Synthetic diamond radiation detector

Nuclear radiation is detected by applying electrical contacts to a synthetic diamond having a nitrogen impurity concentration of 25 to 150 parts per million. An electrical circuit is connected to the contacts and applies a DC bias voltage across the diamond. When the diamond is subjected to nuclear radiation, a change in the current or voltage in the circuit occurs which corresponds to the radiation intensity. The nuclear radiation may be any kind of radiation. The contacts are preferably attached directly to the diamond and are ohmic in nature.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to the detection of nuclear radiation by means of a 
synthetic diamond detector. 
2. Discussion of the Prior Art 
The detection of nuclear radiation using diamond detectors per se is well 
known. According to the literature, it is necessary for diamonds to have 
very low impurity concentrations in order to be suitable for this 
application. In the case of synthetic diamonds, it has been proposed that 
a nitrogen impurity concentration of less than 20 parts per million (ppm) 
is necessary for use in nuclear radiation detectors--see for example, in 
European Patent Publication Number 0052397 (Burgemeister). 
Diamond is composed of the single element carbon and, to date, only 
nitrogen and boron are known with certainty to be incorporated into the 
diamond lattice. Nitrogen atoms may be bonded into the diamond as: 
(a) a single nitrogen atom replacing a carbon atom. This form of nitrogen 
is known as the single substitutional nitrogen; 
(b) a pair of adjacent nitrogen atoms (called an A-aggregate) replacing a 
pair of carbon atoms; or 
(c) a cluster of nitrogen atoms referred to as the B-aggregate or B form of 
nitrogen. The exact molecular form of this nitrogen is still unknown. It 
would appear from the experimental data available that the cluster 
involves four nitrogen atoms. 
Each of the above mentioned forms of bonded nitrogen atoms displays 
characteristic infrared and ultraviolet absorption spectra which are 
signatures of their presence. It should be pointed out that such spectra 
by themselves do not provide quantitative concentrations of the type of 
nitrogen impurities present. 
According to the Robertson, Martin and Fox scale, diamonds are typified by 
the presence and concentration of the nitrogen within the diamond: 
Type Ia Diamond 
About 98% of natural diamonds are of this type. They contain nitrogen as an 
impurity in fairly substantial amounts. The nitrogen appears to be 
distributed into small aggregates. The dominant A and B aggregate, as 
mentioned above, are recognized from the impurity induced infrared 
absorption spectrum and strong absorption in the ultraviolet. Nitrogen in 
the A and B forms is not paramagnetic. 
Type Ib Diamond 
These are very rare in nature (.apprxeq.0.1%). They contain nitrogen on 
isolated substitutional lattice sites. These nitrogen atoms produce 
paramagnetic resonance. 
Type IIa Diamond 
Rare in nature, these diamonds have insufficient nitrogen to be easily 
detected by infrared or ultraviolet absorption measurement. 
Type IIb Diamond 
Extremely rare in nature. These diamonds have such a low concentration of 
nitrogen (lower even than the type IIa) that some of the boron acceptors 
are not compensated in the crystal and as a result the crystal is a p-type 
semiconductor. 
Paramagnetic resonance experiments are based on the property of the 
electron discovered in 1921 by Otto Stern and Walter Gerlack; namely that 
every electron has a magnetic dipole like a miniature bar magnet. 
Usually when a material is formed, the bonding electrons pair off so that 
their magnetic effects cancel each other and consequently most substances 
are non magnetic. In a pure diamond, the electrons behave in this way i.e. 
they pair off to give a non magnetic diamond. However when one carbon atom 
is replaced by a nitrogen atom, the nitrogen atom has 5 valence electrons 
which can be used for bonding. The carbon atom, on the other hand, has 
only four valence electrons. This gives rise to an excess electron which 
has no partner with which to pair. As a result its magnetic properties 
cannot be hidden. 
If such a diamond is placed in a magnetic field, the electron must either 
point along the magnetic field or essentially in the opposite direction. 
Photons of electromagnetic energy are required to swing the electron 
dipoles from pointing along the magnetic field to pointing to the opposite 
direction. This is the basis of the electron spin resonance (ESR) 
measurements. A measurement of the integrated absorption of energy in ESR 
spectroscopy is therefore associated with transition of electrons from a 
lower energy level to a higher level and a measurement of the integrated 
absorption provides a measurement of the number of atoms or molecules 
containing unpaired electrons. 
The total magnetic field experienced by the electron is made up of the 
external magnetic field applied to the specimen plus a small but 
detectable contribution from the nuclei in the neighborhood of the 
electrons. By the ESR technique one can in favorable cases indentify the 
chemical species and the structural arrangement of the impurity. 
In the case of diamond, Smith et al (1959, Phys. Rev., 115, 1546) found 
that electron paramagnetic resonances occur at single substitutional 
nitrogen atoms. It is important to stress that the ESR technique requires 
the presence of the unique unpaired electron and when the technique is 
used for nitrogen determination only the presence of single substitutional 
nitrogen is measured. 
The concentrations of centers with unpaired electrons are invariably quoted 
by ESR spectroscopists as "centers per cc", "centers per meter cube" 
"atomic %" or just "ppm" (implying ppm atomic). This stems from the actual 
technique itself, whereby the number of centers which are responsible for 
the photon absorption is related to that produced by a standard, the 
concentration of which is normally quoted in terms of the number of 
ESR-centers per unit volume. 
It is important to note that unless synthetic diamonds are, after 
synthesis, subjected for a relatively long period of time to high 
pressures, or special precautions are taken to exclude nitrogen from the 
growth enviroment, all synthetic diamonds may be classified as type Ib 
where 99% of the nitrogen atoms are present at substitutional sites. 
Although each diamond type was found to exhibit a characteristic infrared 
spectrum, it was not until Chrenko's work, in which he established the 
correlation between the intensity of the type Ib infrared band and the ESR 
measured nitrogen concentrations, that infrared absorption spectra were 
used to quantify the presence of these centers. Although the method used 
by Burgemeister (EP 0052397) to determine the nitrogen concentration is 
not specified it can be assumed with confidence that because he was using 
synthetic diamond, the quoted nitrogen concentration represented the 
amount of single substitutional nitrogen present in the diamonds used. 
Furthermore it is of interest to note that even Burgemeister quoted such 
nitrogen concentrations in centers per unit volume and in units of atomic 
percentage. Kozlov, on the other hand, selected natural diamonds for use 
as radiation detectors. Although not stated in his patent specification 
(U.S. Pat. No. 3,665,193) it can be inferred from all his subsequent 
publications that natural diamonds used were of the type IIa variety. In 
any event, the criteria used for selecting natural diamonds in this 
application are different from those applicable to synthetic diamonds. 
In the abovementioned Kozlov patent, reference is made to natural diamonds 
having a nitrogen content of less than 10.sup.19 atoms cm.sup.-3. Recent 
work by Loubser and van Wyk (1965, S. A. Journal of Physics, 10, 165) 
shows that the concentration of single substitutional centers in type II 
and intermediate natural diamonds, measured by ESR, is less than 1015 
centers cm.sup.-3, a factor approximately 3000 less than the figure quoted 
by Kozlov. From this it can be deduced that the figure of 10.sup.19 atoms 
cm.sup.-3 quoted by Kozlov represents total nitrogen concentration. 
SUMMARY OF THE INVENTION 
According to the invention a method of detecting nuclear radiation includes 
the steps of applying electrical contacts, preferably ohmic, to a 
synthetic diamond material having a paramagnetic nitrogen impurity 
concentration of 25 to 150 parts per million, connecting an electrical 
circuit to the contacts which applies a DC bias voltage across the diamond 
material, subjecting the diamond material to nuclear radiation, and 
monitoring a current or voltage in the circuit corresponding to the 
radiation dose rate. The current or voltage may correspond to pulses of 
radiation. 
Further according to the invention a nuclear radiation detector comprises a 
synthetic diamond material having a paramagnetic nitrogen impurity 
concentration of 25 to 150 parts per million, a pair of electrical 
contacts, preferably ohmic, applied to the diamond material, and an 
electrical circuit connected to the contacts which includes a DC voltage 
source for applying a bias voltage across the diamond material and means 
for measuring a voltage or current in the circuit. 
Still further according to the invention a synthetic diamond material 
suitable for use in a nuclear radiation detector has a paramagnetic 
nitrogen impurity concentration of 25 to 150 parts per million. 
The nitrogen impurity concentration of the diamond material is preferably 
between 25 and 60 parts per million. 
The radiation may be any nuclear radiation such as alpha, beta, gamma, 
neutron or X-radiation. 
The diamond may be used to monitor radiation administered to patients. 
It is to be noted that nitrogen impurity in this specification is as 
measured by ESR techniques. 
The diamond material is preferably a synthetic diamond particle, but may 
also be a diamond film or diamond-like film on a substrate.

DETAILED DESCRIPTION OF THE INVENTION 
An embodiment of the invention will now be described. A synthetic diamond 
crystal having a paramagnetic nitrogen impurity concentration of 97 ppm, 
as measured by ESR techniques, was used as a nuclear radiation detector. 
The crystal, with a thickness of 0.2 mm, and with ohmic contacts on 
opposite sides thereof was clamped between two gold beads. One of the 
contacts consisted of a silver paint layer, while the other contact was a 
graphite layer. Both layers were heat-cured at 50.degree. C. for two 
hours. A DC bias voltage source was connected in series with the contacts 
and a 100M ohm resistor, as illustrated in FIG. 1. 
The diamond was exposed to gamma radiation from a cobalt 60 source, with a 
response as plotted in FIG. 2. At a 60 volt bias voltage, the detector 
response was found to be substantially linear up to a dose rate of 1,0 
Gy/min, which is a practical dose rate for radiotherapy treatment. An 
important feature of the tested diamond is that its linear response 
extends as low as 2 cGy/hour, which is sufficiently sensitive for accurate 
measurement of small therapeutic radiation doses. FIG. 3 shows the energy 
response of the diamond to alpha particles from an americium 241 source. 
Good linearity in the relationship between particle energy and the voltage 
response of the detector is evident. Similar effects were obtained in a 
vacuum eliminating any ionization effects in the air. 
FIG. 4 shows the variation in sensitivity of diamonds with different 
nitrogen impurity concentrations. It can be seen from the figure that the 
diamonds with nitrogen impurity concentrations of 20 ppm and less have a 
response current which is an order of magnitude greater than that of 
diamonds with nitrogen impurity concentrations of about 65 to 105 ppm. 
However, the difference between the former group and a diamond having a 
nitrogen impurity concentration of about 50 ppm is much less, being about 
5 to 1. 
Compared to prior art detectors, the diamond detector of the invention has 
a much faster settling time, in the region of 1 to 2 seconds, which is 
also less dependent on the intensity of visible light illumination 
incident on the detector. This is illustrated by FIG. 5, which compares 
the response time of a diamond having a low nitrogen concentration (10 to 
20 ppm) with one having a medium nitrogen concentration (30 to 50 ppm). 
The medium-nitrogen stone has a lower maximum response current, but has a 
much faster response. FIG. 6 shows the current and voltage response of a 
medium nitrogen diamond (30 to 50 ppm) under illumination by intense white 
light. As can be seen from the Figure, the diamond is essentially 
insensitive to illumination with white light. 
As shown above, the sensitivity of the diamond to nuclear radiation 
decreases with an increasing nitrogen impurity concentration, while the 
response time reduces with increasing nitrogen concentration. Low-nitrogen 
diamonds exhibit higher sensitivity than those with higher nitrogen 
contents, but have a slow response time, taking several minutes for their 
response to reach its maximum value. They are also sensitive to 
illumination with white light, which markedly alters the response time. 
The linearity of low-nitrogen diamonds is also not very satisfactory over 
a wide range of radiation dose levels. Thus, the diamonds used in the 
practice of the present invention have the advantages, relative to 
diamonds used in the prior art, of being relatively insensitive to white 
light, having a fast response time and good linearity over a wide range of 
radiation dose levels, without sacrificing materially on sensitivity. 
Diamonds according to the invention are typically of a size varying from 1 
mm to 3 mm in diameter. For these diamonds the electrical contacts may be 
attached to the diamond. Smaller diamond particles, i.e. those having a 
size of less than 1 mm, may also be used. For such diamonds it is 
preferably to mount the diamonds between a pair of electrical contacts 
which are firmly pressed into contact with the diamonds. Applications 
envisaged for the diamonds include miniature personal radiation monitors, 
which can conveniently be fabricated in a card format. The card substrate 
would carry a battery or other power source, a diamond radiation detector, 
an electronic measuring circuit and possibly an alarm device to provide a 
warning when a specified radiation dose rate or cumulative dose is 
exceeded. The circuitry may also be such as to detect pulses of radiation. 
The synthetic diamond particles used in the practice of the invention may 
be made by methods known in the art. These methods involve subjecting a 
carbon source in the presence of a suitable diamond catalyst to elevated 
temperature and pressure conditions at which diamond is 
crystallographically stable. Tailoring such methods to produce diamonds of 
a particular and desired nitrogen content is well within the knowledge and 
skills of the man in the art. 
It has been found that the diamonds produced in a single batch have 
reproducible characteristics from stone to stone. 
It will be apparent that although the described synthetic diamonds have a 
paramagnetic nitrogen impurity concentration which is significantly 
greater than that of prior art diamond radiation detectors, the advantages 
obtained outweigh, in many operations, the reduction in absolute 
sensitivity experienced.