GM counter and EMF fields

["Jeffrey A. Leavey (532-4595 / 5-3466)" <certhp@VNET.IBM.COM>]

Jim Barnes replies:
>>
>>#2.  Can pancake geiger detectors be set off by high electromagnetic fields?
>>
>My experience is that yes it can, but not consistently.  Just about any GM probe
>will "pulse" in sync with an RF field (found this out when we were running a
>linac on an FEL).  I have also noted some type of odd "spike" when a probe is
>suddenly shifted in position when frisking metallic objects (I've never heard an
>explanation for that, but suspect some kind of electromagnetic thing).  Magnetic
>fields may also affect the probe's behavior.

>I've seen friskers misbehave when someone was welding nearby; suggests EMF
>interference.

I too have seen the same effect near accelerator kylstrons, but
it's the electronics that's affected, not the GM probe
itself. To see a count, the probe has to see an event to cause
the electron multiplication. RF is non-ionizing, however, like
a taut rubber band, a strong enough EM field might cause a breakdown.
Given that a GM tube is usually all metal, the gas is probably well
shielded from RF. The electronics, however, is very sensitive
to RF interference, pulsed or otherwise based on my experience.

One way to test all this is run thru combinations of RF shielding
the meter and probe, or put either out of the RF area and see
what you get.

Magnetic fields WILL affect GMs and ion chambers -
electrons bend in a mag field and will be turned away from the
anode. Mag fields also penetrate materials better than electric
fields. I have trouble measuring activation of my synchrotron
when the superconducting magnets are on because of this.
There was a paper in HP Journal (I think) within the past two
years or so about GMs and mag fields. Maybe someone can post the
issue number.

Hope this helps.
Jeff
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=
Jeffrey Leavey                          certhp@vnet.ibm.com
X-ray Lithography Program                         IBM Corp.
914-892-4595                       East Fishkill, NY  12533
=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=