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RE: Box Calibration Source & Scatter
Josh:
I agree with Grant. Here's a couple more thoughts for others interested in instrument calibrators in general, and box calibrators specifically.
- This applies to any calibrator. If your attenuators are lead (Pb), over time they will distort from the natural flow of lead and/or banging around. I first recommend tungsten attenuators, or steel as a second option.
- I recommend a probe/detector holder to help eliminate geometry problems. You should have a holder designed specifically for each type of probe. There should be no metal in the probe/detector holder, ala Grant's comments below. I make my holders out of Plexiglas - screws and all.
- Most folk purchase a box calibrator because they have no space for an open range source. Personally, if I can get the space, I opt for an open range calibrator - there are fewer confounding factors.
Here's a line to start an argument: other than with NIST (or like agencies), does anyone really believe you can get a good calibration, on a standard GM or ion chamber, below 1 mR/hr (10 uSv/hr)? I'm not going to give my opinion, but I am curious to see what folks think.
Grimm
-----Original Message-----
From: NIXON, Grant (Kanata) [mailto:GNIXON@MDS.Nordion.com]
Sent: Monday, August 11, 2003 1:19 PM
To: 'Grimm, Lawrence'; 'Josh Mancheiwitz'; radsafe@list.vanderbilt.edu
Subject: RE: Box Calibration Source & Scatter
Hi Josh,
I do not have experience with Shepherd Model 89 Calibrators. However, from
my dosimetry experience with Gammacell blood irradiators, what Larry says is
true. Moreover, placing a large detector like an ion chamber (e.g., RO-7
probe) will perturb the field/isodose lines somewhat in comparison to using
a small detector.
I do not think that a higher curie content would be much of an issue - it is
the geometry and attenuation properties of the stuff inside the chamber that
will likely be the main factors affecting the dose rate you measure.
For that reason, using a pre-calibrated device to measure the central dose
rate, is a bit unsatisfactory because of the perturbation/geometry effects.
Your options? You could (i) perform a proper dose mapping (using film
dosimetry) of the sample chamber for the given geometries, or (ii), perform
a Monte Carlo calculation (e.g., using MCNP) to model the sample chamber
under conditions of measurement.
I have a question/warning: How is the attenuator held in place?
You could be experiencing "beaming" effects due to the use of brass screws
to affix the attenuators! That could indeed be the primary reason for your
dose rate being higher than expected.
I hope that this discussion was of some help to you. Feel free to contact me
if you would like to discuss this more.
Best regards,
Grant
Grant Nixon
Radiation Physicist
Physics Department
Ion Technologies
MDS Nordion
447 March Road
Ottawa, ON K2K 1X8
tel. (613) 592-3400 ext. 2869
fax (613) 591-7422
-----Original Message-----
From: Grimm, Lawrence [mailto:LGrimm@FACNET.UCLA.EDU]
Sent: Monday, August 11, 2003 1:48 PM
To: 'Josh Mancheiwitz'; radsafe@list.vanderbilt.edu
Subject: 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
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