A wiggler such as the IR Demo wiggler, which uses NdFeB as a
permanent magnet material, can show some degradation in field
quality with as little as107 Rad and a dose
of 108 Rad will probably lead to unacceptable
wiggler performance. If we use NdFeB we need to be very careful
with beam loss around the wiggler. G. Neil has suggested that
an integrating radiation detector be placed near the wiggler so
that the dose could be monitored. If the dose gets near allowable
limits, the beam loss must be reduced somehow.
Radiation damage calculation:
How much beam loss can produce a dose of 107
Rad? A quick calculation indicates that the degradation in the
wiggler from a single high current beam loss would be negligible
(the dose from one 100 µsec., 5 mA loss would be one kRad).
A more serious hazard is a low current loss over a very long
time. We might tolerate as much as 500 nA of beam loss for longer
than one minute in one place ( this is 20W of beam power loss
and is equivalent to the tune-up beam current). At 42 MeV the
radiation in the forward direction is 106 Rad/mA/min
at 1 meter from the source . If we say that the beam is lost
10 cm upstream of the wiggler (where the chamber necks down) this
much loss would produce a dose rate in the wiggler of 3 MRad/hr
and might lead to damage of the wiggler in three hours. Note
that the dose rate at 90° to the beam loss would be approximately
90 Rad/hr at one meter from the beam loss (This figure must be
confirmed by Geoffrey Stapleton. He is working on it with P.
Kloeppel). If we place two tenth-value layers of high-Z shielding
(this is four inches of lead) between the neck-down point and
the wiggler we could increase the time before damage up to 300
hours of operation. If the wiggler is damaged at this point,
it could be repaired with about two to three months of downtime.
Proposed MPS Design:
The upstream shielding must be complemented by at least one integrating
ionization chamber near the wiggler entrance. Two chamber are
preferred for redundancy, placed on either side of the wiggler
entrance. If the trip point of a chamber 30 cm from the neck-down
point were set to allow 100 Rad/hr. averaged over all operations,
the wiggler should survive for at least 3000 hours. A dose rate
averaged over some reasonable time (e.g. 10 minutes) can probably
be ten times this average dose rate and still allow operations
without tripping the integrated dose limit. The trip point on
integrated dose should be 2400 Rad/day. The integrated dose in
the monitor should be zeroed each day at midnight. The dose rate
trip level should therefore be approximately 1000 Rad/hr and the
dose trip level, rezeroed daily, should be approximately 2400
Rad. Note that one could set a higher dose limit in the detector
if it were closer but that would imply a very small detector head.
For a 30 cm long detector, the distance should be no less than
30 cm. Finally, note that these numbers are ballpark numbers
right now and we will need the results from Peter Kloeppel's EGS
runs to be sure of them. His results will also indicate a good
position for the detector.
1. NCRP Report No. 51, "Radiation Protection Design Guidelines for 0.1-100 MeV particle accelerator facilities", National Council on Radiation Protection and Measurements, Washington D.C., 1977.