Answer to Question #10174 Submitted to "Ask the Experts"
Category: Accelerators — Instrumentation
The following question was answered by an expert in the appropriate field:
I have taken an output measurement from an accelerator. Every time it shows lots of variation in the electrometer reading. I am using a cylindrical chamber for the measurement which I immediately connected to the electrometer for the leakage measurement. It shows some leakage in the chamber. This chamber was calibrated 10 months ago. Why is there so much variation in the readings?
There are many possibilities that could lead to the electrometer readings that you have observed. Without detailed information or careful follow-up measurements and studies I cannot specify which one, or group, of the following effects are responsible for the unstable readings. I do not know what kind of accelerator, particle type, energy, and intensity is produced; what material is used for the shielding; and what thicknesses or conditions under which the measurements were done. I assume the accelerator is located inside a shielded enclosure, and you are making the measurements outside the shielding while the accelerator is operating. Here are some of the possible causes:
- Variation of the accelerator beam intensity, energy, and targeting.
- Production of short- and long-lived radionuclides in the shielding, from which (if the beam structure allows) you will see partial decays and buildups.
- Even if the detector does not have any saturation or serious dead-time issues, if you are using the detector and electrometer as a rate meter, with a very small time constant, you will see the rate jumping around and that is the nature of the radiation field. Your detector/electrometer should be set to give you a (moving) average after integrating over a reasonable length of time.
If none of these are the problem, then there are further possibilities given below:
- Make sure the levels you are measuring are well above the background. Verify that the background has not changed/increased recently.
- Make sure no one present near where you were making the measurements had recently undergone a medical procedure involving radionuclides.
However, if you suspect you may have detector problems—which involve either or both the ion chamber and the electronics—then here are possible causes of variations in your ion chamber reading:
- Most importantly, check to make sure the detector is not damaged—connecting cables or the electronics do not need repair. It does not matter when the detector was calibrated; it may have been damaged since then. You can check this by testing the detector using a calibrated source to see if you get an acceptable response.
- Radiofrequency (RF) pick up: if the chamber or electronics are not well shielded, you might pick up RF from all sources, accelerator power supplies, nearby motors, welders, or even old walkie-talkies.
- Stability of the power supply for the chamber bias and even the readout electronics is crucial in getting a stable readout.
- If you have a very sensitive electrometer, then connections, moving, or external stresses may affect it.
- Stress on the insulators on the ion chamber caused by tight screws or tightening the cable connectors can induce piezoelectric leakage currents—no radiation is needed.
- Stress on the cables connecting the detector to the electrometer can cause variation in the signals or change the leakage current.
- Variations in humidity levels could affect sensitive electronics such as the electrometer.
- Temperature variation (near an air-conditioning vent) on the chamber or the electrometer can cause variations in the read out.
Kamran Vaziri, PhD