Beta radiography

In Instance 10:94 Karen Potje askes whether there is any risk of
residual activity in a drawing after beta radiography is used to
make an image of its watermark, or whether the process could harm
the drawing.

The answer to both questions is no.  If beta-radiography is carried
out in a cassette, using x-rays to cause the emission of electrons
(beta rays) from a lead screen, the electrons are only emitted while
the x-rays are on.  If the radiography is done using a sheet of
beta-emitting plastic as the electron source then this sheet must be
handled with care, but it does not cause electrons to be emitted by
the drawing.

Personally, I recommend the lead cassette technique, if you have
access to a radiographic facility.  The drawing is placed inside a
radiographic cassette, between the upper lead screen and the film,
and the cassette is exposed to x-rays.  The exposure voltage and
time need to be determined by trial and error, but a high voltage
(~200KV) is desirable, with a copper filter between the tube and the
cassette.

Barry Knight
English Heritage
London

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                  Conservation DistList Instance 10:96
                    Distributed: Monday, May 5, 1997
                       Message Id: cdl-10-96-004
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