Radiation sensor dosimetry circuit

A radiation sensor of the type that acquires a charge or voltage proportil to a dose of radiation. Upon obtaining a predetermined value to the associated voltage proportional to the dose, the voltage is caused to be discharged. A counter keeps track of the number of discharges which is proportional to the total dose of radiation to which the radiation sensor has been exposed. The radiation sensor is thereby prevented from achieving a high charge or voltage which affects the accuracy of the radiation sensor. A calculator and a clock are used to display the value of the dose in any convenient way, such as in relation to tissue dose or average dose rate for a given period of time.

GOVERNMENT INTEREST 
The invention described herein may be manufactured, used, and licensed by 
or for the Government for governmental purposes without the payment to us 
of any royalty thereon. 
FIELD OF THE INVETNION 
This invention relates generally to detecting radiation dose, and more 
specifically to a circuit for displaying an accurate dose. 
BACKGROUND OF THE INVENTION 
There are many instances in which the measurement of the dose of radiation, 
to which either an individual or equipment is exposed, is desirable. This 
is particularly important for a commander in a nuclear battle field to 
determine the radiological status of troops. Besides tactical use in a 
nuclear battle field, there are many commercial applications in which 
environments of ionizing radiation intensity occur in which an accumulated 
dose or dose rate need to be detected and accurately indicated. One type 
of device used to measure radiation dose is a device made from materials 
with different atomic numbers resulting in a difference in the probability 
of production of Compton electrons during exposure to gamma rays. This 
Compton electron emission is used to measure radiation doses. One such 
device is disclosed in U.S. Pat. No. 4,019,058 entitled "Charge Transport 
Tactical Dose Meter", issuing to Kronenberg et al on Apr. 19, 1977, which 
is herein incorporated by reference. Therein disclosed is a quartz fiber 
electroscope shunted by a variable capacitor and coupled to a radiation 
detection capacitor. The radiation detection capacitor is formed of a 
material of relatively large atomic number and a material of lesser atomic 
number. An electric charge is induced by neutron or gamma radiation. A 
variable capacitor is then used to determine the voltage as a result of 
the induced electric charge. The voltage is proportional to the dose. 
This type of radiation sensor is called a proton/electron transfer sensor 
or, hereinafter, PETS. The operation of a PETS dosimeter for detecting 
gamma rays can be appreciated as follows. A capacitor whose one electrode 
is made of a higher atomic number conductor than its other electrode and 
separated by a thin insulator, when exposed to gamma rays, will cause 
energetic electrons to be generated. The production of photoelectrons per 
unit mass of the material is proportional to the atomic number to the 
fifth power. The production of Compton electrons is proportional to the 
atomic number. Therefore, more electrons will be produced in the higher 
atomic number electrode than in the lower atomic number electrode. Many 
electrons will escape from the electrode where they are generated and will 
be absorbed in other parts of the capacitor. Their range (when expressed 
in weight per area) does not depend significantly on atomic number and 
therefore, the higher atomic number electrode will lose more electrons 
than gaining electrons. This will result in the higher atomic number 
electrode acquiring a positive charge and the lower atomic number 
electrode a negative charge. This charge is proportional to the gamma ray 
dose. 
PETS may also be used to measure fast neutrons. A capacitor having one 
electrode made of a conductive low atomic number hydrogenous material 
while its other electrode, separated by a hydrogenous insulator, is made 
of a conductive low atomic number non-hydrogenous material will detect 
fast neutrons. The atomic numbers of all the involved materials being 
similar and low. This device will show a very low response to gamma rays. 
When exposed to fast neutrons, recoil protons will be generated in the 
hydrogenous electrode and in the insulator. Some of these recoil protons 
will escape from the hydrogenous electrode charging it negatively and will 
be absorbed in the non-hydrogenous electrode charging it positively. The 
amount of accumulated charge will be proportional to the fast neutron 
dose. 
The sensitivity of both types of PETS, for detecting gamma or neutron 
radiation, will depend on the area of the electrode, the capacitance of 
the device, and the distance between electrodes. It should be noted that 
this system is free of any dose rate saturation effects and thus can be 
used for steady state as well as pulsed radiation application where short 
pulses delivered at very high rates are encountered. 
While the rate of generation of electric charge on the electrodes of a PET 
is strictly proportional to the radiation dose rate and the electric 
charge between the two electrodes, which increases linearly with dose, 
they are not without inaccuracies. There are inaccuracies and adverse 
effects in PETS as a result of polarization of the dielectric, ionization, 
and electric charge leakage. Therefore, there is a need for reliable, more 
accurate dose information to be derived from radiation sensors that 
operate as a proton/electron transfer sensor or PETS. 
SUMMARY OF THE INVENTION 
The present invention is directed to an improved proton/electron transfer 
sensor and circuit therefor for detecting doses of radiation. The present 
invention comprises a proton/electron transfer type radiation sensor that 
acquires a charge or voltage proportional to the radiation exposure, a 
voltage detector for detecting the acquired voltage, a discharge means for 
discharging the acquired voltage after a predetermined voltage is reached, 
counter means for counting the number of times the voltage is discharged 
from the radiation sensor, and a display means for displaying the dose of 
radiation the radiation sensor has been exposed to based upon the acquired 
charge or voltage. A timer may be included to obtain a dose rate in 
contrast to an overall exposure dose. Additionally, the display of dose 
may be adjusted or calibrated for a direct reading of tissue dose. 
Accordingly, it is an object of the present invention to provide a more 
accurate radiation sensor not influenced by polarization of the 
dielectric, ionization, or electric charge leakage. 
It is another object of the present invention to provide an inexpensive, 
dependable dosimeter. 
It is an advantage of the present invention that a direct readout is 
obtained indicative of dose. 
It is another advantage of the present invention that it has an unlimited 
dose range that will not saturate due to high dose rates. 
It is yet a further advantage of the present invention that the dosimeter 
can measure gamma rays and fast neutrons. 
It is a feature of the present invention that a discharge means is used for 
reducing the voltage after a predetermined value of the voltage developed 
as a result of radiation exposure to minimize adverse polarization 
effects. 
It is a feature of the present invention that a counter is used for 
counting the number of discharges which is directly proportional to dose. 
These and other objects, advantages, and features will become readily 
apparent in view of the following more detailed description.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
FIG. 1 is a block diagram illustrating the present invention. A radiation 
sensor 10 is coupled to a voltage detector 12. The voltage detector 12 is 
coupled to a discharge means 14. A counter 16 is coupled to the voltage 
detector. The counter 16 is coupled to calculation means 20. A clock 18 is 
coupled to calculation means 20. The calculation means 20 is coupled to a 
display 22. 
The radiation sensor 10 is a proton/electron transfer type sensor or PETS. 
The structure of the radiation sensor 10 has the form of two electrodes 
with an insulator therebetween, and thus is a capacitor. A convenient 
configuration of a radiation sensor 10 is that of two electrodes 
sandwiched by an insulator in a device that is rolled to form a cylinder. 
The nature of the electrodes and the insulator depends upon whether the 
radiation sensor is to detect gamma rays or fast neutrons. In either type 
of radiation sensor 10, an electric charge and corresponding voltage will 
develop as a result of the radiation sensor 10 being exposed to either 
gamma rays or fast neutrons. The charge or voltage is directly 
proportional to the radiation dose. The choice of thickness of the 
electrodes, the insulator or dielectric material therebetween, and the 
thickness of the overall device determines the sensitivity of radiation 
sensor 10. 
The operation of the present invention can readily be appreciated with 
reference to FIG. 1. When radiation sensor 10 is exposed to radiation, a 
charge or voltage is developed. This charge or voltage is directly 
proportional to the dose or exposure to radiation of the radiation sensor 
10. Voltage detector 12 detects an increase in voltage caused by the 
exposure of radiation sensor 10 to a dose of radiation. At a predetermined 
voltage, discharge means 14 is activated to discharge or drain off the 
voltage caused by the dose of radiation to which the radiation sensor 10 
has been exposed. Counter 16 counts or keeps track of the number of 
discharges caused by the discharge means 14. The number of discharges 
counted by the counter 16 is directly proportional to the dose of 
radiation experienced by the radiation sensor 10. Calculation means 20 
calculates a value in any desired unit for display by display 22. Clock 18 
is coupled to the calculation means 20 so that a display of a dose rate is 
obtained if desired. The dose rate can be calculated based on the time 
interval between the discharges as measured by the counter 16. 
Any number of conventional circuits can be easily fabricated to practice 
the present invention. By way of example only, the following are 
illustrative circuits embodying the present invention. 
FIG. 2 is a schematic representation of a basic circuit for practicing the 
present invention. A radiation sensor 10 having a capacitance C is 
attached to a resistor 24. The resistor 24 has a contact 28 at the other 
end. A switch 26 is used to complete the circuit with resistor 24 or 
contact 30. A coil 32 is placed adjacent contact 30. A controller 33 is 
connected to the switch 26 and the coil 32 as well as a capacitor 34 
having a capacitance C.sub.1. Resistors 36 and 38 are also in the circuit 
and the radiation sensor 10 is biased by bias voltage 40. In operation, 
the switch 26 is normally closed contacting contact 28. On exposure to 
radiation, the radiation sensor 10 will develop a charge charging 
capacitor 34 resulting in a voltage. When controller 33 detects a 
predetermined voltage associated with capacitor 34, the controller 33 
energizes coil 32 causing switch 26 to close on normally open contact 30. 
When switch 26 contacts contact 30, the capacitor 34 discharges. The 
controller 33 then keeps track of this discharge and advances a counter, 
not shown. After discharging, the switch 26 is placed into the normally 
closed position contacting contact 28. During continued exposure of 
radiation by radiation sensor 10, a charge and voltage continues to 
accumulate on capacitor 34. To prevent the first dose required for the 
first count from being larger than for subsequent counts, the radiation 
sensor 10 is biased by bias voltage 40. This assures that all counts will 
have an equal dose associated therewith. By selecting the predetermined 
voltage at which the capacitor 34 is discharged, the value for capacitor 
34, and the capacitance value of the radiation sensor 10, the dose per 
count or discharge can be selected at any desired value. A convenient 
predetermined value for the voltage to reach before discharge has been 
determined to be in the range of 20 millivolts. If the radiation is 
delivered in the form of a single high intensity pulse instead of being 
delivered in the mode of steady state, the voltage on the sensor rises 
responsively to the delivered dose, and may reach a substantial value. 
This voltage is then discharged by rapidly occurring steps of 20 mV and 
the number of these steps is recorded. Although this voltage may achieve a 
high value, it will not generate an adverse polarization effect in the 
dielectric because its duration is too short. 
FIG. 3 illustrates the increase in voltage over time as a result of 
exposure of the radiation sensor 10 illustrated in FIG. 2 to radiation. 
Line 42 represents the voltage value over time. During a constant exposure 
of radiation, the voltage increases to a predetermined threshold of 20 
millivolts. A discharge then takes place, followed by a continued increase 
of voltage until another discharge occurs. The peaks 44 illustrate 
successive discharges which, when counted, are directly proportional to 
the exposure dose of the radiation. 
FIG. 4 illustrates another circuit for practicing the present invention. 
The radiation sensor 10 is connected to a reed relay 46. The reed relay 46 
is connected through a resistor 47 to a series of transistors and 
capacitors in parallel. Transistor 48 is connected in parallel with 
capacitor 50 having a value of C.sub.1. Transistor 52 is in series with 
capacitor 54 having a value of C.sub.2. Transistor 52 and capacitor 54 are 
in parallel with transistor 48 and capacitor 50. Transistor 56 is in 
series with capacitor 58 having a value of C.sub.3. Transistor 56 and 
capacitor 58 are in parallel with transistor 48, capacitor 50 and the 
series connected transistor 52 and capacitor 54. A buffer amplifier 60 is 
used between one node of resistor 47 and an analog to digital converter 
62. The analog to digital converter is coupled to a micro-controller 64. 
The micro-controller 64 controls the reed relay 46 and the transistors 48, 
52, and 56. The micro-controller 64 is also coupled to a EPROM memory 
device 66 which in turn is coupled to a display. 
In operation, referring to FIG. 2, the micro-controller closes the reed 
switch 46. Upon radiation sensor 10 being exposed to radiation, a voltage 
will be generated and builds up through resistor 47 onto capacitor 50. 
When the voltage of capacitor 50 exceeds a predetermined value, the 
micro-controller 64 causes transistor 52 to close. The value of capacitor 
54 is nine times that of the value of capacitor 50. This causes the 
voltage to drop by a factor of ten. The voltage will continue climbing as 
sensor 10 is continued to be exposed to radiation. Should the voltage 
continue to climb and exceed a predetermined voltage, the micro-controller 
64 causes transistor 56 to close. Capacitor 58 has a value ninety times 
capacitor 50. Therefore, the voltage drops by a factor of ten. As the 
radiation sensor 10 continues to be exposed to radiation, the voltage 
continues to climb. When the voltage exceeds another predetermined 
voltage, the value is stored in memory 66 and the micro-controller 64 
causes transistor 48 to close, discharging capacitors 50, 54, and 58. The 
accumulated dose can then be displayed by display 68 or the dose 
measurement continued and an accumulating value maintained by memory 66. 
FIG. 5 illustrates another electronic circuit embodying the principals of 
the present invention. A radiation sensor 10 is connected to a switch 70. 
Switch 70 is normally closed. Switch 70 couples the radiation sensor 10 to 
an operational amplifier 72. The operational amplifier 72 in turn is 
coupled to a dual comparitor 74. The dual comparitor is coupled to a 
switch control 76 and a binary counter 78. The binary counter 78 can be 
reset by reset switch 80. In operation, when the voltage generated by the 
radiation sensor 10 obtains a predetermined value, the dual comparitor 74 
causes the activation of switch control 76 which will discharge a 
capacitor and advance the binary counter 78. The binary counter 78 keeps 
track of the number of discharges and thereby is directly proportional to 
the dose to which the radiation sensor 10 has been exposed. 
Accordingly, from the above, it should readily be appreciated that the 
present invention advances the accuracy and reliability of radiation dose 
indicators that can be used for measuring and monitoring moderate to high 
intensity steady state or pulsed radiation. This can be applied to 
tactical applications as well as commercial applications such as radiation 
processing plants, including sterilization of medical supplies and 
materials processing. 
Additionally, although the preferred embodiment has been illustrated and 
described, it will be obvious to those skilled in the art that various 
modifications may be made without departing from the spirit and scope of 
this invention.