Monitoring/Shielding of 50fs 100 MeV e-

[Rob Tayloe <tayloe@ibm.estd.battelle.org>]

On Tue, 25 Mar 1997 Bruce Busby <bbusby@umich.edu> wrote:

> 
> Of interest to me here is a researcher who is developing a laser based
> accelerator. They predict that their next unit may be capable of producing
> essentially monoenergetic 100 MeV electrons in 50-100 fs pulses repeated
> with up to a 1 hz frequency.  I would appreciate any advice you could
> provide about monitoring equipment  Clearly energy dependance, saturation,
> recombination and calibration are big issues.  Fortunately, the lab is
> reasonably well funded.
> 
> Certainly shielding is also a big issue.  The beam produced will probably
> be produced with an initial narrow cone (5 cm spread a 50 cm distance).
> Any advice for materials, amounts, shielding codes or references would be
> appreciated.

Battelle assisted a private corporation with dosimetry and shielding analysis at a
similar facility.  In the early phases of the design, the simple one-dimensional
DSN code ANISN with the HILO-86 cross section set was used.  The HILO-86
cross sections are available for a number of materials for neutron energies
to 400 MeV and photon energies to 20 MeV.  These codes and cross sections
are available from the Radiation Safety Information Computational Center (RSICC)
as code package CCC-254 and DLC-119, respectively.  A PC version is available
for ANISN that will read the ANISN format cross sections (i.e., HILO-86).

Later in the design and early operational phases of the facility, we used a
modified version of the point kernel code QAD-CGGP (available from RSICC as
code package CCC-493, albeit without our modifications).  Our modifications
included use of empirically derived attenuation coefficients for neutrons and
photons.  The neutrons were treated as three groups, a giant resonance group,
and two high energy groups.  The IAEA Tech Report 188 is a good source of
information.  There are many other references that I can provide.  We also
modified QAD to allow us the generate contour plots of expected dose rates
outside the shielding.  We were able to make a very accurate model of the
shielding using QAD.  The generated contour plots were imported into a CAD
program and overlaid on a drawing of the facility.

We found that the dose rates outside the shield was essentially too low
to measure and thus compared calculated versus measurements inside the
LINAC vault.  Not unexpectedly the point kernel code was not always
in good agreement with the measurements in the vault.  We used a number of
instruments and dosimeters to measure the neutron and photon radiation,
but had the best success and repeatability with CR-39 for neutrons
and LiF TLDs for photons.  We also had good luck with the bubble detectors from
Siemens.  

Use of the bubble detectors depended on having a reasonably good idea
of the neutron field andselecting a detector sensitivity and exposure time that 
would yield between 25 and 100 bubbles.  More bubbles are difficult to count 
manually.  We sat around a coffee table and counted bubbles in the bubble detectors
after making an exposure.  Olympic scoring was used for each detector; that is we
 threw out the high and low counts and averaged the remaining counts.

Much better agreement was obtained with the measured neutron fields inside the
LINAC vault when the calculated doses were obtained using the MCNP code (available
from RSICC as CCC-200).  MCNP uses the monte-carlo technique and the run times
can be quite long.  MCNP allows one to make a very accurate model of the shielding
and other significant objects (i.e., magnets).

We used a code called DCTDOS (RSICC code CCC-520) to calculate the dose reduction
in the labyrinths to the LINAC and storage ring vaults.  We had good agreement with
the measurements using CR-39 and LiF TLDs.

One of the difficulties we encountered was in defining the source locations and
intensities.  Beam loss can (and DOES!) occur in locations other than intended, 
particularly in the early phases of operation.  Surveying the beamline for residual
activity following a beam injection or operation is a good way to locate sources.

I would be happy to provide more information upon request.  More information on
RSICC may be found on their WWW page at:

	http://epicws.epm.ornl.gov/

Rob Tayloe <tayloe@battelle.org>
Columbus, OH
phone 614-424-5796