The use of MOSFETs for detecting radiation is well known. It is understood that the threshold voltage of a MOSFET, or as it is sometimes called an insulated gate field effect transistor (IGFET) varies with exposure to radiation and therefore provides a useful building block in the design of dosimeters. The theory behind the use of these MOSFET dosimeters has been described in a number of papers some of which have been authored by the inventor. A paper entitled "Radiation Dosimetry with MOS.Sensors" by Ian Thomson, R. E. Thomas and L. Berndt, published in "Radiation Protection Dosimetry", Vol. 6, No. 1, pp. 121-124, December, 1983, and a paper entitled "Semiconductor MOSFET Dosimetry", published in the proceedings of the Health Physics Society 1988 Annual Meeting presents the theory behind MOSFET dosimeters, as well as experimental results with response to radiation of different kinds. These papers provide useful information on the background of this application.
Various configurations of MOSFETs have been implemented in the prior art in order to measure the amount of radiation dose received, while at the same time overcoming the numerous problems which limit the accuracy and stability of these devices. A recent implementation of a direct reading dosimeter using IGFETs is described in the inventor's U.S. Pat. No. 5,117,113 dated May 26, 1992. This patent discloses a radiation dosimeter having a pair of IGFETs 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 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.
There are a number of applications that require the use of a dosimeter to measure radiation. The sensitivity of a MOSFET to radiation is dependent upon the thickness of the gate oxide and the gate bias. By placing a small voltage, example 3 to 10 volts, on the gate of a MOSFET the sensitivity of the sensor can be enhanced.
MOSFETs of the type disclosed in the U.S. Pat. No. 5,117,113 would have to be mounted on a suitable package or substrate. Generally this takes the form of a standard IC package such as an 8 pin dual in line package (DIP). This type of packaging is adequate for the applications such as portable dosimeters or even badge type dosimeters.
A standard 8 pin DIP packaging is too bulky in uses such as in vivo radiation measurement. It is generally desirable for the dosimeter to be attached to a patient or inserted into a patient. In the latter case, this may require the use of a catheter where sterile conditions apply. U.S. Pat. No. 4,976,266 to Huffman et al. discloses a method and apparatus for in vivo radiation measurements which uses a MOSFET dosimeter. A disadvantage of the Huffman device is that it requires the separation of the MOSFETs by having a single MOSFET in a probe and another matched MOSFET outside the probe in order to provide temperature compensation. A compensation circuit is connected with this matched unirradiated MOSFET to operate at a current designed to eliminate temperature dependence of the device. However the human body has a much higher temperature than ambient temperature and it is likely that Huffman does not achieve the temperature compensation which is required of this type of application since any flexing of the catheter, particularly near the MOSFET, is likely to break the lead wire to the MOSFET. A further disadvantage with the Huffman device is the mounting of the MOSFET within the catheter with an epoxy which itself is limited in application. Further, the MOSFET is rigidly mounted which further limits the possible uses of the dosimeter. Since the threshold voltage of the MOSFET increases with cumulative dose and cannot be re-set to its original value, the dosimeter is generally discarded after a number of exposures. In the case of current MOSFET dosimetry systems when a saturation level of 20,000 cGy is reached the dosimeter is discarded. This means that for radiotherapy exposures of 200 cGy, the dosimeter may be used up to 100 times before it is discarded. In the case of higher dose exposures, such as in bracytherapy, the dosimeter may be used only 20 times or less.
A major disadvantage of Huffman and current devices is that they are not intended for large scale manufacture and thus the cost of these dosimeters does not justify use on a routine basis.
Furthermore there is a requirement for radiation workers to wear dosimeters at their extremities where these extremities are likely to receive higher doses than their whole body badges. Examples include technicians who work in the manufacture of radioisotopes, and technicians in nuclear medicine departments of hospitals who administer radioisotopes, physicians and nurses who work in the X-ray beam with fluoroscopy procedures and nuclear plant workers.
The annual dose allowed to extremities is 50 rems, as opposed to 2 rems for whole body since the extremities are less susceptible to negative effects of radiation than the organs in the body. The current requirements for lowest detectable levels for extremity dosimeters is 250 mrem (0.25 rem,) and the maximum is 10 rem as determined by the "U.S. Department of Energy-Standard for the Performance Testing of Extremity Dosimetry" draft May 4, 1991 (Rev 4). (Radiotherapy dose units used earlier were in cGy and we assume for simplicity that 1 rem=1 cGy.)
The most commonly used extremity dosimeter is a TLD, which is fixed to the wrist or fingers of the worker with adhesive tape or special finger ring type holders. Some disadvantages of this approach are:
(i) TLD extremity dosimeters suffer from the same drawbacks as other TLD dosimeters in that they can only be read with a special laboratory instrument. The crystals must be removed from their holders and manually handled. This is a relatively expensive slow process and, once read, the dose information is erased. In addition, identification can be lost once the crystal is removed from its holder which may contain an identification tag. In some applications, daily dose readings are carried out, thus requiring the facility to have twice as many dosimeters as workers since reading is carried out in a laboratory. PA1 (ii) Extremity TLD dosimeters are generally too large to wear at the fingertips, which is the area of highest dose in most handling operations. A finger ring or wrist type is most commonly used in this application. PA1 (iii) One of the main types of radiation of interest for extremity dosimetry is beta particles. These particles travel a short distance in material. Placement on the wrist or on a finger ring will not give an accurate measure of the dose to finger tips or thumbs. The dose at the wrist, for example, may be orders of magnitude different from that at the thumb. PA1 (iv) Normal TLD crystals are too thick to adequately measure all energies of beta particles. The performance of TLD dosimeters is, therefore, not uniform with different types of radiation such as beta particles and X-rays. There also exists the need for a dosimeter which can be attached to a person's extremities (e.g. fingers, hands, head,) to measure personal radiation dose. PA1 a pair of insulated gate field effect transistors integrated into the same semiconductor substrate each having a gate, source and drain; and PA1 an elongate flexible member for supporting the transistors at a first end thereof, and for electrically connecting the gate, source and drains of each of the transistors to a second end remote to the first end, the second end being adapted for connection to external circuitry. PA1 flexible member having a plurality of electrical connection tracks extending between first and second ends thereof; PA1 a semiconductor radiation sensor supported on the first end of the flexible member, the sensor having a pair of insulated gate field effect transistors integrated into the same substrate each having a gate, source and drain, and the electrical connection tracks for connecting to a respective one of the source, drain and gate; and PA1 means connectable to the second end of the flexible member for differentially biasing the transistors so that one of the transistors is more sensitive to ionising radiation than the other of the transistors during exposure of the transistors to radiation. PA1 (i) inserting into the body a flexible radiation probe having a flexible member including a plurality of electrical connection tracks extending between first and second ends thereof and a radiation sensor supported at a first end of the flexible member, the sensor having a pair of insulated gate field effect transistors integrated into the same substrate each having a gate, source and drain, the electrical connection tracks connecting the source, drain and gate to the second end; PA1 (ii) periodically biasing the transistors differentially and reading out the differential threshold voltage between the transistor through the flexible member.
There therefore still exists a need for a miniature dosimeter that is capable of being used, for example, in vivo radiation measurement or which can be bent into any configuration to conform the sensor to its measuring environment. The sensor must also be capable of compensating for a wide variation in temperature and must also be sufficiently cost effective to justify its large scale use.
There are also times when it may be inconvenient to have many wires leading from the flexible circuit to direct reading circuitry. For instance, in radiotherapy, it is desirable to keep material on the patient to a minimum. At present thermoluminescence TLDs are used to measure the dose received by a patient. However, they often take hours to read and consequently pose further discomfort to an already uncomfortable situation.
There also exists a need in, for example, radiosurgery for a dosimeter which has high spatial resolution.