Thimble Ion Chambers

Frequently Asked Questions

I have a non-floating electrometer. Can I use it with an Exradin chamber?

While the vast majority of electrometers are of the floating design, there are some older electrometers with a non-floating design. While effective use of Exradin chambers can be achieved in plastic phantoms or in a water tank with the assistance of a thin, sealed sleeve around the chamber body, usage of a non-floating electrometer is discouraged due to a resulting shock hazard.

What is the typical Exradin Ion Chamber leakage?

All Exradin chambers should exhibit leakages on the order of 10 x 10^-15 amps or less.

How often should I send my ion chamber in for calibration?

• The IEC (International Electrotechnical Commission) recommends ion chamber calibration every two years to assure optimal performance.
  • IEC 60731 – Medical electrical equipment – Dosimeters with ion chambers such as used in radiotherapy.
  • IEC 61674 – Medical electrical equipment – Dosimeters with ion chambers and/or semi-conductor detectors as used in X-ray diagnostic imaging.
• Standard Imaging works with the University of Wisconsin ADCL to perform calibrations, but also provides free inspection and testing services, as well as expedited repair service, if necessary. Click here to initiate a calibration and service appointment.
  • Standard Imaging’s goal is a two week turn around upon receipt of equipment, although actual time may vary depending on the UW ADCL’s schedule.

I am seeing drift or leakage with my electrometer + ion chamber system. Is there something wrong with my chamber, electrometer, or both?

• You may need to zero your electrometer. Refer to your electrometer’s user manual for more details.
  Check all cables for crimping or damage, and allow settling time after setup or movement.
• Crimping, damage, and significant movement such as unwinding from a spool can cause or contribute to system leakage.
• It is recommended to use as short of an extension cable as possible. In general, the longer the cable, the longer the signal settling time and the longer for bias voltage to dissipate.
• A common cause is dust that has collected on the inside of the connector on the chamber cable, extension cable, and/or electrometer. Standard Imaging has created a tech note containing a triax connector cleaning procedure. This procedure should be periodically performed or whenever there are concerns of drift or leakage. Heavier product use may require more frequent cleanings. In addition, utilize dust caps on chamber and electrometer triax connectors whenever possible.

Determine which part of the electrometer + extension cable + chamber system is exhibiting the leakage or causing drift, in this order of troubleshooting:

1. Electrometer: With nothing attached and the dust cap in place, warm up the electrometer and check for stability. Perform a 60 second charge collection and determine if the signal shown is within the manufacturer’s stated acceptable limits.
2. Extension cable: If it has been determined that the electrometer is not exhibiting leakage or drift, connect the desired extension cable (with no bias applied at time of connection, no chamber attached, but dust cap on), and determine if the signal level settles in the cable manufacturer’s stated settling time estimate.
3. Exradin Ion Chamber: If it has been determined the electrometer is not exhibiting leakage or drift, connect the desired chamber directly to electrometer (with no extension cables attached), and determine if the signal level settles within 5-10 minutes and the leakage of the chamber is less than 10 x 10-15 amps.

• If these techniques do not resolve the drift or leakage problems, contact Standard Imaging representative or your electrometer’s manufacturer for assistance. Click here for contact information.

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