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X-ray diffraction investigations of pre-irradiation annealed cellulose nitrate plastic track detectors
Radiat, Phys, Chem. Vol. 16, pp. 379-383 Pergamon Press Ltd.. 1980. Printed in Great Britain
X-RAY DIFFRACTION INVESTIGATIONS OF PRE-IRRADIATION ANNEALED CELLULOSE NITRATE PLASTIC TRACK DETECTORSt C. A. MAJID, A. U. AHMAD, R. A. AKBER and H. A. KHAN:[: Pakistan Institute of Nuclear Science and Technology, Pinstech, Nilore, Rawalpindi, Pakistan
(Received 16 October 1979) Abstract--X-ray diffraction studies of CA80-15 cellulose nitrate plastic Solid State Nuclear Track Detectors (SS-NTD) annealed at various temperatures have been carried out. These results along with the thermogravimetric analysis of unannealed detectors show that (a) the thermal decomposition of cellulose nitrate is a function of the annealing temperature, (b) the decomposed products are retained within the detector material up to a temperature of about 145°C and (c) the inter-chain separation increases with the annealing temperature and the interatomic binding is correspondingly weakened.
INTRODUCTION THE Solid State Nuclear Track Detectors (SSNTD) are now extensively applied as nuclear radiations recorders in various fields of Science and Technology.°) In almost all the applications of SSNTD one has to scan the detectors for one or more of the track parameters like density, dimensions, direction and angular distribution, etc. These parameters can be later interpreted to draw important conclusions in any desired direction during a specific research programme. It is now established that the exposures carried out under changed environmental conditions like temperature, atmosphere and background radiations, etc. may result in considerable deviations in the ultimately obtained track parameters. Similar effects of environmental conditions have been observed for the pre/post treatment of the SSNTD. ~t-~) A knowledge of the exact environmental conditions prevailing before, during, and after the exposure of an SSNTD is, therefore, necessary for a "true" interpretation of the track analysis during a sophisticated experiment. The importance of understanding the basic factors responsible for track parameter variations with changes in the environmental conditions is also quite evident.
tWork sponsored by IAEA under research contract No. 1662/RB. The financial support is gratefully acknowledged. ~Chief Scientific Investigator of the contract.
It has been observed that pre-irradiation annealing of the plastic solid state nuclear track detectors changes the etched track characteristics. In our present work attempts have been made to find out the basic reasons for this behaviour of the track parameters. To this end, the modifications caused in the material structure of CA80-15 cellulose nitrate plastic track detectors with annealing temperatures have been observed. An X-ray diffractometer has been employed for this purpose. Thermogravimetric analysis of CA80-15 has also been carried out to support the results. E X P E R I M E N T A L DETAILS Several CA80-15 specimens of 15 x 10 mm2 were taken from a common sheet. Each specimen was annealed at a certain fixed temperature for ten minutes in a furnace. A temperature range up to 15ff'C was covered with suitable time intervals. After annealing, they were taken out at normal room temperature and pressure conditions. The X-ray diffraction studies of these specimens were performed on an X-ray diffractometer using Ni-fitered Cu Ka radiation with wavelength A = 1.5418,A. Constant power level, amplifier gain and collimating slit width were maintained throughout the analysis. The specimens were held on a specimen-holder of the diffractometer in such a way that an identical surface area (15x 10ram2) was exposed to the X-rays. In order to obtain better results, each experiment was repeated at least twice and with both the faces of the specimens alternately exposed to the incident radiations. The corresponding diffraction patterns were recorded from a diffraction angle 20 = 4°-2 0 = 65° at a constant diffractometer speed of 0.25° per rain and the results for the specimens were analysed for peak positions and area under the peaks, etc.
379
C. A. MAJIDet al.
380
The thermogravimetric analysis was carried out to observe any loss of weight of CA80-15 at elevated tem~ peratures. An unannealed specimen was heated from room temperature (about 24°(?) onwards up to 200°(? at a heating rate of l°C per rain. RESULTS AND DISCUSSION The relative intensity of the diffracted X-rays vs the diffraction angle is shown in Fig. l o As indicated, the curves have been plotted for a number of annealing temperatures. Within the range of diffraction angle covered in the present investigations, and for all samples, annealed as well as un-annealed, only one broad peak was observed. There were, however, several very small sharp peaks, superimposed upon the broad peak and scattered throughout the angular range covered in these studies. There was no sharp peak of measureable intensity observed at least in the range of 20 covered in the present studies. Figure 2 shows the thermogravimetric analysis results. There exist two regions of weight loss of the specimen during heating. Region I started at about 30°C and saturated around 70°C. The total loss of weight in this region was about 2%. The
drastic loss of weight took place in Region II which commenced from about 145°C and ultimately complete damage of the specimen was observed around 190°C Keeping in view the physics of the X-ray diffraction patterns, Fig. 1 can be used for the explanation of the behaviour of the cellulose nitrate S S N T D towards track parameters as a function of the pre-irradition annealing temperature. Existence of a very broad peak and absence of sharp lines of considerable intensity suggests that the cellulose nitrate specimens under the present observation are amorphous polymer samples. ~4~ Small sharp lines superimposed on the broad peak are due to camphor which was present (as a plasticiser) to the extent of about 25%, Separate diffraction pattern of powdered camphor was taken to comfirm the results (Fig. 3). Intensity of the scattering pattern l ( s ) for an amorphous polymer may be written (4~ h
I(s) = N~= f f ( l
- e -2u) +
N{A'F. + JP.,12z(s)}
(1)
80
60
(Z
>- 1.0 i-Z
z
20
0 10
15
20
DIFFRACTION ANGLE , 2 0
25
30
(DEGREES )
FIO. i. X-ray diffraction peaks due to CA80-15 cellulose nitrate plastic Solid State Nuclear Track Detectors (SSNTD) annealed for 10rain at various temperatures. The insert shows the area under the scattered X-ray peaks as a function of annealing temperature.
e -2"
X-ray diffraction investigations
381
I00 ~REGION
I l
-0..,,
i,.,.I J Q.
II
v) --
UJ 80 "1"
HEATING R A T E : I C ° / M I N .
0
ILl 6O 45
85
125
TEMPERATURE
165
205
C°
FIG. 2. Thermogravimetric analysis results for an un-annealed CA80-15 specimen. The specimen was heated from room temperature (about 24°C) to about 200°C at a rate of l°C/min. The two regions of weight loss have been marked as Regions I and II.
100
ii
80
_
/
"x
¢t: POWDERD CAMPHOR
tt
b:CA 80-15 ( 2 & ~ )
~x%%
#
/ I ( I I I
6O
I
\
In
~0
\
\ \k %
_z I
/
x \ ~ \ x\
/
\b
i
20
9
13
17
21
25
2~
DIFFRACIION ANGLE 20 (DECREES)
FIG. 3. X-ray diffraction pattern of powdered camphor. The broken line shows the X-ray diffraction pattern of CA80-15 and has been drawn for the sake of comparison.
Various expressions explained as follows N n s
in
equation
(1)
can
be
number of molecules of the specimen contributing to the scattering number of atoms in unit group 4~r sin 0 ,~ , 0 being the angle of diffraction,
and A, the incident radiation wavelength atomic scattering factor; f~ depends upon the total charge of the atom F,. scattering factor averaged over all orientations IF.I 2 spherically averaged structure factor for the unit group Z(s) interference function f~
382
C. A. MAJID et al.
M number of nearest neighbour unit groups A a factor taking into account any distorttion effects. For the purpose of analysis, the area under the X-ray diffraction peaks may be taken as a measure of the intensity of scattering: ~ Insert of Fig. 1 shows the area under the broad peaks as a function of annealing temperature. A gradual fall in this area (and hence the intensity of the scattered beam) is quite evident. A possible explanation of the fall in intensity is a random scission of cellulose nitrate molecules with temperature. This results in a decrease in the number of atoms connected with the cellulose nitrate chains per unit volume of the sample: ~'-8> This parameter occurs in equation (1) as the number of unit groups of the assembly contributing to the scattering of X-rays. The new compounds formed as the consequence of the thermal decompositiont of the cellulose nitrate and initially too little to form independent intensity peaks of any significant value. Their contribution forms perhaps the general background in the scattered intensity. However, for higher annealing temperatures a large number of small kinks appear above and throughout the broad diffraction peak of cellulose nitrate. As the glass transition temperature of cellulose nitrate is 152°C/9~ no transitions from a disordered to order form to produce crystalline cellulose nitrate are expected below this temperature. "°~ The small sharp peaks can, therefore, only be due to the thermal decomposition products of cellulose nitrate. These newly formed materials and the reduced chain length, therefore, results in a changed overall composition of the material which may possess a different behaviour towards track registration and etching characteristics. As regards the changes in the intensity of the small sharp peaks due to camphor which were superimposed in the diffraction patterns of cellulose nitrate; the intensity of these lines was found to diminish with annealing temperature. Our thermogravimetric measurements, however, show that the depletion of camphor does not occur due to its escape from the sample. One possible explanation is that the camphor is used up by the decomposition products of cellulose nitrate to form some new compounds. Another observation worth noting is the shift in the maxima of the peak position of diffracted
X-rays with the annealing temperature. A detailed study in this direction revealed that peak position gradually shifted from 2 0 = 18.6--17.4° for annealing temperature 24--131PC, the corresponding change in d-spacing being 4.75-5.10°A. For amorphous polymers the first diffraction peak is considered to be the contribution of iateratomic vectors between adjacent chains. Position of the peak maximum can be used to find the frequently occuring interchain separation R as ~) (2)
R = constant x d-spacing
The actual value of the constant multiplier on the RHS of equation (2) lies between 1.11 and 1.25) ~) When a solid "relaxes" in three dimensions the interatomic (or interchain) separation R tends to be lengthened and consequently the atomic binding forces are weakened. The above mentioned observation of the gradual shift of the diffraction peak maximum towards lower 2 0 values (or high d-spacing), therefore, suggests an increase in the interatomic distance with annealing temperature. A consequent decrease in the atomic binding forces and hence the stopping power of the material is quite evident. This effect may result in an increase in length on the damage trails due to the passage of charged particles. CONCLUSIONS The following conclusions can be drawn from the above mentioned experimental work. 1. The annealing of CA80-15 cellulose nitrate plastic SSNTD results in the thermal decomposition of the amorphous cellulose nitrate. 2. For a given annealing time the magnitude of the decomposition depends upon the annealing temperature. 3. The annealing up to a temperature of about 145°C does not result in any significant loss of weight of the sample. The thermal decomposition products are, therefore, retained within the sample up to 145°C. 4. A very rapid rate of loss of the sample starts above 145°C which results in an ultimate "burning" of the entire sample. 5. The interchain separation of cellulose nitrate becomes larger with the annealing temperature resulting in weakened interatomic binding and decreased stopping power. REFERENCES I. R. L. FLIESCHER, P. B. PRICE and R. M. WALKER,
tA detailed account of thermal decomposition has been given in Ref. 8.
Nuclear Tracks in Solids; Principles and Applications. University of California Press, U.S.A., 1975.
X-ray diffraction investigations 2. H. A. KHAN and I. AHMAD, Nuclear Instruments and Methods 1975, 131, 89. 3. H. A. KHAN, Nuclear Instruments and Methods 19/5,125, 419. 4. B. K. VAINSHTEIN, Diffraction of X-rays by Chain Molecules. Elsevier, London, 1966. 5. H. P. KLUG and L. E. ALEXANDER, X-ray Diffraction
Procedures /or Polycrystalline Materials. Wiley, London, 19/4.
and Amorphous
383
6. H, MURAOUR and M. W. GRAFFLIN, Bull. Soc. Chim 193 I, 49, 276. 7. H. MURAOUR, Bull. Soc Chim. 1932, $1,1094. 8. L. PmLUPS, Nature 1947, 160, 753. 9. H. F. MARK, J. J. JR. McKETrA, D. F. OTXMER and A. STANDEN, (Editors), Encyclopedia of Chemical Technology. Wiley, New York, 1970. 10. C. A. M~ID, P. R. PRAOER, N. H. FLETCHER and J. M. BRETrEL, J. Non Crystal Solids 19/4 16, 365.
X-RAY DIFFRACTION INVESTIGATIONS OF PRE-IRRADIATION ANNEALED CELLULOSE NITRATE PLASTIC TRACK DETECTORSt C. A. MAJID, A. U. AHMAD, R. A. AKBER and H. A. KHAN:[: Pakistan Institute of Nuclear Science and Technology, Pinstech, Nilore, Rawalpindi, Pakistan
(Received 16 October 1979) Abstract--X-ray diffraction studies of CA80-15 cellulose nitrate plastic Solid State Nuclear Track Detectors (SS-NTD) annealed at various temperatures have been carried out. These results along with the thermogravimetric analysis of unannealed detectors show that (a) the thermal decomposition of cellulose nitrate is a function of the annealing temperature, (b) the decomposed products are retained within the detector material up to a temperature of about 145°C and (c) the inter-chain separation increases with the annealing temperature and the interatomic binding is correspondingly weakened.
INTRODUCTION THE Solid State Nuclear Track Detectors (SSNTD) are now extensively applied as nuclear radiations recorders in various fields of Science and Technology.°) In almost all the applications of SSNTD one has to scan the detectors for one or more of the track parameters like density, dimensions, direction and angular distribution, etc. These parameters can be later interpreted to draw important conclusions in any desired direction during a specific research programme. It is now established that the exposures carried out under changed environmental conditions like temperature, atmosphere and background radiations, etc. may result in considerable deviations in the ultimately obtained track parameters. Similar effects of environmental conditions have been observed for the pre/post treatment of the SSNTD. ~t-~) A knowledge of the exact environmental conditions prevailing before, during, and after the exposure of an SSNTD is, therefore, necessary for a "true" interpretation of the track analysis during a sophisticated experiment. The importance of understanding the basic factors responsible for track parameter variations with changes in the environmental conditions is also quite evident.
tWork sponsored by IAEA under research contract No. 1662/RB. The financial support is gratefully acknowledged. ~Chief Scientific Investigator of the contract.
It has been observed that pre-irradiation annealing of the plastic solid state nuclear track detectors changes the etched track characteristics. In our present work attempts have been made to find out the basic reasons for this behaviour of the track parameters. To this end, the modifications caused in the material structure of CA80-15 cellulose nitrate plastic track detectors with annealing temperatures have been observed. An X-ray diffractometer has been employed for this purpose. Thermogravimetric analysis of CA80-15 has also been carried out to support the results. E X P E R I M E N T A L DETAILS Several CA80-15 specimens of 15 x 10 mm2 were taken from a common sheet. Each specimen was annealed at a certain fixed temperature for ten minutes in a furnace. A temperature range up to 15ff'C was covered with suitable time intervals. After annealing, they were taken out at normal room temperature and pressure conditions. The X-ray diffraction studies of these specimens were performed on an X-ray diffractometer using Ni-fitered Cu Ka radiation with wavelength A = 1.5418,A. Constant power level, amplifier gain and collimating slit width were maintained throughout the analysis. The specimens were held on a specimen-holder of the diffractometer in such a way that an identical surface area (15x 10ram2) was exposed to the X-rays. In order to obtain better results, each experiment was repeated at least twice and with both the faces of the specimens alternately exposed to the incident radiations. The corresponding diffraction patterns were recorded from a diffraction angle 20 = 4°-2 0 = 65° at a constant diffractometer speed of 0.25° per rain and the results for the specimens were analysed for peak positions and area under the peaks, etc.
379
C. A. MAJIDet al.
380
The thermogravimetric analysis was carried out to observe any loss of weight of CA80-15 at elevated tem~ peratures. An unannealed specimen was heated from room temperature (about 24°(?) onwards up to 200°(? at a heating rate of l°C per rain. RESULTS AND DISCUSSION The relative intensity of the diffracted X-rays vs the diffraction angle is shown in Fig. l o As indicated, the curves have been plotted for a number of annealing temperatures. Within the range of diffraction angle covered in the present investigations, and for all samples, annealed as well as un-annealed, only one broad peak was observed. There were, however, several very small sharp peaks, superimposed upon the broad peak and scattered throughout the angular range covered in these studies. There was no sharp peak of measureable intensity observed at least in the range of 20 covered in the present studies. Figure 2 shows the thermogravimetric analysis results. There exist two regions of weight loss of the specimen during heating. Region I started at about 30°C and saturated around 70°C. The total loss of weight in this region was about 2%. The
drastic loss of weight took place in Region II which commenced from about 145°C and ultimately complete damage of the specimen was observed around 190°C Keeping in view the physics of the X-ray diffraction patterns, Fig. 1 can be used for the explanation of the behaviour of the cellulose nitrate S S N T D towards track parameters as a function of the pre-irradition annealing temperature. Existence of a very broad peak and absence of sharp lines of considerable intensity suggests that the cellulose nitrate specimens under the present observation are amorphous polymer samples. ~4~ Small sharp lines superimposed on the broad peak are due to camphor which was present (as a plasticiser) to the extent of about 25%, Separate diffraction pattern of powdered camphor was taken to comfirm the results (Fig. 3). Intensity of the scattering pattern l ( s ) for an amorphous polymer may be written (4~ h
I(s) = N~= f f ( l
- e -2u) +
N{A'F. + JP.,12z(s)}
(1)
80
60
(Z
>- 1.0 i-Z
z
20
0 10
15
20
DIFFRACTION ANGLE , 2 0
25
30
(DEGREES )
FIO. i. X-ray diffraction peaks due to CA80-15 cellulose nitrate plastic Solid State Nuclear Track Detectors (SSNTD) annealed for 10rain at various temperatures. The insert shows the area under the scattered X-ray peaks as a function of annealing temperature.
e -2"
X-ray diffraction investigations
381
I00 ~REGION
I l
-0..,,
i,.,.I J Q.
II
v) --
UJ 80 "1"
HEATING R A T E : I C ° / M I N .
0
ILl 6O 45
85
125
TEMPERATURE
165
205
C°
FIG. 2. Thermogravimetric analysis results for an un-annealed CA80-15 specimen. The specimen was heated from room temperature (about 24°C) to about 200°C at a rate of l°C/min. The two regions of weight loss have been marked as Regions I and II.
100
ii
80
_
/
"x
¢t: POWDERD CAMPHOR
tt
b:CA 80-15 ( 2 & ~ )
~x%%
#
/ I ( I I I
6O
I
\
In
~0
\
\ \k %
_z I
/
x \ ~ \ x\
/
\b
i
20
9
13
17
21
25
2~
DIFFRACIION ANGLE 20 (DECREES)
FIG. 3. X-ray diffraction pattern of powdered camphor. The broken line shows the X-ray diffraction pattern of CA80-15 and has been drawn for the sake of comparison.
Various expressions explained as follows N n s
in
equation
(1)
can
be
number of molecules of the specimen contributing to the scattering number of atoms in unit group 4~r sin 0 ,~ , 0 being the angle of diffraction,
and A, the incident radiation wavelength atomic scattering factor; f~ depends upon the total charge of the atom F,. scattering factor averaged over all orientations IF.I 2 spherically averaged structure factor for the unit group Z(s) interference function f~
382
C. A. MAJID et al.
M number of nearest neighbour unit groups A a factor taking into account any distorttion effects. For the purpose of analysis, the area under the X-ray diffraction peaks may be taken as a measure of the intensity of scattering: ~ Insert of Fig. 1 shows the area under the broad peaks as a function of annealing temperature. A gradual fall in this area (and hence the intensity of the scattered beam) is quite evident. A possible explanation of the fall in intensity is a random scission of cellulose nitrate molecules with temperature. This results in a decrease in the number of atoms connected with the cellulose nitrate chains per unit volume of the sample: ~'-8> This parameter occurs in equation (1) as the number of unit groups of the assembly contributing to the scattering of X-rays. The new compounds formed as the consequence of the thermal decompositiont of the cellulose nitrate and initially too little to form independent intensity peaks of any significant value. Their contribution forms perhaps the general background in the scattered intensity. However, for higher annealing temperatures a large number of small kinks appear above and throughout the broad diffraction peak of cellulose nitrate. As the glass transition temperature of cellulose nitrate is 152°C/9~ no transitions from a disordered to order form to produce crystalline cellulose nitrate are expected below this temperature. "°~ The small sharp peaks can, therefore, only be due to the thermal decomposition products of cellulose nitrate. These newly formed materials and the reduced chain length, therefore, results in a changed overall composition of the material which may possess a different behaviour towards track registration and etching characteristics. As regards the changes in the intensity of the small sharp peaks due to camphor which were superimposed in the diffraction patterns of cellulose nitrate; the intensity of these lines was found to diminish with annealing temperature. Our thermogravimetric measurements, however, show that the depletion of camphor does not occur due to its escape from the sample. One possible explanation is that the camphor is used up by the decomposition products of cellulose nitrate to form some new compounds. Another observation worth noting is the shift in the maxima of the peak position of diffracted
X-rays with the annealing temperature. A detailed study in this direction revealed that peak position gradually shifted from 2 0 = 18.6--17.4° for annealing temperature 24--131PC, the corresponding change in d-spacing being 4.75-5.10°A. For amorphous polymers the first diffraction peak is considered to be the contribution of iateratomic vectors between adjacent chains. Position of the peak maximum can be used to find the frequently occuring interchain separation R as ~) (2)
R = constant x d-spacing
The actual value of the constant multiplier on the RHS of equation (2) lies between 1.11 and 1.25) ~) When a solid "relaxes" in three dimensions the interatomic (or interchain) separation R tends to be lengthened and consequently the atomic binding forces are weakened. The above mentioned observation of the gradual shift of the diffraction peak maximum towards lower 2 0 values (or high d-spacing), therefore, suggests an increase in the interatomic distance with annealing temperature. A consequent decrease in the atomic binding forces and hence the stopping power of the material is quite evident. This effect may result in an increase in length on the damage trails due to the passage of charged particles. CONCLUSIONS The following conclusions can be drawn from the above mentioned experimental work. 1. The annealing of CA80-15 cellulose nitrate plastic SSNTD results in the thermal decomposition of the amorphous cellulose nitrate. 2. For a given annealing time the magnitude of the decomposition depends upon the annealing temperature. 3. The annealing up to a temperature of about 145°C does not result in any significant loss of weight of the sample. The thermal decomposition products are, therefore, retained within the sample up to 145°C. 4. A very rapid rate of loss of the sample starts above 145°C which results in an ultimate "burning" of the entire sample. 5. The interchain separation of cellulose nitrate becomes larger with the annealing temperature resulting in weakened interatomic binding and decreased stopping power. REFERENCES I. R. L. FLIESCHER, P. B. PRICE and R. M. WALKER,
tA detailed account of thermal decomposition has been given in Ref. 8.
Nuclear Tracks in Solids; Principles and Applications. University of California Press, U.S.A., 1975.
X-ray diffraction investigations 2. H. A. KHAN and I. AHMAD, Nuclear Instruments and Methods 1975, 131, 89. 3. H. A. KHAN, Nuclear Instruments and Methods 19/5,125, 419. 4. B. K. VAINSHTEIN, Diffraction of X-rays by Chain Molecules. Elsevier, London, 1966. 5. H. P. KLUG and L. E. ALEXANDER, X-ray Diffraction
Procedures /or Polycrystalline Materials. Wiley, London, 19/4.
and Amorphous
383
6. H, MURAOUR and M. W. GRAFFLIN, Bull. Soc. Chim 193 I, 49, 276. 7. H. MURAOUR, Bull. Soc Chim. 1932, $1,1094. 8. L. PmLUPS, Nature 1947, 160, 753. 9. H. F. MARK, J. J. JR. McKETrA, D. F. OTXMER and A. STANDEN, (Editors), Encyclopedia of Chemical Technology. Wiley, New York, 1970. 10. C. A. M~ID, P. R. PRAOER, N. H. FLETCHER and J. M. BRETrEL, J. Non Crystal Solids 19/4 16, 365.