Re: Ionization at 80eV

[Eric C Wagner <wagnerer@engin.umich.edu>]

I've worked with one of these before so perhaps I can clarify the
situation. A fixed mass spec unit is placed between the vacumn chamber
that is being tested and the vacumn pump. Someone then goes around with
small source of Helium gas and sprays it over sections of the vacumn
chamber. When the mass spec starts to detect the He it triggers an alarm.
Eventually you can find where the leak in the vacumn system is by where
the He is directed when the alarm is triggered.

The 80eV would be the potential the He ions are accelerated over in the
mass spec unit. So we don't have electrons here but essentially very low
energy alpha's.

Eric Wagner

Date: Fri, 16 May 1997 12:36:10 -0700
From: "Pickett, Bruce D" <Bruce.Pickett@PSS.Boeing.com>
Subject: Ionization at 80 eV?

>Someone called our office with questions about the safety of a device they
>are using. Apparently, this is some type of a mass spec device that is used
>as a helium leak detector, and which has an electrical potential across a
>cathode and anode. We were told that the "amount of radiation produced is 80
>electron volts of ionized radiation"; we don't know if this information comes
>from a label on the device, or the operator's manual, or somewhere else.
>
>In talking this over in our office here, we're not certain that ionization
>would even occur at such a low potential as 80 eV (we're normally working
>with thousands to hundreds of thousands of times this potential). However, we
>do agree that even if ionization does occur, any resulting x-rays would be so
>soft that they would be stopped by a small amount of air, so our conclusion,
>based on the little we know of this device, is that there would be no 
>ionizing radiation hazard associated with its use.
>
>Now, in order to satiate our curiosity, can anyone on RADSAFE tell us what
>the lowest electrical potential would be that would result in ionization and
>the formation of x-rays. I presume that we would need to known the ionization 
>potential of the materials in the device's anode, but knowing almost nothing
>of the device itself, we could only guess at what this would be made of.
>
>Bruce Pickett
>Radiation Health Protection
>The Boeing Company


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