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Detection of nuclear radiation by scintillation counting using synthetic diamond
Appl. Radiat. ht. Vol. 40, No. 8, pp. 657-661, ht. J. Rodiat. Appl. Instrum. Part A Printed in Great Britain. All rights reserved
1989
0883-2889/89
$3.00 + 0.00
Copyright ‘c 1989 Maxwell Pergamon Macmillan
plc
Detection of Nuclear Radiation by Scintillation Counting Using Synthetic Diamond T. L. NAM,’ P. J. FALLON,’
R. J. KEDDY,‘* H. J. VAN RIJN*
and JOANNE
F. ANDREWS2
‘Schonland Research Centre, University of the Witwatersrand, of South Africa and 2De Beers Diamond Research Laboratory, (Receioed 30 January
P.O. WITS, Johannesburg, Johannesburg, Republic
2050 Republic of South Africa
1989)
The scintillation characteristics of boron activated synthetic diamonds, produced from an iron free solvent/catalyst, when subjected to nuclear ionizing radiations have been investigated. Despite the fact that the optical coupling between the crystal and the surface of the photomultiplier which was used in the
experimental arrangement was far from ideal, the crystals, when exposed to irradiation by a-particles, exhibited good energy resolution and linear dose rate response characteristics.
Introduction Luminescence in a material may be defined as the emission of electromagnetic radiation, usually in the U.V. to i.r. region, that is not attributable to incandescence. The features common to all forms of luminescence are: (a) the occurrence of some processes whereby an atom, molecule or, more importantly in our case, a “center” (an aggregate of atoms or defects in a crystal) is excited to a higher energy state, and (b) its radiative de-excitation with the emission of a photon after the lapse of some period of time. The distinction between various types of luminescence i.e. “fluorescence” and “phosphorescence”, is normally based on the time dependence of the emission of the photon but a more meaningful distinction may be based on the temperature-dependence of the luminescence decay time (Bull, 1986). However, when luminescence is present only during excitation it is “fluorescence” and if the excited state lifetime is short then this luminescence can be termed “scintillation”. In the case of impurity-activated crystals the scintillation process, by means of which the incident energy is converted to luminescence emission, is a complex sequence of events and is certainly a function of the impurity atoms which are present within the crystal lattice. The interaction of nuclear radiation with any material creates charges carriers by ionization within the material. The number of charge carriers in the
*Author
for correspondence. 657
form of electrons and holes created per incident nuclear particle is proportional to the energy of the particle. If suitable luminescence centers are present within the bulk of the material, the electrons and holes may recombine at these centers to emit photons. The intensity of the light thus emitted is therefore proportional to the energy of the incident particle. The total number of such events will give an indication of the flux of nuclear particles. By coupling the attenuating material to a photomultiplier the magnitude (energy) and number (flux) of the incident nuclear particles may be recorded. This is, of course, the phenomenon of scintillation counting, a technique widely applied in the real-time detection of nuclear radiation. Many crystals including certain diamonds exhibit this scintillation property and the scintillation properties of natural diamonds when subjected to irradiation by nuclear particles has been reported (cf. Ralph, 1959; Dean et al., 1960; Miller, 1966). Other general properties of diamond, e.g. tissue equivalence, nontoxicity and stability against most corrosive fluids including body fluids (Keddy et al., 1987), places diamond in a very unique position on the list of scintillators, particularly if in vivo applications are contemplated. A further advantage of diamond as a scintillator is that, unlike other scintillators, it exhibits a higher X- to p-particle scintillation response (Dean et al., 1960). Despite all these advantages however, diamond has not featured as a practical scintillation radiation detector. Specimens of natural diamond have been shown to exhibit energy responses which varied from reasonable to poor to virtually non-existent and this is confirmed in
658
T. L.
NAM
al.
et
this work. The omission to use natural diamond as a radiation detector is explained by the fact therefore that, besides being expensive, no two stones can be guaranteed to have the same scintillation characteristics. Synthetic diamonds offer new possibilities however and we report here on the scintillation performance of specially produced synthetic diamonds when irradiated by a beam of r-particles obtained from 14’Am radioactive sources. The properties of the synthetic stones are compared to those of natural diamonds and to other synthetic stones that did not have the same selective chemistry during the manufacturing process. PM
TUBE
Experimental (A) Phosphor
.
preparation
The natural diamonds used for this experiment were classified as type I and type IIa (Robertson rt al.. 1934). They were all polished on opposite parallel faces. Four specimens of each type were used. They varied in size from 0.05 to 0.20ct per stone. The synthetic stones were not polished because of their small size. They were used “as-is” i.e. the only treatment which they underwent after removal from the pressure cells was that they were chemically cleaned. They were manufactured by tightly controlling all aspects of the synthesis parameters and two suites of diamonds were prepared. One suite was synthesized from a solvent/catalyst which was iron based and the second suite from an iron free solvent/catalyst. Both suites were boron doped. The resulting diamonds were all 3&40 U.S. mesh size (0.60.4 mm). (B)
Scintillation
I
Procedures
and phosphorescence
measurements
The scintillation response from the diamonds was measured with a 9750A EM1 photomultiplier tube. The output pulses from the photomultiplier tube were fed, via an Ortec model I 13 preamplifier to a Nucleus Spectrum 88 Multichannel Analyser operating in the pulse-height analysis mode and set to 1024 channels. To establish the thermoluminescence characteristics of the crystals, measurements were made with a Toledo-654A reader modified to permit maximum read-temperatures to be adjustable up to 5OO’C. The reader was interfaced to the Nucleus analyser and for both the thermoluminescence and phosphorescence decay measurements the analyser was switched to the multiscaling mode. All the raw data were stored on floppy disks and the analysis of the data was carried out on an off-line microcomputer. Figure 1 shows a block diagram of the experimental arrangement. The scintillation response of the diamonds to different dose rates was determined by irradiating the specimens with a collimated beam of r-particles obtained from “‘Am radioactive sources of various activities. The collimator, of 0.2 mm di-
I[\\\\\\I
Fig. ment
DIAhlOXD I\\\\\1
+
CRYST41
COLLIMATOR a source
I. Schematic
representation of experimental arrangeused for determining the scintillation properties of diamond.
ameter. was made ments were carried
of aluminium and all measureout at atmospheric pressure.
Results For those natural diamonds that displayed energy resolution, Figs 2 and 3 show typical responses. These energy spectra were obtained without (Fig. 2) and with (Fig. 3) pulse height discrimination respectively and the presence of a large component of low energy pulses is clearly seen by comparing the two spectra. Many natural stones were tried that exhibited no energy resolution whatsoever. One natural stone also displayed a regional inhomogeneity in energy resolution meaning that on the same polished surface, isolated areas responded differently under the z-beam
Natural
Diamond
OS Scinrilmciion
Counter
6000
Loooi 0
200 Channels
400
I -1
,/ 600
(Energy)
Fig. 2. Pulse height distribution response for scintillation from a natural diamond obtained without pulse height discrimination.
659
Detection of nuclear radiation Natural
Diamond
I
1,
os
Scintillation
I
I
400
500
Iron
Counter
I
Free
Synthetic
Diamond
200
I
0 100
200
300 Channels
600
irradiation, some areas showing energy resolution others displaying none. The property of energy resolution in the natural diamonds tested was found to be independent of the diamond type. The FWHM of the
peaks obtained from those diamonds that did display energy resolution was of the order of 180 channels with the maximum of the peak in channel 385. In the synthetic diamond suites however, all of the iron-based stones indicated no energy resolution whatever (cf. Fig. 4) and, additionally, exhibited an extended after glow phosphorescence. The synthetic diamonds from the iron free solvent on the other hand, despite non-optimum experimental conditions (poor coupling between scintillator and photomultiplier tube, atmospheric pressure experimental condienergy displayed encouragingly good tion), resolution characteristics (FWHM = 44 channels, maximum in channel 270) as shown in Fig. 5. This a -particle energy resolution compared favourably to that obtained from a commercially available CaF,(Eu) crystal when exposed to an 24’Am source. The statistical spread of pulse heights, especially on the lower energy side, can be attributed to particle
Iron
Based
200 Channels
(Energy)
Fig. 3. Pulse height distribution response for scintillation from a natural diamond as in Fig. 2, but with pulse height discrimination.
5*10’
1
100
700
Synthetic
400
300 (Energy)
Fig. 5. Typical pusle height distribution response for scintillation from iron-free synthetic diamonds.
energy degradation and to light scattering as a result of the poor optical coupling between the crystal and the photosurface of the photomultiplier and unpolished crystal surfaces. A typical dose rate response obtained from the diamond scintillator is shown in Fig. 6. Figure 7 shows typical thermoluminescence glowcurves that relate to both natural and synthetic diamonds. These glow-curves were obtained 15 s after an exposure of the diamonds to a dose of 0.30 Gy from a ‘%-source. A linear heating rate of 155°C s-’ was used for the read-out cycle. It can be seen (cf. Fig. 7a) that the natural and iron based synthetic diamonds exhibit very broad structureless peaks which indicates the presence of a number of traps with varying depths. The form of the glow-curve from the iron free synthetic diamonds by contrast (cf. Fig. 7b) shows a much narrower peak indicating a single trap or fewer traps more localized in energy. The phosphorescence component of the specimens was measured 5 s after excitation at different subsequent time intervals. Figure 8 shows the decay curves for both the iron-free and iron-based synthetic crystals.
Diamond
Response
I
p\”
of
Diamond
to
Change
in
Alpha
Activity
1
400
f
v) 3*10’ E 0’
i
/
t (
o
/v
t
4rlO’ 300
b a
2tlO’
-
l*lO’
-
0
w 200 E a 0 100 ‘:I
100
200 Channels
300
400
(Energy)
Fig. 4. Typical pulse height distribution response for scintillation from iron based synthetic diamonds.
I
0
100 Source
200 Activity
(microcurie
400
300 per
sq.cm)
Fig. 6. Scintillation response of iron free synthetic diamond crystals as a function of the a-particle dose-rate.
T. L. NAM et al Curves
of
Sytlthetic
and
200
Natural
400
300
I eri1perature
Diamond
Cr)
Phosphorescence
-, -----...
Decoy
Curves
-_l-l_ i
500
“C
Fig. 8. Phosphorescence
decay curves diamonds.
from
the synthetic
/-
/-
_ 100
200
300
Temperature
Fig. 7. Thermoluminescence iron based synthetic stones.
“C
glow-curves. (a) Natural and (b) Iron free synthetic stones.
Discussion The reason for the poor energy resolution in scintillators can be attributed to the presence of trapping centres (Birks, 1964). It has been shown (Murray and Meyer, 1961) that the ratio of the number of electron-hole pair recombinations (no) which give rise to luminescence to the number of (n,) electron-hole pairs created through ionization may be described by the relation: k,n, ‘$1 n, = k,n,+ k,n, where n, is the trap concentration. k, n, and k,ne are electron-hole capture and recombination rate parameters respectively, and n, is proportional to the energy deposited per unit length by the ionizing particle The trapping centres remove a certain fraction of the charge carriers produced by the incident radiation. This physical process of trapping and the delayed photon emission by the subsequent depopulation of the shallow traps gives rise to the varying degrees of phosphorescence resulting in energy degra-
dation and poor resolution. The presence of phosphorescence renders the use of any diamond (synthetic or natural) unsuitable as a scintillation detector because, in addition to a non-linear response to incident radiation, these diamonds exhibit relatively large background count rates even after the cessation of incident radiation. From the equation it can be seen that only by improving the crystal quality i.e. reducing the trap concentration (n,), does it appear possible to improve the linearity of response and reduce the value of the intrinsic resolution. Increases in the values of the parameters n, and k, will degrade these properties. Our experimental experience supports this model. Unlike the iron-free synthetic diamonds, the natural diamonds and the iron based synthetics displayed a large concentration of trapping levels with low activation energies (cf. Fig. 7a). As a result of detrapping, the lifetimes of the charge carriers trapped at these levels were relatively short. The corresponding increase in the rate trapping parameters k, would explain the poor energy resolutions displayed by these crystals. From the glow-curves it can be seen that the iron-free synthetic diamonds are not inhibited by shallow trapping levels. Only a single peak occurring at a temperature of approx. 300 C is displayed by these crystals. This represents. as was previously pointed out, localized levels or a level with large activation energy and as a result the effect of charge carriers trapped at this level on the overall energy resolution is minimal. Any increase in the concentration of deep trapping levels however can be expected to give rise to a decrease in the sensitivity of the crystal to the incident radiation. A comparison of the rate at which the charge carriers depopulate at room temperature after the irradiation of the crystal can be seen in Fig. 8 showing the phosphorescence decay curves. The level of the phosphorescence intensity for the iron-free diamonds was found to be very much lower in magnitude when compared to the iron containing crystals. Despite the very different solvent/‘catalyst used in the two syn-
Detection thetic
diamond
processes,
the
specimens
from
of nuclear both
procedures displayed phosphorescence curves with very similar shapes. This suggests that the shallow traps are likely to be lattice traps which arise as a result of matrix distortions and which are augmented by the presence of included metals. This is especially so when the nitrogen impurity concentration is lowered. By selecting the solvent/catalyst to be iron-free, crystals with a relatively low concentration of shallow traps and with good scintillation properties can be produced.
Conclusion One of the impurities within the diamond lattice which is responsible for the trapping centres and consequent poor scintillation energy resolution has been identified. It has been shown that by eliminating the presence of iron related impurities in the manufacturing process, synthetic diamonds can be produced which as scintillation detectors exhibit a most satisfactory energy resolution and dose rate response for a-particles. Unlike natural diamonds, the use of synthetic diamonds offers the advantage that, by controlling the synthesis process, reproducible material with desirable characteristics can be obtained. The principal advantage of such detectors is the high
661
radiation
z- to p-particle response ratio. This would make them priority candidates for use in a neutron environment or for a-particle detection especially where discrimination against b-radiation is required. With their tissue equivalence and other attributes they are also excellent candidates for in tliuo measurements. Acknowledgements-The authors wish to thank Dr R. J. Caveney, Dr G. J. Davies and Dr R. C. Burns of the Diamond Research Laboratory for the enthusiastic and encouraging support of this project. Professor J. P. F. Sellschop of the Schonland Research Centre is thanked for the loan of the suites of natural diamonds and Dr U. Karfunkel, also of the Schonland Centre, for helpful discussions. Mr J. Grobbelaar of the Diamond Research Laboratory is thanked for his efficient technical help and support.
References Birks J. B. (1964) IEEE Trans. NW/. Sci. NS-11, no. 3, 4. Bull R. K. (1986) Nucl. Tracks Radial. Meas. 11, 105. Dean P. J., Kennedy P. J. and Ralph J. E. (1960) Proc. Phys. Sot. 76, 670. Keddy R. J., Nam T. L. and Burns R. C. (1987) Phys. Med. Biol. 32, 75 1. Miller T. G. (1966) Nucl. Instrum. Methods 43, 338. Murray R. B. and Meyer A. (1961) Phys. Reu. 122, 814. Ralph J. E. (1959) Proc. Phys. Sot. London 73, 233. Robertson R., Fox J. J. and Martin A. E. (1934) Phil. Trans. R. Sot. A 232, 463.
1989
0883-2889/89
$3.00 + 0.00
Copyright ‘c 1989 Maxwell Pergamon Macmillan
plc
Detection of Nuclear Radiation by Scintillation Counting Using Synthetic Diamond T. L. NAM,’ P. J. FALLON,’
R. J. KEDDY,‘* H. J. VAN RIJN*
and JOANNE
F. ANDREWS2
‘Schonland Research Centre, University of the Witwatersrand, of South Africa and 2De Beers Diamond Research Laboratory, (Receioed 30 January
P.O. WITS, Johannesburg, Johannesburg, Republic
2050 Republic of South Africa
1989)
The scintillation characteristics of boron activated synthetic diamonds, produced from an iron free solvent/catalyst, when subjected to nuclear ionizing radiations have been investigated. Despite the fact that the optical coupling between the crystal and the surface of the photomultiplier which was used in the
experimental arrangement was far from ideal, the crystals, when exposed to irradiation by a-particles, exhibited good energy resolution and linear dose rate response characteristics.
Introduction Luminescence in a material may be defined as the emission of electromagnetic radiation, usually in the U.V. to i.r. region, that is not attributable to incandescence. The features common to all forms of luminescence are: (a) the occurrence of some processes whereby an atom, molecule or, more importantly in our case, a “center” (an aggregate of atoms or defects in a crystal) is excited to a higher energy state, and (b) its radiative de-excitation with the emission of a photon after the lapse of some period of time. The distinction between various types of luminescence i.e. “fluorescence” and “phosphorescence”, is normally based on the time dependence of the emission of the photon but a more meaningful distinction may be based on the temperature-dependence of the luminescence decay time (Bull, 1986). However, when luminescence is present only during excitation it is “fluorescence” and if the excited state lifetime is short then this luminescence can be termed “scintillation”. In the case of impurity-activated crystals the scintillation process, by means of which the incident energy is converted to luminescence emission, is a complex sequence of events and is certainly a function of the impurity atoms which are present within the crystal lattice. The interaction of nuclear radiation with any material creates charges carriers by ionization within the material. The number of charge carriers in the
*Author
for correspondence. 657
form of electrons and holes created per incident nuclear particle is proportional to the energy of the particle. If suitable luminescence centers are present within the bulk of the material, the electrons and holes may recombine at these centers to emit photons. The intensity of the light thus emitted is therefore proportional to the energy of the incident particle. The total number of such events will give an indication of the flux of nuclear particles. By coupling the attenuating material to a photomultiplier the magnitude (energy) and number (flux) of the incident nuclear particles may be recorded. This is, of course, the phenomenon of scintillation counting, a technique widely applied in the real-time detection of nuclear radiation. Many crystals including certain diamonds exhibit this scintillation property and the scintillation properties of natural diamonds when subjected to irradiation by nuclear particles has been reported (cf. Ralph, 1959; Dean et al., 1960; Miller, 1966). Other general properties of diamond, e.g. tissue equivalence, nontoxicity and stability against most corrosive fluids including body fluids (Keddy et al., 1987), places diamond in a very unique position on the list of scintillators, particularly if in vivo applications are contemplated. A further advantage of diamond as a scintillator is that, unlike other scintillators, it exhibits a higher X- to p-particle scintillation response (Dean et al., 1960). Despite all these advantages however, diamond has not featured as a practical scintillation radiation detector. Specimens of natural diamond have been shown to exhibit energy responses which varied from reasonable to poor to virtually non-existent and this is confirmed in
658
T. L.
NAM
al.
et
this work. The omission to use natural diamond as a radiation detector is explained by the fact therefore that, besides being expensive, no two stones can be guaranteed to have the same scintillation characteristics. Synthetic diamonds offer new possibilities however and we report here on the scintillation performance of specially produced synthetic diamonds when irradiated by a beam of r-particles obtained from 14’Am radioactive sources. The properties of the synthetic stones are compared to those of natural diamonds and to other synthetic stones that did not have the same selective chemistry during the manufacturing process. PM
TUBE
Experimental (A) Phosphor
.
preparation
The natural diamonds used for this experiment were classified as type I and type IIa (Robertson rt al.. 1934). They were all polished on opposite parallel faces. Four specimens of each type were used. They varied in size from 0.05 to 0.20ct per stone. The synthetic stones were not polished because of their small size. They were used “as-is” i.e. the only treatment which they underwent after removal from the pressure cells was that they were chemically cleaned. They were manufactured by tightly controlling all aspects of the synthesis parameters and two suites of diamonds were prepared. One suite was synthesized from a solvent/catalyst which was iron based and the second suite from an iron free solvent/catalyst. Both suites were boron doped. The resulting diamonds were all 3&40 U.S. mesh size (0.60.4 mm). (B)
Scintillation
I
Procedures
and phosphorescence
measurements
The scintillation response from the diamonds was measured with a 9750A EM1 photomultiplier tube. The output pulses from the photomultiplier tube were fed, via an Ortec model I 13 preamplifier to a Nucleus Spectrum 88 Multichannel Analyser operating in the pulse-height analysis mode and set to 1024 channels. To establish the thermoluminescence characteristics of the crystals, measurements were made with a Toledo-654A reader modified to permit maximum read-temperatures to be adjustable up to 5OO’C. The reader was interfaced to the Nucleus analyser and for both the thermoluminescence and phosphorescence decay measurements the analyser was switched to the multiscaling mode. All the raw data were stored on floppy disks and the analysis of the data was carried out on an off-line microcomputer. Figure 1 shows a block diagram of the experimental arrangement. The scintillation response of the diamonds to different dose rates was determined by irradiating the specimens with a collimated beam of r-particles obtained from “‘Am radioactive sources of various activities. The collimator, of 0.2 mm di-
I[\\\\\\I
Fig. ment
DIAhlOXD I\\\\\1
+
CRYST41
COLLIMATOR a source
I. Schematic
representation of experimental arrangeused for determining the scintillation properties of diamond.
ameter. was made ments were carried
of aluminium and all measureout at atmospheric pressure.
Results For those natural diamonds that displayed energy resolution, Figs 2 and 3 show typical responses. These energy spectra were obtained without (Fig. 2) and with (Fig. 3) pulse height discrimination respectively and the presence of a large component of low energy pulses is clearly seen by comparing the two spectra. Many natural stones were tried that exhibited no energy resolution whatsoever. One natural stone also displayed a regional inhomogeneity in energy resolution meaning that on the same polished surface, isolated areas responded differently under the z-beam
Natural
Diamond
OS Scinrilmciion
Counter
6000
Loooi 0
200 Channels
400
I -1
,/ 600
(Energy)
Fig. 2. Pulse height distribution response for scintillation from a natural diamond obtained without pulse height discrimination.
659
Detection of nuclear radiation Natural
Diamond
I
1,
os
Scintillation
I
I
400
500
Iron
Counter
I
Free
Synthetic
Diamond
200
I
0 100
200
300 Channels
600
irradiation, some areas showing energy resolution others displaying none. The property of energy resolution in the natural diamonds tested was found to be independent of the diamond type. The FWHM of the
peaks obtained from those diamonds that did display energy resolution was of the order of 180 channels with the maximum of the peak in channel 385. In the synthetic diamond suites however, all of the iron-based stones indicated no energy resolution whatever (cf. Fig. 4) and, additionally, exhibited an extended after glow phosphorescence. The synthetic diamonds from the iron free solvent on the other hand, despite non-optimum experimental conditions (poor coupling between scintillator and photomultiplier tube, atmospheric pressure experimental condienergy displayed encouragingly good tion), resolution characteristics (FWHM = 44 channels, maximum in channel 270) as shown in Fig. 5. This a -particle energy resolution compared favourably to that obtained from a commercially available CaF,(Eu) crystal when exposed to an 24’Am source. The statistical spread of pulse heights, especially on the lower energy side, can be attributed to particle
Iron
Based
200 Channels
(Energy)
Fig. 3. Pulse height distribution response for scintillation from a natural diamond as in Fig. 2, but with pulse height discrimination.
5*10’
1
100
700
Synthetic
400
300 (Energy)
Fig. 5. Typical pusle height distribution response for scintillation from iron-free synthetic diamonds.
energy degradation and to light scattering as a result of the poor optical coupling between the crystal and the photosurface of the photomultiplier and unpolished crystal surfaces. A typical dose rate response obtained from the diamond scintillator is shown in Fig. 6. Figure 7 shows typical thermoluminescence glowcurves that relate to both natural and synthetic diamonds. These glow-curves were obtained 15 s after an exposure of the diamonds to a dose of 0.30 Gy from a ‘%-source. A linear heating rate of 155°C s-’ was used for the read-out cycle. It can be seen (cf. Fig. 7a) that the natural and iron based synthetic diamonds exhibit very broad structureless peaks which indicates the presence of a number of traps with varying depths. The form of the glow-curve from the iron free synthetic diamonds by contrast (cf. Fig. 7b) shows a much narrower peak indicating a single trap or fewer traps more localized in energy. The phosphorescence component of the specimens was measured 5 s after excitation at different subsequent time intervals. Figure 8 shows the decay curves for both the iron-free and iron-based synthetic crystals.
Diamond
Response
I
p\”
of
Diamond
to
Change
in
Alpha
Activity
1
400
f
v) 3*10’ E 0’
i
/
t (
o
/v
t
4rlO’ 300
b a
2tlO’
-
l*lO’
-
0
w 200 E a 0 100 ‘:I
100
200 Channels
300
400
(Energy)
Fig. 4. Typical pulse height distribution response for scintillation from iron based synthetic diamonds.
I
0
100 Source
200 Activity
(microcurie
400
300 per
sq.cm)
Fig. 6. Scintillation response of iron free synthetic diamond crystals as a function of the a-particle dose-rate.
T. L. NAM et al Curves
of
Sytlthetic
and
200
Natural
400
300
I eri1perature
Diamond
Cr)
Phosphorescence
-, -----...
Decoy
Curves
-_l-l_ i
500
“C
Fig. 8. Phosphorescence
decay curves diamonds.
from
the synthetic
/-
/-
_ 100
200
300
Temperature
Fig. 7. Thermoluminescence iron based synthetic stones.
“C
glow-curves. (a) Natural and (b) Iron free synthetic stones.
Discussion The reason for the poor energy resolution in scintillators can be attributed to the presence of trapping centres (Birks, 1964). It has been shown (Murray and Meyer, 1961) that the ratio of the number of electron-hole pair recombinations (no) which give rise to luminescence to the number of (n,) electron-hole pairs created through ionization may be described by the relation: k,n, ‘$1 n, = k,n,+ k,n, where n, is the trap concentration. k, n, and k,ne are electron-hole capture and recombination rate parameters respectively, and n, is proportional to the energy deposited per unit length by the ionizing particle The trapping centres remove a certain fraction of the charge carriers produced by the incident radiation. This physical process of trapping and the delayed photon emission by the subsequent depopulation of the shallow traps gives rise to the varying degrees of phosphorescence resulting in energy degra-
dation and poor resolution. The presence of phosphorescence renders the use of any diamond (synthetic or natural) unsuitable as a scintillation detector because, in addition to a non-linear response to incident radiation, these diamonds exhibit relatively large background count rates even after the cessation of incident radiation. From the equation it can be seen that only by improving the crystal quality i.e. reducing the trap concentration (n,), does it appear possible to improve the linearity of response and reduce the value of the intrinsic resolution. Increases in the values of the parameters n, and k, will degrade these properties. Our experimental experience supports this model. Unlike the iron-free synthetic diamonds, the natural diamonds and the iron based synthetics displayed a large concentration of trapping levels with low activation energies (cf. Fig. 7a). As a result of detrapping, the lifetimes of the charge carriers trapped at these levels were relatively short. The corresponding increase in the rate trapping parameters k, would explain the poor energy resolutions displayed by these crystals. From the glow-curves it can be seen that the iron-free synthetic diamonds are not inhibited by shallow trapping levels. Only a single peak occurring at a temperature of approx. 300 C is displayed by these crystals. This represents. as was previously pointed out, localized levels or a level with large activation energy and as a result the effect of charge carriers trapped at this level on the overall energy resolution is minimal. Any increase in the concentration of deep trapping levels however can be expected to give rise to a decrease in the sensitivity of the crystal to the incident radiation. A comparison of the rate at which the charge carriers depopulate at room temperature after the irradiation of the crystal can be seen in Fig. 8 showing the phosphorescence decay curves. The level of the phosphorescence intensity for the iron-free diamonds was found to be very much lower in magnitude when compared to the iron containing crystals. Despite the very different solvent/‘catalyst used in the two syn-
Detection thetic
diamond
processes,
the
specimens
from
of nuclear both
procedures displayed phosphorescence curves with very similar shapes. This suggests that the shallow traps are likely to be lattice traps which arise as a result of matrix distortions and which are augmented by the presence of included metals. This is especially so when the nitrogen impurity concentration is lowered. By selecting the solvent/catalyst to be iron-free, crystals with a relatively low concentration of shallow traps and with good scintillation properties can be produced.
Conclusion One of the impurities within the diamond lattice which is responsible for the trapping centres and consequent poor scintillation energy resolution has been identified. It has been shown that by eliminating the presence of iron related impurities in the manufacturing process, synthetic diamonds can be produced which as scintillation detectors exhibit a most satisfactory energy resolution and dose rate response for a-particles. Unlike natural diamonds, the use of synthetic diamonds offers the advantage that, by controlling the synthesis process, reproducible material with desirable characteristics can be obtained. The principal advantage of such detectors is the high
661
radiation
z- to p-particle response ratio. This would make them priority candidates for use in a neutron environment or for a-particle detection especially where discrimination against b-radiation is required. With their tissue equivalence and other attributes they are also excellent candidates for in tliuo measurements. Acknowledgements-The authors wish to thank Dr R. J. Caveney, Dr G. J. Davies and Dr R. C. Burns of the Diamond Research Laboratory for the enthusiastic and encouraging support of this project. Professor J. P. F. Sellschop of the Schonland Research Centre is thanked for the loan of the suites of natural diamonds and Dr U. Karfunkel, also of the Schonland Centre, for helpful discussions. Mr J. Grobbelaar of the Diamond Research Laboratory is thanked for his efficient technical help and support.
References Birks J. B. (1964) IEEE Trans. NW/. Sci. NS-11, no. 3, 4. Bull R. K. (1986) Nucl. Tracks Radial. Meas. 11, 105. Dean P. J., Kennedy P. J. and Ralph J. E. (1960) Proc. Phys. Sot. 76, 670. Keddy R. J., Nam T. L. and Burns R. C. (1987) Phys. Med. Biol. 32, 75 1. Miller T. G. (1966) Nucl. Instrum. Methods 43, 338. Murray R. B. and Meyer A. (1961) Phys. Reu. 122, 814. Ralph J. E. (1959) Proc. Phys. Sot. London 73, 233. Robertson R., Fox J. J. and Martin A. E. (1934) Phil. Trans. R. Sot. A 232, 463.