ThePhysicsofRadiology-chapter5-2 放射物理 英文版 教学课件.pptVIP

Measurement of Radiation: InstrumentationTechniques Part Two Summary * * BME College Sherman Sheen(辛学刚) The Physics of Radiology Operational amplifiers Example. Suppose the amplifier of Figure has an open loop gain of 105 and an input voltage of 1.0 volts is applied to the circuit. Let R = 1.0 MΩ, and Rf = 10.0 MΩ. Calculate the output voltage and the error voltage. Output voltage Error voltage Practical Devices for Measuring Radiation Position the center of the ion chamber with its buildup cap at the point where one wishes to determine the exposure rate. 2. Set the electrometer to measure current. 3. Ground the sensitive lead of the electrometer with switch S and use the zero adjustment to null the reading. Open S and observe the leakage current, IL. 4. Open S and turn on the unit. A steady current reading should appear on the electrometer. Observe it for a few seconds and record its average value I. If I is large compared with IL the calibration is complete. If it is not, an attempt should be made to reduce the leakage current by carefully cleaning insulation at the end of the cable. 5. The exposure rate is Nx(I－IL). 6. This should now be corrected for temperature and pressure A calibration can equally well be performed using the electrometer as a charge measuring device and the same answer should be obtained. One would proceed as follows: 1. Set the electrometer to measure charge. 2. Ground S and null the device, open S. 3. Turn on the unit for 10 s and measure the charge Q1. 4. Ground the electrometer and readjust the zero, open S. 5. Turn on the Unit for 70 s and measure a charge Q2. 6. The Charge produced in the ion in 1 min is Q2-Q1. 7. Measure the leakage QL in 1 min with the machine off. It should be negligible compared to Q2-Q1. If it is not, clean the insulator. Guarded Construction 1、the central electrode, as before, is imbedded 　in a good insulator ２、insulator is cover