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HDR maze door
>From: Larry Langrill <rophy@concentric.net>
>
>Patrick,
>We are building a simulator room to also be a workable HDR room. Since
>most of the activity (human) in the room will be simulations using only
>diagnostic xray I wanted to keep the doors to the room standard weight
>xray doors ( 1/8" lead) to make access easy. The room won't be done
>for another 4 to 5 months so final measurements are not available. The
>design has both a patient entry door on a fairly long maze and a staff
>door closer to the simulator and HDR unit on a short (5 foot long) maze.
>
>I've made some measurements in our existing linac / HDR vault which has
>a maze. The 5.4Ci Ir-192 source was 4 meters from the throat of the
>maze. I measured 200 mrem/hr at this position. (I calculate about 170
>mrem/hr for this distance, but this ion chamber isn't exactly calibrated
>for Ir-192) About half a meter into the maze, i.e., out of the
>primary radiation, I read 10 mrem/hr. Further, about a meter into the
>maze I read 5 mrem/hr. I also positioned a 1/8" by 1 foot square piece
>of lead between the ionization chamber and what I assumed to be the
>major regions of scatter emanation, and, for both of the 'in-maze'
>locations I found this lead reduced the reading by a factor of four
>(down to 26%). So in our new room with a full strength 10Ci source
>I'm expecting no more than 1 to 2 mrem/hr outside our staff door and
>much less outside the patient entry door.
>The amount of scatter I measured just into the maze agrees well (within
>half an order of magnitude!) with NCRP 51 reflection coefficients for
>concrete, which shows for 90 degree scatter and 300 - 400 KeV incident
>energy, a coefficient of about 0.03.
>I also found with this 1/8" piece of lead about 1.5 meters from the
>"bare" source that the transmission was only 45%. This means that the
>HVL is at most 3 mm of lead, whereas NCRP 49 reports it as 6 mm, so be
>careful!
>Good luck!
>
>Larry Langrill
>MidMichigan Medical Center
>Midland, MI 48670
>517-839-3452
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