[Sneap] High radiation at low terminal voltage

[David Weisser]

Hi Pertti,

The only way to get x-rays is by electrons being accelerated inside the
accelerator tube.  When the electrons stop on another tube electrode or on
an aperture in the terminal, x-rays are produced.  Nothing done in the
insulating gas space could be the cause of the x-rays. If the accelerator
tube was vented and then evacuated, turbulence in the tube could have moved
some particle from a low electric field region to a high field region which
would then result in its emitting electrons.

Now getting rid of the electron source is much harder than saying it exits.
Often, voltage conditioning will do the trick with a few tank sparks to
shake things up.  It sounds like the source is so prolific that you can't
get to high enough terminal volts to spark the machine.  You might be able
to locate the source or sources, by shorting out say half the machine and
seeing if the x-rays disappear.  You might be able to then put volts on the
bad section and condition it away.

You might also be able to affect the emitter by putting a bit of hydrogen
gas into the tube from the ion source and running up the volts.  You might
need to get the vacuum in the 10-4 torr range or higher to ignite a
discharge to process the emitting site.  Alternatively, a bit of gas in the
tube (10-5 torr) could well quench the electron source and enable operation
which would be impossible at good vacuum.

If you really get desperate, you could vent the tube and then evacuate it
hoping to get rid of the emitter without creating a bunch of new ones. Very
risky though.

A possible tank opening related cause of the x-rays could be if some of the
magnets on the tube electrodes were knocked off during the tank opening.

Best of luck,

David


********************************************
D.C. Weisser
Nuclear Physics Department
Research School of Physical Sciences and Engineering
Australian National University
Canberra A.C.T. 0200
AUSTRALIA

Tel:    + 61 (0)2 6125 2080
Fax:    + 61 (0)2 6125 0748
Mobile: + 61 (0)414 249 209

david.weisser at anu.edu.au

-----Original Message-----
From: sneap-bounces at tunl.duke.edu [mailto:sneap-bounces at tunl.duke.edu]On
Behalf Of Dr. Pertti O. Tikkanen
Sent: Thursday, 17 January 2008 6:51 AM
To: Symposium of Northeastern Accelerator Personnel
Subject: [Sneap] High radiation at low terminal voltage


Hello fellow sneapers,

In our vertical 5-MV-, N2/CO2-insulated tandem we are having severe
problems in reaching any HV higher than about 2 MV. Good ideas for
locating the problem are welcome!
Here is a description of how the symptoms developed:

-After a tank opening in September 2007 we conditioned the machine up to
4,3 MV and did run some experiments where the tandem performed perfectly.
There wasn't any need to go to higher TV's.
The just-installed green Siegling belt ran also well. This time we had
left out the inner equipotential bars. A bad mistake, perhaps?
-The problems started to appear after a few days AMS run at 3.2 MV. In
that run, the charging current was relatively high, as we used about
50-microamps of corona probe current. It wasn't exceptionally high,
however.
-Subsequently, when starting the tandem for another experiment, much
higher radiation levels, tens of micro-Sv/hour (compared to what we
usually have, only a few uSv/h) were observed already at 3.5 MV. It was
impossible to go any higher than about 3.8 MV because of high radiation
levels and very unstable TV.
-As the insulating gas was a bit more humid than usual, we dried it once
more to reach a dew point of about -69 Celsius. However, this made the
problem even worse; the radiation started to inrease heavily at voltages
lower than 3 MV's.
-There were almost no tank sparks. At least none were related to increased
radiation level.
-Before opening the tank, we monitored the radiation to find the location
of its source. All that we found was that it was concentrated near the
terminal.
-The tank was opened, but we didn't find any apparent reason for the extra
radiation. The only difference compared to normal was that there were a
lot of sparking tracks at the edges of the terminal shield. That's the
place where the electric field is the strongest, I suppose.
-We thought that there are some extra particles in the insulating gas and
installed new filter elements into our pumping lines. Measured also the
particle concentration in the gas. It was very small.
-The resistors were checked carefully, a few were found defective (in
random places), all surfaces were cleaned carefully, the belt was found in
good condition. Looked for extra items like loose wires etc. in the
column, found none. So everything looked just OK and we closed the tank,
only to find out that
-The high radiation level was still present!
-We got an idea that there was some chemical contaminant in the gas
because we had recently installed new absorbant in our gas dryer.
-Replaced the insulating gas with a new mixture of N2/CO2. Furthermore,
the gas dryer was regenerated carefully at higher temperature.
-After all this we are in a situation that a lot of radiation is generated
at about 2.2 MV or even at lower TV's.

What did we miss?
We are anyway going to open the tank again and try to locate the source of
the problem.

Best wishes and best regards,

Pertti

% Pertti O. Tikkanen, PhD         email: pertti.tikkanen(at)helsinki.fi
% Senior Laboratory Manager

% Accelerator Laboratory                        TEL:   +358 9 191 50006
% P.O. Box 43 (Pietari Kalmin katu 2)           FAX:   +358 9 191 50042
% FIN-00014 UNIVERSITY OF HELSINKI, FINLAND     GSM:   +358 40 72 65386

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