[Date Prev][Date Next][Thread Prev][Thread Next][Date Index][Thread Index]
RE: Box Calibation Source & Scatter
John:
You say you measure (confirm the dose rate) with an MDH. What size detector are you using? If a larger detector is used for confirmation of the dose, it is quite possible that it is extending into areas of the box outside of the 100% dose area (center). Shepherd irradiator dose rates will vary up to 25% from the center of the box to the walls. Shepherd calibrates and demonstrates where the isodose lines lie by using ion chambers that have a sensitive volume of about 0.6 and 0.18 cm3. You really need tiny ion chambers like this to measure the isodose lines. The ion chambers should have build-up caps. I remember Radcal made some ion chambers like this. Not sure if MDH makes them, but I would guess they do as they make similar instruments.
The higher activity source will give more buildup, but I would hope the initial calibration would account for this. If not, someone screwed up the calibration in the first place.
I've done the calibration of a Shepherd box irradiator with the tiny ion chambers. It's a pain in the butt to do - typically taking a full day to 1-1/2 days. Of course the ion chambers should be NIST calibrated.
Larry Grimm, Senior HP
UCLA EH&S/ Radiation Safety Division
* lgrimm@admin.ucla.edu
* Phone: (310) 206-0712 Fax: (310) 206-9051 Cell: (310) 863-5556
* On Campus Mail: 501 Westwood Plaza, 4th Floor, MS 951605
* Off Campus Mail: UCLA Radiation Safety Div, 501 Westwood Plaza
4th Fl, Box 951605, Los Angeles, CA 90095-1605
* If this email is not RSD business, the opinions are mine, not
UCLA's.
-----Original Message-----
From: Josh Mancheiwitz [mailto:hptech99@YAHOO.COM]
Sent: Sunday, August 10, 2003 4:16 AM
To: radsafe@list.vanderbilt.edu
Subject: Box Calibation Source & Scatter
Comrades,
If anyone has information concerning scattered photons
produced within Sheperd Model 89 Calibrators please
reply. My laboratory owns and operates three of these
units and we recognized that the calibration charts do
not reflect the actual dose rates at lower level
geometries (1 - 10 mR/hr points). One of the
calibrators has an approximately 15 mCi Cs-137 source
for lower dose rate calibrations, the other two are
equipped with 130 mCi sources which can also be
attenuated (per the cal chart) to acheive lower dose
rates. However, the actual dose rates are not equal at
these low levels. When verified with Eberline Ion
Chamber dose rate instruments, and Eberline GM dose
rate instruments, the higher curie (130 mCi) sources
produce dose rates 10 to 40 % higher than expected.
The calibrators are verified yearly using MDH and
electrometer readings down to approx 60 mR/hr and fall
within +-5%. My theory is that the higher curie
content sources produce enough scattered photons
within the box calibrator to affect the instrument
response at the low range.
If anyone knows of a positive way to prove or disprove
the scatter theory, please respond.
PS: Please hold all criticisms and respond with
helpful anecdotes only.
Thanks,
Josh Mancheiwitz
HPTECH99@yahoo.com
__________________________________
Do you Yahoo!?
Yahoo! SiteBuilder - Free, easy-to-use web site design software
http://sitebuilder.yahoo.com
************************************************************************
You are currently subscribed to the Radsafe mailing list. To unsubscribe,
send an e-mail to Majordomo@list.vanderbilt.edu Put the text "unsubscribe
radsafe" (no quote marks) in the body of the e-mail, with no subject line.
You can view the Radsafe archives at http://www.vanderbilt.edu/radsafe/
************************************************************************
You are currently subscribed to the Radsafe mailing list. To unsubscribe,
send an e-mail to Majordomo@list.vanderbilt.edu Put the text "unsubscribe
radsafe" (no quote marks) in the body of the e-mail, with no subject line.
You can view the Radsafe archives at http://www.vanderbilt.edu/radsafe/