problem of neutron dosimetry using SSNTDs.
1. Introduction
incidence: their responses using both chemical trochemical etching were then observed.
At present solid state nuclear track detectors, or SSNTDs for short, are widely used in the field of neutron detection. Particularly good results have been attained using a plastic solid state nuclear track detector, CR-39, the organic compound polyallyl-diglycolcarbonate. In spite of some unexplained phenomena, these detectors are already in practical use as neutron dosimeters. However much work is still in progress for a full characterization of the CR-39 response to various neutron irradiation conditions (i.e. energy and angle of incidence) [l]. Several researchers dealing with that problem have recently started using proton beams instead of neutron beams because of their more accurate energy and precise angle of incidence [2-41. This approach is useful as regards the problem of SSNTD application to neutron dosimetry: within the energy range from 0.1 to 10 MeV the contribution of recoil protons to track formation is by far more important than that of carbon and oxygen recoils or alphas originating from the 0i6(n, cy)Ci3 reaction [5,6]. The aim of this work was to test the efficacy of a 3 MeV Van de Graaff proton accelerator to study the response characteristics of plastic SSNTDs. An additional interest in utilizing this accelerator was its ability to produce proton beams of energy below 0.5 MeV, the energy range which requires the greatest attention when dosimetric applications are considered. We therefore arranged an experimental setup to irradiate the plastic detectors at different energies and angles of 0168-583X/91/$03.50
0 1991 - Elsevier Science Publishers
and elec-
2. Experimental setup 2.1. Irradiation The irradiation of the CR-39 detectors was performed using proton beams with an energy range from 0.3 to 3.0 MeV obtained using the KN 3000 Van de Graaff accelerator installed at the Department of Physics of the University of Florence. Since the required optimum track density over the detector surface must not be higher than 200 tracks/mm2, even the minimum beam current (0.1 nA) obtainable using this accelerator, was too high. A special arrangement was thus introduced into the irradiation chamber as depicted in figs. la and lb. A thin carbon target (60 pg/cm2) was positioned perpendicularly to the beam direction. The detector was therefore indirectly hit only by those protons undergoing a 135O angle backscatter from the carbon target. Since the probability of multiple interaction in the target was very low, the scattering process did not introduce any appreciable degradation of the original proton energy which was, however, reduced by a factor 0.75. Additionally, this allowed for the irradiation of detector areas larger than the actual beam section (= 1 mm2) with virtually parallel protons as long as the diameter of the irradiated area was much smaller (= 5
B.V. (North-Holland)
L. Bernardi et al. / Response of CR-39 track detectors to 3 Mel/protons
62
relation to that of the CR-39 detector. This permitted the control, in real time, of both the energy spectrum of the 135” angle scattered protons and the counting rate over its known area (which was collimated to 32 mm2). The energies used to irradiate the CR-39 detectors were 0.24, 0.40, 0.79, 1.54, and 2.30 MeV. Other samples were irradiated using protons with reduced energy (0.15 MeV) by superimposition of a 1.5 pm thick Mylar foil over the CR-39 surface. In this latter condition the energy degradation was approximatively 15% FWHM.
Si detector
2.2. Chemical
a
CR 39
b Fig. 1. Experimental setup. a) Horizontal section, b) vertical section.
mm) than the distance between the detector and the carbon target. Under these conditions, the time required to obtain the optimum track density was about 30 s. This amount of time was adequate to allow for our manual control of the irradiation process. A silicon surface barrier detector was positioned within the vacuum chamber in a symmetrical position in
Fig. 2. Examples
of tracks
from
normally
incident
and electrochemical
etching procedure
After being irradiated the samples were separated into two groups. One group underwent chemical etching under those conditions (KOH 6M, 60 a C) which provide the best ratio between the track and the bulk etch velocity [7]. The second group underwent electrochemical etching with a short chemical pre-etching. The electrochemical etching consisted of a chemical etching in the presence of an oscillating electrical field (30 kV/cm (RMS) at 2 kHz), according to methods already reported in the literature [8,9] as being the best empirically determined. The two groups were observed step by step in order to follow the track evolution as the etching process was taking place. At regular time intervals, (see table 1) the samples were extracted from the thermostatic bath, immediately washed and then observed by means of the “System III Image Analyzer Computer” connected to an optical microscope. This arrangement allowed for the measurement of several parameters of the track shape (i.e. area, maximum and minimum diameter, perimeter length, etc.), in addition to the calculation of the track density. Figs. 2a and 2b show the tracks obtained by chemical etching. Figs. 3a-3d show a sequence of the track
protons: a) 0.79 MeV protons. (19 h them. etch.).
(19 h them.
etch.),
b) 2.30 MeV protons.
63
L. Bernardi et al. / Response of CR-39 track detectors to 3 MeVprotons
Fig. 3. Electrochemically
etched tracks from 0.40 MeV normally
Table 1 Mean diameter (pm) of normally standard error of the mean. Etching time (hours) 0.5 1 3 5 7 9 11 13 15 17 19
incident
protons
vs chemical
incident
etching
protons.
Etching
time at different
time: a) 1 h. b) 2 h. c) 3 h. d) 5 h.
energies.
Values
are reported
Energy (MeV) 0.15
0.24
0.40
0.79
1.54
_ _ _
_ _ _ 3.9 +0.5 6.0f0.5 6.7+0.8 8.5 + 1 _
4.5 +0.5 6.4kO.5 7.6kO.8 8.9*1
_
2.30
_ 5.4*0.1 7.5 + 0.2 9.650.3 1o.s+o.4 13.1 &OS 15.1+ 0.6 16.8 f 0.7
3.2kO.l 5.6 f 0.2 7x+0.3 9.5 + 0.4 11.8F0.4 13.9*0.4 16.750.5
_ 4.4kO.l 6.0+0.3 7.8kO.3 9.2 i 0.4 10.9 f 0.5 13.7*0.5
4.4 f 0.1 5.2 f 0.2 6.6kO.4 7.7kO.4 8.9*0.4 11.1*0.5
with their
L. Bernardi et al. / Response of CR-39 track detectors to 3 MeVprotons
64
Table 3 Calculated V,/V,
4
3
zo-
$
E”
.;
15-
q
0.79 MeV
o
1.54
n
2.30 MeV
MeV
values for three different proton energies.
Energy (MeV)
v,/vb
0.79 1.54 2.30
1.45 1.24 1.18
2 f
10 -
S-
0
1;
:
1; Etching
2b time
(hours)
Fig. 4. Mean diameter of tracks from normally incident protons vs chemical etching time for three different energies. (0.79, 1.54,2.30 MeV).
formation etching.
obtained
by means
of the electrochemical
In fig. 4 an almost linear dependence of the mean track diameter vs etching time can be seen. This allows for the calculation of the ratio between the etch velocity (V,) along the track and the bulk etch velocity (V,) (table 3) according to the simple model published by Somogyi [ll] which considers the etch velocity as being constant along the entire track.Our results demonstrate that this model is applicable because of the small variation (about 20%) of the calculated V,/V,, values within the energy range 0X-2.3 MeV. Furthermore, the experimental results on track detectability reported in table 2 for protons incident at an angle of 45 o agree with the theoretical detection condition: (V,/V,)
3. Results The results obtained after the chemical etching are summarized in tables 1 and 2. In table 1, the track diameter vs. etching time is reported for different energies of normally incident protons. In table 2 the track area vs etching time and proton energy is reported for protons incident at an angle of 45O. The data show that protons with energy above 0.8 MeV are no longer detected if the angle of incidence is 45 O, This result agrees with experimental data already reported in the literature [3,10].
cos e > 1,
(0 being the angle of incidence) if in this inequality the actual V/V, values of table 3 (normally incident protons) are introduced. The tracks detectability obtained by application of the electrochemical etching does not differ from that resulting from tables 1 and 2, as obtained by chemical etching. In particular also in this case it was impossible to detect protons incident at an angle of 45 o at energies above 0.8 MeV. This is in accordance with the assumption that pits obtained by chemical etching are necessary before the electrical field is allowed to enhance the material damage through the breakdown phenomena.
Table 2 Mean area (urn*) of tracks from protons incident at an angle of 45” vs chemical etching time for different proton energies. Values are reported with their standard error of the mean. Etching time (h) 0.5 1 3 5 1 9 11 13 15 17 19
Energy (MeV) 0.15
- a)
0.24
11+2 16+5 _ _
0.40
_ 10*0.5 23kl 5052 7453 90_+3 96+5 100+5
0.79
_ 11 f0.6 19&l 31+2 45+3 60+5 78+5
‘I Tracks are recognizable but too much blurred for a significant assessment of their area.
1.54
2.30
_
_
_ _ _ _
L. Bernardi et a/. / Responseof CR-39 track detectorsto 3 Me Yprotons
1500
lOB0
500
2000
500
AREA Pm2
1000
1500
60"
0
1000
500
2000
AREA I*m2
1500
2000
(4
0
500
1000
1500
2000
AREA pm2
AREA pm* Fig. 5. Comparision of track size distribution obtained with 0.40 MeV protons and electrochemical etching: a) 45 ’ incidence: tracks counted without any selection on the track shape. b) 45 ’ incidence: tracks counted according to the condition FF > 0.8. c) Normal incidence: tracks counted without any selection on the track shape. d) Normal incidence: tracks counted according to the condition FF 7 0.8.
The strong dependence of proton detectability on the angle of incidence is a serious drawback for the use of CR-39 as a neutron dosimetric tool. Actually, recoil protons generated from neutron interaction are scattered in any direction (from 0 o to 90 “). A possible approach to this drawback might be the restriction of track count-
ing to only those tracks which are produced by recoil protons which almost normally hit the detector surface. The spots of tracks from normally incident protons seem to be, in fact, characterized by a rather identifying circular shape when compared to others. This characteristic is also present, though to a lower extent, in the tracks produced by electrochemical etching, which, on the other hand, are best suited for automatic counting. One of the features of the System III Image Analyzer is the possibility of calculating the so called “shape factor” FF of each detected track, defined as: FF=
4n x AREA (PERIMETERI
.
Therefore it is possible to select the counting of only
the tracks with an almost circular shape, according to the condition that the shape factor FF is preset above a certain value. The effect of selective counting in accordance with the track spot shape is depicted in fig. 5. It can be seen that a remarkable reduction of the number of counted tracks produced by protons incident at an angle of 45 o occurred after the condition FF > 0.8 was introduced. On the other hand, the number of counted tracks remained almost unaltered for the normally incident protons, whether the shape factor condition FF > 0.8 was applied or not. Finally, our results reported in tables 1 and 2 show a maximum of detectability (tracks detectable over different angles of incidence) at an energy level of about 0.5 MeV. Since the mean energy transferred from a neutron to a proton in an elastic scattering is one half of the initial neutron energy, we expect a m~mum of neutron detectability at an energy level of about 1 MeV. Actually, data on neutron detectability vs neutron energy as evaluated from the experimental data of Azimi-Carakani
L. Bernardi et al. / Response of CR-39 track detectors to 3 MeYprotons
66
et al. [12,13] corrected for the fluence to dose equivalent conversion function [14] seem to confirm this consideration.
4, Conclusions The complete irradiation
The energy spread was 20 keV FWHM. Finally, due to the Iittle energy spread of the beams produced by a Van de Graaff accelerator, its use can be relevant in studies on the physico-chemical processes involved during the chemical or electrochemical etching of the detector surface.
procedure using a 3 MeV
Van de Graaff accelerator was rather simple. Each sample was irradiated and changed for another within a few minutes. Simple arrangements could be introduced within the vacuum chamber to change the samples in a
shorter time than has been previously described. Nevertheless the i~adiation procedure used in this study is suitable for some practical applications, such as the quality control of new batches of plastic detectors, where irradiation with protons of very few different energies and angles of incidence are required. The vacuum chamber used in this study was designed to allow for the verification of the spread of proton energy before - as we11as during - each irradiation by means of the silicon detector. This was additionally useful to verify, after the irradiation, whether or not the desired amount of protons per mm’ actually reached the target. Theoretical calculations by Sadaka and others [15] have been extensively developed with the aim of establishing an accurate correlation between the absorbed dose and the number of tracks in a plastic detector of appropriate geometry. However these calculations require precise knowledge of the detection limit angle as a function of the energy of the recoil protons, Up to now only limited information is available on the subject and additional experimental work is necessary. The Van de Graaff accelerator could be useful for research appiications in this still limited field. A 3 MeV Van de Graaff accelerator allows for an accurate study of SSNTD response to protons from a maximum energy of 2-3 MeV to a ~nimum of 0.2 MeV. However further decreases in the proton energy are possible by superimposing a thin foil over the plastic detector. During some preliminary studies, protons of 70 keV have been obtained with a thin Mylar foil.
The authors wish to thank Dr. Luigi Tommasino and Dr. Giancarlo Torri of ENFA-DISP for their theoretical advice and assistance during the etching and counting procedure. The authors wish also to thank Dr. Augustine Kyere for the helpful discussions during the preliminary studies for .this work. References [I] L. Tommasino, Nucl. Instr. and Meth. A255 (1987) 293. 121 W.Cr.Cross, Nuel. Tracks 12 (1986) 533. [3] W.C. Cross, A. Arneja and H. Ing, Nucl. Tracks 12 (1986) 649. 141 A.K. Kyere, A. Cecchi and C. Gori, Report Univ. of Plorence DFF 80 (1988). 151 K.N. Beckurts and EK. Wirtz, Neutron Physics (Springer Verlag Berlin, 1964). 161 K. James, Matiull~ and S.A. Durrani, Rad. Prot. DOS. 19 (1987) 5. [7] S.A. Durrani and R.K. Bull, Solid State Nuclear Track Detection (Pergamon Press, Oxford, 1987). [8) L. Tommasino and KG. Harrison, Rad. Prot. DOS 10 (1985) 219. 191 I.. Tommasino, Rad. Fret. DOS. 17 (1986) 219. [lo] Matiuil~ and S.A. Durrani, Rad. Prot. DOS.19 (1987) 15. 1111 G. Somogyi, and S.A. Szalay, Nucl. Ins@. and Meth. 109 (1973) 211. [El D. Azimi-Can&am, B. Flares, I_. Tommasino and G. Toni, I&ados Cendos Report 1987-01, KfK-4305 (1987). 1131 L. Bernardi, L. Tommasino, G. Torri and D. AzimiGarakani, Eurados Cendos Report in press (1989). 1141 ICRP Publication No. 21 (Pergamon, Oxford-LondonNew York, 1976). [15] J.L. Descossas, S. Sadaka, J.C. Vareille, L. Makovicka and J.L. Teyssier, Nucl. Tracks 12 (1986) 543.