- Email: [email protected]
Enhancement in the energy resolution of cellulose nitrate track detectors for alpha particles
Nuclear
Instruments
0 North-Holland
and Methods Publishing
173 (1980)
103-109
Company
ENHANCEMENT IN THE ENERGY RESOLUTION
OF CELLULOSE NITRATE TRACK DETECTORS FOR
ALPHA PARTICLES
Gulzar HUSSAIN and Hameed Ahmed KHAN Nuclear Engineering
Division, Pakistan Institute of Nuclear Science and Technology
(PINSTECH),
Nilore, Rawalpindi, Pakistan
Different irradiation and etching modes have been investigated to improve the energy resolution properties of cellulose nitrate track detectors when used as alpha particle spectrometer. In this connection CA80-15 and LR-115 plastics were employed for the separation of alpha particle peaks from 23gPu and 238U sources. Enhancement in the energy resolution has been observed when the detectors are etched from the reverse side, or if some degrading foils of optimum thickness are interposed between the source and the detector surface to be analysed. In both the above mentioned modes of enhancement in energy resolution, the regions of maximum damage (the Bragg peaks) occurring along the particle trajectories are attacked immediately with the starting of the etching process. The peaks in the Bragg curve occur at different places when different energy alpha particles pass through the detector. Drastically different rates of energy losses at these places are responsible for the diametric separation of the damage trails.
1. Introduction
with etchants from the side opposite to that of the irradiated one of the detector (called back etching). Here the detector thickness slightly exceeds the range of the particle so that the etchant enters that portion of the damage trail where the ionization rate is varying rapidly near the end of the range. The long range tracks (due to high energy alphas) are developed earlier and faster than those having low energies. In the present work efforts have been made to compare the energy resolutions achieved by the front and the back etching techniques in LR-115 and CA80-15 detectors for 23gPu and 238U alpha particles. Also, the energy resolution has been studied as a function of the etching time for three different energies of the alpha particles.
Solid state nuclear track detectors are well known for recording the tracks of heavy charged particles. The tracks can be etched preferentially as compared to the bulk material, rendering them visible under an optical microscope. The basic etching characteristics of these detectors have been studied extensively in numerous laboratories [l]. The energy of heavy charged particles like fission fragments has been determined by measuring the diameters of the perpendicularly falling particles on glass detectors. The diameter of an etch pit is found to be a function of the incident radiation damage along the track. The damage thus produced is proportional to the specific energy loss and thereby dependent on the particle energy. A group of charged particles of varying energies incident on the detector can be discriminated by studying the diameters of the etch pits. Normally the tracks are etched from the irradiated side of the detector (hereafter called front etching). The tracks belonging to a-particles of different energies are revealed at different etching times. The low energy ones appear first, while the high energy ones appear at a later stage. In this mode of etching large-size etch pits will be formed for low energy alpha particles while for high energy ones, smaller etch pits will result. Recently a major improvement in the energy resolution has been achieved by attacking the detectors
2. Method Plastic detectors of peelable LR-115 (a cellulose nitrate of thickness 13 pm) were irradiated with collimated beams of 23gPu and 238U alpha particles. For comparison the two exposures were carried out at two different points in the same detector. The irradiation times were 2 min and 25 h for 23gPu and 238U respectively. The uranium source being a weaker one, the exposure had to be done for a longer period. The energies of the main alpha particles emitted from the two sources (23gPu and 238U) were approximately 5.2 MeV and 4.2 MeV, 103
III. METHODOLOGY
G. Hussain, H.A. Khan /Enhancement
104
I F2
LR-115
NO. 2
INCOMING
NO
I
I I
_ LR-115
ALPHA
*0
1 -
COLIMATOR
LR-115
-
Fig. 1. Irradiation geometry for an assembly of two sheets of LR-I 1.5.
respectively. The detectors were stripped off their back supports and the surfaces which were not required to be etched were joined to the back supports with araldite. This process could facilitate “front” and “back” etching separately. Etching was done up to 120 min in 30% NaOH at 50°C. Diameters of the tracks were measured with a micrometer eyepiece attachment under a magnification of 500X. Another exposure was made as shown in fig. 1. A sheet of LR-115 was peeled off and was put in contact with an unpeeled sheet. The assembly of the two sheets was then put in front of the collimator such that the alpha particles penetrated the two sheets. These detectors were etched for 90 min in 30% NaOH at 50°C (without applying araldite on any of the surfaces) both from front and back sides. Measurements were made by focussing the microscope on Fr, B, and Fz surfaces, where Fr was equivalent to the front etching and B corresponded to the back etching. The etch pit diameter distributions were obtained and were then fitted with Gaussian curves. In another experiment, the energy of the 23gPu alpha particles was degraded by interposing 8 pm and 16 I.tm thick Makrofol-G foils between the source and the detectors (LR-115 and CA80-15). A third set of exposures was made with bare detectors (without any degrading). All the three sets were etched in 30% NaOH at 50°C for 120 min at appropriate intervals. Diametric measurements were made with a magnification of 1250X.
3. Experimental
PARTICLE
FI
23g Pu *
9
in the energy resolution
results and discussion
One can establish some general characteristics of the front and back etching curves for the tracks of alpha particles of different energy.
r$j-TJ
Fig. 2. Evolution of alpha damage profile in LR-115.
Let the R + R” charged distance particle of etch
thickness of the detector = h (fig. 2) = h, where R is the distance travelled by the particle from the irradiation side, R” is the from the back side to the point where the ends in the detector. The time for revelation pits through front etching is
T= (R - R’)&
=
h-R;-R’=h-(Ry”tR’) g
(1) g
where R’ is the length of the track and Vs is the bulk etch velocity. The time for relevation of etch pit through “back etching” is Z+=R”/V,=(h-R)/V,.
(2)
R > R’ t R” as the particle ends near the other extreme of the detector, which shows that the tracks will be developed much earlier through “back etching” than through “front etching”. In order to predict the time of appearance of tracks of different energies, it is necessary to have a knowledge of the range-energy relation, and the maximum expected range of the particles. The critical rate of energy loss (-ti/dx) developed by Fleischer et al. [2] and the restricted energy loss criterion of Benton [3] established a minimum threshold value of dE/dx below which a normally incident charged particle will not appear after etching. The Bragg curve represents the ionization rate of alpha particles. Alpha particles with lower energy will be slowed down quickly to the threshold value of (-ti/d_x),n and therefore their tracks will be revealed much earlier than the higher energy alphas. On further etching, the low energy tracks will start disappearing and the higher energy ones will start appearing. The diameter, D, is correlated with the energy loss of the particle [4] by D2aCiEldX. The
energy
loss
(given
by
the
Bethe-Bloch
105
G. Hussain, H.A. Khan /Enhancement in the energy resolution I
I
2.8
!
1 13 IJt.4 THICK
t-
LR-ll!
2.4 “E 2.0 ” L z 1E
i
2 G
1.i
: 0.e
F
49
b t
0.4 I I I
1
0.1
12
0
L
DISTANCE
I
16
TRAVERSEDt
20 Pm 1
Fig. 3. Plot of the rate of energy loss dE/ti (MeV mg-’ cm2) against the distance traversed (pm) by the alpha particles in 13 pm thick LR-115.
formula) is inversely proportional to the energy of the particle. Therefore the low energy alphas will give rise to large diameter etch pits. Fig. 3 is a plot of d.E/dx vs ranges of the 239Pu and 238U alpha particles in LR-115 [5]. It is clear from the graph that the energy loss rate is the maximum near the end of the range. If the damage trails of the charged particles are attacked chemically at the point where the ionization rate is varying rapidly, the
tracks will be developed quickly. The two peaks are about 5 pm apart. In “front etching”, the etchant attacks all the tracks up to the starting of the peaks, with practically the same velocity. In the “back etching” the rising part of one peak and the top of the other peak are etched simultaneously, resulting in etching the different rate of energy loss regions. In back etching, the etchant sees the tail of one peak and the maximum of the other peak immediately. As the ionization loss is considerably different at these places, enhanced resolution is achieved. The diametric distribution of the tracks etched through the front surface only is shown in fig. 4. Here the graph due to 238U crosses over that due to 239Pu and the two peaks are separated by 4.8 arbitrary units. The full width at half-maximum in the diametric distributions may be taken as a criterion to measure the resolution. After 120 mm etching in 30% NaOH at 5O”C, a resolution of 0.23 and 0.47 was attained for 239Pu and 238U respectively. Fig. 5 gives the diameter distribution obtained by etching through the back surface only. Here 238U curve has less crossover and the two peaks are 7.8 arbitrary units apart. The resolution achieved is equal to 0.19 and 0.26 for 239Pu and 238U respectively. Comparison of the two graphs (figs. 4 and 5) reveals that better resolution is attained by back etching. As already mentioned a second set of detectors was irradiated with one peeled detector on top of an unpeeled one (fig. 1). Here, since the combined thickness of the two detector sheets was greater than the ranges of the incident alphas, it was expected that the particles would come to rest in the second sheet.
32 z::
RESOLUTION
(23gp~)=o.23
E 24
RESOLUTION
(236”
kO.47
0” =
16
: I 3 =
0
0
0
4
12
16
20 DIAMETER
Fig. 4. Diametric distribution
2L ( ARBIT.
20
32
36
LO
41
UNIT)
of front etched damage trails. Etching was carried out for 120 min in 30% NaOH at 50°C. III. METHODOLOGY
106
G. Hussain, H.A. Khan /Enhancement
in the energy resolution
0 0
8
L
16
12
20 DIAMETER
24
28
1 ARBIT
UNIT
32
36
LO
Fig. 5. Diametric distribution of back etched alpha tracks. Etching was done for 120 min 2 3 9Pu alphas and 238U alphas have been clearly resolved as compared to~those in fig. 4.
Etching of the detectors was carried out from both directions simultaneously for 90 min in 30% NaOH at 50°C. In the first detector, through tracks were obtained. Here, etching of Fi and B surfaces was equivalent to front and back etching respectively. The track diameter distribution of the Fi surface has been given in fig. 6. The figure shows that the peaks are 3 divisions apart and the resolutions achieved are 0.20 and 0.30 for 239Pu and 238U, respectively. Fig. 7 shows the etch pit diameter distribution of surface B. The two peaks are quite resolved, the distance between the peaks being 6.2 div. The resolution attained is 0.13 and 0.20 for 239Pu and 238U
LB
u
in 30% NaOH at 50°C. Peaks due to
respectively. Crossover of 238U is very little. In fig. 8 the diametric response of 23gPu alphas produced tracks in the second sheet (LR-I 15 # 2, F2 surface) has been drawn. Here after 90 min etching, 238U alpha tracks, being of lower energy, disappeared due to the etched-out layer. Therefore only 239Pu alphas tracks could be seen and consequently their distribution was plotted. Here the resolution is 0.22. Table 1 represents the resolution results. In another set, LR-115 and CA80-15 were exposed to the collimated beam of 23gPu at three different energies. The resolution was studied at different etching times for all the three energies. Figs. 9 and 10
c 0
L
8
12
16 DIAMETER
Fig. 6. Track diameter
distribution
in F1 surface.
52
1
20 I ARBIT.
Etching
2L
20
32
36
LO
UNIT )
time was 90 min in 30% NaOH at 50°C.
I __
-0
I
RESOLUTION
L
12%
I= 0.20
12
8
I
16
20
DIAMETER
Fig. 7. Etch pit diametric
distribution
in surface
2L
1 ARBIT
28
32
36
40
UNIT)
B. Here the etching
time was 90 min in 30% NaOH at 50°C.
5E
1E
E
t 0
L
Fig. 8. Representation of diametric 90 min in 30% NaOH at 50°C.
a
response
12
of alpha
28
20
2L
DIAMETER
I ARBIT.
UNIT )
tracks
in Fa surface
16
32
in the second
sheet
36
LO
of LR-115.
Etching
was done for
Table 1 S. No.
1 2 3 4 5
Etching
surface
Front etching Back etching Fr surface B surface Fz surface
Etching time (min)
Resolution 239Pu
238~
120 120 90 90 90
0.23 0.19 0.20 0.13 0.22
0.47 0.26 0.30 0.20 -
Crossover
Peak to peak distance 4.8 7.8 3.0 6.2 -
(arbit. (arbit. (arbit. (arbit.
unit) unit) unit) unit)
Much crossover Less crossover Much crossover Less crossover _
G. Huswin, H.A. Khan /Enhancement
108
0
30
in the energy resolution
60 ETCHING TIME IMINUTES)
90
120
Fig. 9. Variation of resolution with etching time at different alpha particle energies in CA80-15.
represent the resolutions achieved in CA80-I.5 and LR-11.5 respectively. One can conclude that the resolution is a function of the detector type, energy of alphas and the etching time.
4. Conclusions The back etching technique can be effectively used for enhancing the energy resolution of plastic track
i
150
100 U
YU
bU ETCHING
TIME
12u
(MINUTES)
Fig. 10. Change in restilution with etching time for different energy alpha particles registered in LR-115.
G. Hussain, H.A. Khan /Enhancement
detectors. Normally incident alpha particles of different energies vary in the position of the maximum rate of energy loss in the dielectric medium. The maxima occur at different places. The etchant at a certain time attacks the maximum corresponding to the peak of the Bragg ionization curve, simultaneously, it etches the rising or falling edge of the other peak. In front etching this point is attained after etching out a layer of the detector, while in back etching it is achieved immediately. Etching at a point where the difference in the rate of energy loss is drastic, produces a sharp increase in the resolution. Intercepting the alpha particles with an optimum thick Makrofol-G foil also causes resolution enhancement.
in the energy resolution
109
We express our thanks to Dr. S.A. Hasnain for providing experimental facilities. The technical assistance of Mr. N. Zaman is also gratefully acknowledged.
References [ 1 ] H.A. Khan and S.A. Durrani, Nucl. Instr. and Meth. 114 (1974) 291. [2] R.L. Fleischer, P.B. Price, R.M. Walker and E.L. Hubbard, Phys. Rev. 133 (1964) 1443. [3] E.V. Benton and W.D. Nix, Nucl. Instr. and Meth. 67 (1969) 343. [4] G. Somogyi, Nucl. Instr. and Meth. 42 (1966) 312. [5] J. Tripier, G. Remy, J. Ralarosy, M. Debeauvais, R. Stein and D. Huss, Nucl. Instr. and Meth. 115 (1974) 29.
III. METHODOLOGY
Instruments
0 North-Holland
and Methods Publishing
173 (1980)
103-109
Company
ENHANCEMENT IN THE ENERGY RESOLUTION
OF CELLULOSE NITRATE TRACK DETECTORS FOR
ALPHA PARTICLES
Gulzar HUSSAIN and Hameed Ahmed KHAN Nuclear Engineering
Division, Pakistan Institute of Nuclear Science and Technology
(PINSTECH),
Nilore, Rawalpindi, Pakistan
Different irradiation and etching modes have been investigated to improve the energy resolution properties of cellulose nitrate track detectors when used as alpha particle spectrometer. In this connection CA80-15 and LR-115 plastics were employed for the separation of alpha particle peaks from 23gPu and 238U sources. Enhancement in the energy resolution has been observed when the detectors are etched from the reverse side, or if some degrading foils of optimum thickness are interposed between the source and the detector surface to be analysed. In both the above mentioned modes of enhancement in energy resolution, the regions of maximum damage (the Bragg peaks) occurring along the particle trajectories are attacked immediately with the starting of the etching process. The peaks in the Bragg curve occur at different places when different energy alpha particles pass through the detector. Drastically different rates of energy losses at these places are responsible for the diametric separation of the damage trails.
1. Introduction
with etchants from the side opposite to that of the irradiated one of the detector (called back etching). Here the detector thickness slightly exceeds the range of the particle so that the etchant enters that portion of the damage trail where the ionization rate is varying rapidly near the end of the range. The long range tracks (due to high energy alphas) are developed earlier and faster than those having low energies. In the present work efforts have been made to compare the energy resolutions achieved by the front and the back etching techniques in LR-115 and CA80-15 detectors for 23gPu and 238U alpha particles. Also, the energy resolution has been studied as a function of the etching time for three different energies of the alpha particles.
Solid state nuclear track detectors are well known for recording the tracks of heavy charged particles. The tracks can be etched preferentially as compared to the bulk material, rendering them visible under an optical microscope. The basic etching characteristics of these detectors have been studied extensively in numerous laboratories [l]. The energy of heavy charged particles like fission fragments has been determined by measuring the diameters of the perpendicularly falling particles on glass detectors. The diameter of an etch pit is found to be a function of the incident radiation damage along the track. The damage thus produced is proportional to the specific energy loss and thereby dependent on the particle energy. A group of charged particles of varying energies incident on the detector can be discriminated by studying the diameters of the etch pits. Normally the tracks are etched from the irradiated side of the detector (hereafter called front etching). The tracks belonging to a-particles of different energies are revealed at different etching times. The low energy ones appear first, while the high energy ones appear at a later stage. In this mode of etching large-size etch pits will be formed for low energy alpha particles while for high energy ones, smaller etch pits will result. Recently a major improvement in the energy resolution has been achieved by attacking the detectors
2. Method Plastic detectors of peelable LR-115 (a cellulose nitrate of thickness 13 pm) were irradiated with collimated beams of 23gPu and 238U alpha particles. For comparison the two exposures were carried out at two different points in the same detector. The irradiation times were 2 min and 25 h for 23gPu and 238U respectively. The uranium source being a weaker one, the exposure had to be done for a longer period. The energies of the main alpha particles emitted from the two sources (23gPu and 238U) were approximately 5.2 MeV and 4.2 MeV, 103
III. METHODOLOGY
G. Hussain, H.A. Khan /Enhancement
104
I F2
LR-115
NO. 2
INCOMING
NO
I
I I
_ LR-115
ALPHA
*0
1 -
COLIMATOR
LR-115
-
Fig. 1. Irradiation geometry for an assembly of two sheets of LR-I 1.5.
respectively. The detectors were stripped off their back supports and the surfaces which were not required to be etched were joined to the back supports with araldite. This process could facilitate “front” and “back” etching separately. Etching was done up to 120 min in 30% NaOH at 50°C. Diameters of the tracks were measured with a micrometer eyepiece attachment under a magnification of 500X. Another exposure was made as shown in fig. 1. A sheet of LR-115 was peeled off and was put in contact with an unpeeled sheet. The assembly of the two sheets was then put in front of the collimator such that the alpha particles penetrated the two sheets. These detectors were etched for 90 min in 30% NaOH at 50°C (without applying araldite on any of the surfaces) both from front and back sides. Measurements were made by focussing the microscope on Fr, B, and Fz surfaces, where Fr was equivalent to the front etching and B corresponded to the back etching. The etch pit diameter distributions were obtained and were then fitted with Gaussian curves. In another experiment, the energy of the 23gPu alpha particles was degraded by interposing 8 pm and 16 I.tm thick Makrofol-G foils between the source and the detectors (LR-115 and CA80-15). A third set of exposures was made with bare detectors (without any degrading). All the three sets were etched in 30% NaOH at 50°C for 120 min at appropriate intervals. Diametric measurements were made with a magnification of 1250X.
3. Experimental
PARTICLE
FI
23g Pu *
9
in the energy resolution
results and discussion
One can establish some general characteristics of the front and back etching curves for the tracks of alpha particles of different energy.
r$j-TJ
Fig. 2. Evolution of alpha damage profile in LR-115.
Let the R + R” charged distance particle of etch
thickness of the detector = h (fig. 2) = h, where R is the distance travelled by the particle from the irradiation side, R” is the from the back side to the point where the ends in the detector. The time for revelation pits through front etching is
T= (R - R’)&
=
h-R;-R’=h-(Ry”tR’) g
(1) g
where R’ is the length of the track and Vs is the bulk etch velocity. The time for relevation of etch pit through “back etching” is Z+=R”/V,=(h-R)/V,.
(2)
R > R’ t R” as the particle ends near the other extreme of the detector, which shows that the tracks will be developed much earlier through “back etching” than through “front etching”. In order to predict the time of appearance of tracks of different energies, it is necessary to have a knowledge of the range-energy relation, and the maximum expected range of the particles. The critical rate of energy loss (-ti/dx) developed by Fleischer et al. [2] and the restricted energy loss criterion of Benton [3] established a minimum threshold value of dE/dx below which a normally incident charged particle will not appear after etching. The Bragg curve represents the ionization rate of alpha particles. Alpha particles with lower energy will be slowed down quickly to the threshold value of (-ti/d_x),n and therefore their tracks will be revealed much earlier than the higher energy alphas. On further etching, the low energy tracks will start disappearing and the higher energy ones will start appearing. The diameter, D, is correlated with the energy loss of the particle [4] by D2aCiEldX. The
energy
loss
(given
by
the
Bethe-Bloch
105
G. Hussain, H.A. Khan /Enhancement in the energy resolution I
I
2.8
!
1 13 IJt.4 THICK
t-
LR-ll!
2.4 “E 2.0 ” L z 1E
i
2 G
1.i
: 0.e
F
49
b t
0.4 I I I
1
0.1
12
0
L
DISTANCE
I
16
TRAVERSEDt
20 Pm 1
Fig. 3. Plot of the rate of energy loss dE/ti (MeV mg-’ cm2) against the distance traversed (pm) by the alpha particles in 13 pm thick LR-115.
formula) is inversely proportional to the energy of the particle. Therefore the low energy alphas will give rise to large diameter etch pits. Fig. 3 is a plot of d.E/dx vs ranges of the 239Pu and 238U alpha particles in LR-115 [5]. It is clear from the graph that the energy loss rate is the maximum near the end of the range. If the damage trails of the charged particles are attacked chemically at the point where the ionization rate is varying rapidly, the
tracks will be developed quickly. The two peaks are about 5 pm apart. In “front etching”, the etchant attacks all the tracks up to the starting of the peaks, with practically the same velocity. In the “back etching” the rising part of one peak and the top of the other peak are etched simultaneously, resulting in etching the different rate of energy loss regions. In back etching, the etchant sees the tail of one peak and the maximum of the other peak immediately. As the ionization loss is considerably different at these places, enhanced resolution is achieved. The diametric distribution of the tracks etched through the front surface only is shown in fig. 4. Here the graph due to 238U crosses over that due to 239Pu and the two peaks are separated by 4.8 arbitrary units. The full width at half-maximum in the diametric distributions may be taken as a criterion to measure the resolution. After 120 mm etching in 30% NaOH at 5O”C, a resolution of 0.23 and 0.47 was attained for 239Pu and 238U respectively. Fig. 5 gives the diameter distribution obtained by etching through the back surface only. Here 238U curve has less crossover and the two peaks are 7.8 arbitrary units apart. The resolution achieved is equal to 0.19 and 0.26 for 239Pu and 238U respectively. Comparison of the two graphs (figs. 4 and 5) reveals that better resolution is attained by back etching. As already mentioned a second set of detectors was irradiated with one peeled detector on top of an unpeeled one (fig. 1). Here, since the combined thickness of the two detector sheets was greater than the ranges of the incident alphas, it was expected that the particles would come to rest in the second sheet.
32 z::
RESOLUTION
(23gp~)=o.23
E 24
RESOLUTION
(236”
kO.47
0” =
16
: I 3 =
0
0
0
4
12
16
20 DIAMETER
Fig. 4. Diametric distribution
2L ( ARBIT.
20
32
36
LO
41
UNIT)
of front etched damage trails. Etching was carried out for 120 min in 30% NaOH at 50°C. III. METHODOLOGY
106
G. Hussain, H.A. Khan /Enhancement
in the energy resolution
0 0
8
L
16
12
20 DIAMETER
24
28
1 ARBIT
UNIT
32
36
LO
Fig. 5. Diametric distribution of back etched alpha tracks. Etching was done for 120 min 2 3 9Pu alphas and 238U alphas have been clearly resolved as compared to~those in fig. 4.
Etching of the detectors was carried out from both directions simultaneously for 90 min in 30% NaOH at 50°C. In the first detector, through tracks were obtained. Here, etching of Fi and B surfaces was equivalent to front and back etching respectively. The track diameter distribution of the Fi surface has been given in fig. 6. The figure shows that the peaks are 3 divisions apart and the resolutions achieved are 0.20 and 0.30 for 239Pu and 238U, respectively. Fig. 7 shows the etch pit diameter distribution of surface B. The two peaks are quite resolved, the distance between the peaks being 6.2 div. The resolution attained is 0.13 and 0.20 for 239Pu and 238U
LB
u
in 30% NaOH at 50°C. Peaks due to
respectively. Crossover of 238U is very little. In fig. 8 the diametric response of 23gPu alphas produced tracks in the second sheet (LR-I 15 # 2, F2 surface) has been drawn. Here after 90 min etching, 238U alpha tracks, being of lower energy, disappeared due to the etched-out layer. Therefore only 239Pu alphas tracks could be seen and consequently their distribution was plotted. Here the resolution is 0.22. Table 1 represents the resolution results. In another set, LR-115 and CA80-15 were exposed to the collimated beam of 23gPu at three different energies. The resolution was studied at different etching times for all the three energies. Figs. 9 and 10
c 0
L
8
12
16 DIAMETER
Fig. 6. Track diameter
distribution
in F1 surface.
52
1
20 I ARBIT.
Etching
2L
20
32
36
LO
UNIT )
time was 90 min in 30% NaOH at 50°C.
I __
-0
I
RESOLUTION
L
12%
I= 0.20
12
8
I
16
20
DIAMETER
Fig. 7. Etch pit diametric
distribution
in surface
2L
1 ARBIT
28
32
36
40
UNIT)
B. Here the etching
time was 90 min in 30% NaOH at 50°C.
5E
1E
E
t 0
L
Fig. 8. Representation of diametric 90 min in 30% NaOH at 50°C.
a
response
12
of alpha
28
20
2L
DIAMETER
I ARBIT.
UNIT )
tracks
in Fa surface
16
32
in the second
sheet
36
LO
of LR-115.
Etching
was done for
Table 1 S. No.
1 2 3 4 5
Etching
surface
Front etching Back etching Fr surface B surface Fz surface
Etching time (min)
Resolution 239Pu
238~
120 120 90 90 90
0.23 0.19 0.20 0.13 0.22
0.47 0.26 0.30 0.20 -
Crossover
Peak to peak distance 4.8 7.8 3.0 6.2 -
(arbit. (arbit. (arbit. (arbit.
unit) unit) unit) unit)
Much crossover Less crossover Much crossover Less crossover _
G. Huswin, H.A. Khan /Enhancement
108
0
30
in the energy resolution
60 ETCHING TIME IMINUTES)
90
120
Fig. 9. Variation of resolution with etching time at different alpha particle energies in CA80-15.
represent the resolutions achieved in CA80-I.5 and LR-11.5 respectively. One can conclude that the resolution is a function of the detector type, energy of alphas and the etching time.
4. Conclusions The back etching technique can be effectively used for enhancing the energy resolution of plastic track
i
150
100 U
YU
bU ETCHING
TIME
12u
(MINUTES)
Fig. 10. Change in restilution with etching time for different energy alpha particles registered in LR-115.
G. Hussain, H.A. Khan /Enhancement
detectors. Normally incident alpha particles of different energies vary in the position of the maximum rate of energy loss in the dielectric medium. The maxima occur at different places. The etchant at a certain time attacks the maximum corresponding to the peak of the Bragg ionization curve, simultaneously, it etches the rising or falling edge of the other peak. In front etching this point is attained after etching out a layer of the detector, while in back etching it is achieved immediately. Etching at a point where the difference in the rate of energy loss is drastic, produces a sharp increase in the resolution. Intercepting the alpha particles with an optimum thick Makrofol-G foil also causes resolution enhancement.
in the energy resolution
109
We express our thanks to Dr. S.A. Hasnain for providing experimental facilities. The technical assistance of Mr. N. Zaman is also gratefully acknowledged.
References [ 1 ] H.A. Khan and S.A. Durrani, Nucl. Instr. and Meth. 114 (1974) 291. [2] R.L. Fleischer, P.B. Price, R.M. Walker and E.L. Hubbard, Phys. Rev. 133 (1964) 1443. [3] E.V. Benton and W.D. Nix, Nucl. Instr. and Meth. 67 (1969) 343. [4] G. Somogyi, Nucl. Instr. and Meth. 42 (1966) 312. [5] J. Tripier, G. Remy, J. Ralarosy, M. Debeauvais, R. Stein and D. Huss, Nucl. Instr. and Meth. 115 (1974) 29.
III. METHODOLOGY