[Sneap] X-rays from ion source

[David Weisser]

SNEAP,

 

We have finally fixed a long standing problem with X-rays from our sputter
ion sources. Both sources, an NEC multi-sample SNICS and a SNICS II modified
for a gas cathode, inject 150 kV pre-acceleration tubes and had caused more
than 50 micro Sieverts of X-rays detected by monitors about 150 cm from the
ground end of the tubes and about 30 cm from the axes.  In spite magnetic
field traps at the negative ion extraction point, radiation persisted that
appeared to be caused by electrons launched beyond the trap in the volume of
the tubes.  These electrons were stopped on various bits of stainless steel
slit supports, stainless steel flanges and, worst of all, on the tantalum
shroud of the beam profile monitor.  Since the X-ray intensity increases
with Z2, it was important to mask the high Z material with lower Z material.
Beryllium would be best and carbon next best but the former involved too
many safety issues and the latter is too expensive as glassy carbon.

 

We installed an aluminium aperture and baffle in front of the beam profile
monitor to ensure that all electrons within a 150 mm diameter are stopped on
aluminium but leaving a 20 mm diameter hole on axis. The radiation has been
reduced to less than 5 micro-Sieverts.  This now allows us to work in the
ion source room while one of the sources is in operation providing more
development opportunities.

 

Most labs don't accelerate the beam to as high an energy before magnetic
analysis and so electrons and the X-rays they produce on stopping, are not a
significant problem. There are a few machines though where this is problem.
We hope that this cheap solution may be of some use to them.  We'll give
more details at SNEAP in Salt Lake City.

 

David, Howard, Tom and Alan.

 

 

David Weisser                                     

Nuclear Physics Department

Research School of Physical Sciences and Engineering

Australian National University

Canberra ACT AUSTRALIA 

 

Office +61 (0)2 6125 2080

Home   +61 (0)2 6231 9211

Mobile +61 (0)414 249 209

 

david.weisser at anu.edu.au

 

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