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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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