Product Review - UDI-1G and GM-tube

Review of UDI-1G and GM-tube fold-back testing

Testing of the UDI-1G and GM-tubes was carried out with the collaboration of Sellafield Sites Ltd. to investigate the reduction in count rate of a system exposed to very high levels of gamma radiation, above 10Sv/h.

The system in question consisted of a CMS Interlock SIL monitor, a UDI-1G and a GM-1/314 detector. Radiation and synthetic pulse input tests were carried out to discover the source of the issue and identify remedial steps.

Radiation tests were carried out at the calibration gamma facilities at Nuvia in Winfrinth, at NPL and at the HPIC facility at Sellafield. Bench testing was also carried out at Lab Impex and at Sellafield. Tests confirmed that the CMS Interlock SIL was not the source of the problem, and that a modification to the UDI-1G for certain applications would be desirable.

The issue is now more fully understood and has led to some recommendations for this equipment provided by Lab Impex Systems. A technical note will be issued to advise current customers on the actions to be taken, but it should be noted that no correctly set system in the field has failed to danger.

Two problems were identified, one concerning electronic dead time and aliasing at a particular harmonic count rate, and the second concerning cable length or, more accurately, cable capacitance. These are discussed in more detail below.

Tests have shown that the GM-1/314 detector is the most susceptible to the fold-back and that GM-1/324 and GM-1/304 detectors are significantly less likely to exhibit the severity of reduced signal seen in the GM-1/314.

Lab Impex recommend that cable length should be limited to 10 metres between UDI and detector. The cable length from UDI to CMS is less important as this signal is conditioned in a balanced RS422 format.

We would like to thank Debra Rook, Eliot Williams, Alan Clare and Ashley Finlayson of Sellafield Ltd. as well as Peter Burgess of Nuvia and Mike Bates at Centronic for their help with testing, discussing and provision of data.

Further details of the two issues discovered during the testing, relating to cable length and the monostable output of the UDI at high dose rates, are provided below. More detailed documentation will be available upon request.

Shaping Time and monostable output:

The UDI-1G has a monostable output pulse width tuned by a capacitor value to match the detector dead time. With count rates above the detector specification, the detector dead time effectively reduces and the monostable output time dominates. This can result in situations where monostable output pulses occur too close together to each other to be discriminated as individual events by the counting electronics. A simple modification to the UDI for situations where over-range count conditions are possible solves this issue, by ensuring that the monostable output pulse always occurs inside the shaping time of the UDI amplifier.

Cable Length:

It is commonly accepted that a GM-1/314 tube pulse height reduces to around one seventh of the normal operating pulse height when overloaded. This reduced pulse height appears as an oscillating signal. Radiation interacts with the detector as soon as it becomes active from the previous pulse, causing a repetitive, reduced pulse height chain. To ensure that these pulses are detected, a discriminator set at one tenth (or slightly lower) of the full pulse height is usually used.

However, testing of the GM tubes with different length cable highlighted another potential problem. The signal amplitude at overload can be sufficiently attenuated by long cables so that it falls below the threshold level. This is caused by cable capacitance and only becomes a problem for systems with UDI to detector cables longer than 10 metres, where the detector is in over-load.

This issue has been resolved by the limiting of GM-1/314 detector systems to a 10 metre cable length; when the application under consideration can lead to a dose rate at the installed detector of greater than 3 Sv/hr in the case of the GM-1/314.　 Any intended application, beyond our issued specifications, must be discussed with us and appropriate testing/substantiation undertaken before use.

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