Direct reading dosimeter

• US 5,117,113 A
• Filed: 07/06/1990
• Issued: 05/26/1992
• Est. Priority Date: 07/06/1990
• Status: Expired due to Term

• Current Assignee: Best Theratronics Incorporated
• Original Assignee: Thomson & Nielsen Electronics Limited
• Inventors: MacKay, Gary F., Brown, Martin P., Thomson, Ian
• Primary Examiner(s): Hannaher, Constantine
• Application Number:
  US07/549,156
• Time in Patent Office: 690 Days
• Field of Search: 357/30 I, 357/30 G, 250/370.07, 250/370.14
• US Class Current: 250/370.07
• CPC Class Codes: G01T 1/026 Semiconductor dose-rate meters

Abstract

The invention relates to a radiation dosimeter having a pair of insulated gate field effect transistors integrated into the same silicon substrate, in which each of the transistors are operable in a bias mode and a test mode. A circuit element for biasing each of the transistors, during said test mode is provide, so that one of the transistors is more sensitive to ionizing radiation than the other of the transistors. A circuit element is provided for determining, during the test mode, the difference in the threshold voltages of the transistors, whereby the difference voltage is indicative of the radiation dose, and a circuit element is provided for continuously switching the transistors between the bias mode and the test mode, whereby the period of operation of the transistors in the test mode time period is small in comparison to the period of operation of the transistors in the bias mode.

Claims

1. A radiation dosimeter comprising:

a pair of insulated gate field effect transistors integrated into the same silicon substrate, each said transistor being operable in a bias mode and a test mode;

means for biasing, during said bias mode, each said transistor so that one of said transistors is more sensitive to ionizing radiation than the other of said transistors;

means for determining, during said test mode, the difference in the threshold voltages of said transistors, said difference voltage being indicative of the radiation dosage; and

means for continuously switching said transistors between said bias mode and said test model, the period of operation of said transistors in said test mode time period being small in comparison to the period of operation of said transistors in said bias mode.

2. A radiation dosimeter of claim 1 comprising digital voltmeter means for continuously measuring and displaying the differential threshold voltage between said transistors, said differential threshold voltage being representative of the radiation dosage.

3. The radiation dosimeter of claim 1 in which the means for determining the difference in threshold voltage of said transistors includes a single constant current source and constant current source switching means for switching the current source between said transistors for temperature stability during testing.

4. The radiation dosimeter of claim 3 in which means for determining the difference in threshold voltage of said transistors further includes sample and hold means.

5. The radiation dosimeter of claim 4 in which the means for continuously switching said transistors and constant current source switching means are comprised of CMOS switching devices.

6. The radiation dosimeter of claim 5 in which the insulated gate field effect transistor is an MOS P-channel device on an n-doped silicon substrate.

7. The radiation dosimeter of claim 6 in which the biasing means are sources of positive voltage of magnitude 6 volts and 3 volts respectively, applied to the gates of the transistors, making the transistor biased with the greater positive voltage more sensitive to radiation.

8. The radiation dosimeter of claim 7 in which the radiation sensor comprises a dual insulated gate field effect transistor component chip;

a layer of epoxy covering said chip;

first and second thin sheets of tin sandwiching said epoxy covered insulated gate field effect transistor component chip; and

means for electrically connecting said dual transistors to external circuitry.

9. The radiation dosimeter of claim 8 further comprising means for resetting the difference in threshold voltage to zero.

10. The radiation dosimeter of claim 9 further comprising means for continuously displaying the difference in threshold voltage, indicative of radiation dosage.

11. The radiation dosimeter of claim 10 further comprising alarm means for providing a warning when the radiation dosage has exceeded a predetermined level.

12. The radiation dosimeter of claim 11 in which the duration of the test period is approximately 3 ms and the test period duration is approximately 2% of the duration of the combined bias and test periods.

13. The radiation dosimeter of claim 1 in which the means for continuously switching said transistors and the constant current source switching means are CMOS switching devices.

14. The radiation dosimeter of claim 1 in which the means for determining the difference in threshold voltage of said transistors further includes sample and hold means.

15. A radiation dosimeter comprising:

a pair of insulated gate field effect transistors integrated into the same silicon substrate, each having a gate, a source and a drain;

means for differentially biasing said transistors so that one of said transistors is more sensitive to ionizing radiation than the other of said transistors during exposure of said transistors to radiation;

means for determining the differential threshold voltage between the transistors, said differential voltage being representative of radiation dosages, said determining means including a single current source means and current source switching means for switching the current source between said transistors for providing temperature stability during testing.

16. A radiation dosimeter comprising;

a pair of insulated gate field effect transistors integrated into the same silicon substrate, each having a gate, a source and a drain, said transistors being operable in a bias mode and a test mode;

means for biasing said transistors during said bias mode so that one of said transistors is more sensitive to ionizing radiation than the other of said transistors;

means for determining during said test mode the differential threshold voltage indicative of the radiation dosage, said determining means including;

a single current source for providing a source-drain current for both of said transistors; and

current source switching means for connecting said single current source to the one of said transistors being tested so that both transistors are tested using the same current source so as to minimize temperature instability in the testing mode;

means for switching said transistors between said bias mode and said test mode at a predetermined frequency and for maintaining said transistors in each of said modes for predetermined intervals of time wherein the test mode time interval is a small relative to the bias mode time interval.

17. A method of continuously determining a radiation exposure dosage comprising the steps of:

(a) biasing one transistor in a dual insulated gate field effect transistor radiation dosimeter with respect to the other of said transistors so that one of said transistors is more sensitive than the other of said transistors to ionizing radiation during a predetermined bias period during which said dosimeter is exposed to radiation;

(b) switching said transistors into a test mode for a predetermined time to sample and hold the threshold voltage of one of said transistors;

(c) switching back to said bias mode for said bias period;

(d) switching back to said test mode for said test period to sample and hold the threshold voltage of the other of said transistors;

(e) determining the difference in said threshold voltages of said transistors;

(f) displaying said difference in threshold voltages, said difference in threshold voltages being representative of said radiation dosage; and

(g) continuously repeating the steps a) to f).

18. A method of measuring the differential voltage in a dual insulated gate field effect transistor radiation dosimeter to minimize temperature dependence comprising the steps of:

(a) connecting a constant current source to one transistor to provide a constant source-drain current;

(b) measuring the threshold voltage of that transistor;

(c) connecting said same constant current source to the other transistor to provide a constant source-drain current;

(d) measuring the threshold voltage of the said other transistor; and

(e) continuously repeating the steps a) to d).

19. A method of providing a continuous direct reading of accumulated radiation comprising the steps of:

(a) operating a dual insulated gate field effect transistor radiation dosimeter in a bias mode for a first predetermined period of time during which the transistors are differentially biased so that one is more sensitive to ionizing radiation than the other;

(b) operating said dual insulated gate field effect transistor radiation dosimeter in a test mode for a second predetermined period of time which is a fraction of said first period of time, including the steps of;

i. turning one of said transistors ON and the other of said transistors OFF;

ii. connecting a single constant current source to said one transistor so as to provide a constant source-drain current; and

iii. sampling the threshold voltage of said one transistor;

(c) switching said transistors to said bias mode according to step (a) said first predetermined period of time;

(d) operating said dual insulated gate field effect transistor radiation dosimeter in a test mode for said second predetermined period of time, including the steps of;

i. turning the other one of said transistors ON and said one of said transistors OFF;

ii. connecting a single constant current source to the other of said transistors so as to provide a constant source-drain current; and

iii. sampling the threshold voltage of said other of said transistors;

(e) determining the difference between the sampled threshold voltage of said one transistor and the sampled threshold voltage of the other of said transistors; and

(f) providing a signal representative of said difference to a visual display to provide an indication of the accumulated radiation dose.

20. A radiation dosimeter comprising:

first and second insulated gate field effect transistors integrated into a single silicon substrate, each said transistor having a gate, a source and a drain;

a bias circuit for turning OFF both transistors by positive bias voltages applied to their respective gates, the bias voltage of one of said transistors being greater than the bias voltage of the other of said transistor;

a test circuit in which either of the transistors can be turned off the other turned on and connected to a constant current source to provide a constant source drain current and sample and hold circuitry to sample and hold the threshold voltage of the turned on transistor;

a switching circuit to connect the transistors into the bias circuit for a predetermined bias period during which the transistors are exposed to radiation and then to connect the transistors into the test circuit for a predetermined test period to sample to threshold voltage of one of the transistors;

a timing circuit to alternately switch between the bias and test circuitry for predetermined periods of time, the test circuitry time being significantly less than the bias circuitry time; and

digital voltmeter means to measure and continuously display the difference between the threshold voltages and indicative of accumulated radiation dose.

21. A method of continuously determining a radiation exposure dosage comprising the steps of:

(a) biasing one transistor in a dual insulated gate field effect transistor radiation dosimeter with respect to the other of said transistors so that one of said transistors is more sensitive than the other of said transistors to ionizing radiation during a predetermined bias period during which said dosimeter is exposed to radiation;

(b) switching said transistors into a test mode for a predetermined time to sample and hold the threshold voltages of the transistors;

(c) determining the difference in said threshold voltages of said transistors;

(d) displaying said difference in threshold voltages, said difference in threshold voltages being representative of said radiation dosage; and

(e) continuously repeating the steps a) to d).

22. A radiation sensor comprising:

a dual insulated gate field effect transistor component chip;

a layer of epoxy covering said chip;

means for electrically connecting said dual transistors to external circuitry; and

first and second thin sheets of tin sandwiching said epoxy covered insulated gate field effect transistor component chip;

23. The radiation sensor of claim 22 in which the thin sheets of tin are each 0.5 mm in thickness.

24. The radiation sensor comprising:

a dual insulated gate field effect transistor component chip;

a layer of epoxy covering said chip;

means for electrically connecting said dual transistors to external circuitry;

said transistor being bonded to a ceramic package and said package being covered by a high Z material lid.