Re: Thallium Activated Xtals and Neutron Dse Rate Measurements

[Ted de Castro <tdc@xrayted.com>]

Mike Malaxos wrote:
> 
> Hello All,
> 
> Neutron Dose rate Measurement. An attempt was made to check the calibration
> of an old neutron detector consisting of a slow neutron BF3 detector mounted
> inside a 200 mm diameter block of moderator using a 50 mCi Am241 Be Neutron

This is a flux moderator - instrument response will be proportional the
neutron flux.


> source.  The detector had good gamma rejection and the source specifications
> listed the neutron dose rate at 100 cm as 1.2 microSievert/h.

One of our nominal 50 mCi AmBe sources is 0.27 mR/hr at 100 cm at 150 cm
above a concrete slab (15.5% scatter from slab).  (neutron flux is 1.85
n/cm^2/sec).

Good gamma rejection is the nature of a BF3 and tells me you probably
have it set up correctly.


> 
> The detector and source were mounted 300 cm above a cement floor. I
> understand that the distance above the floor should be more than 1000 cm for
> sufficient scatter reduction.

Actually 300 cm is quite good - scatter should be about 6% - or there
abouts.

I have a spreadsheet setup to do the calculations including the scatter
according to the energy dependent form of the Jenkins formula.  BUT you
will need to determine the q of the source.


> A cross check with a snoopy monitor which was not in recent calibration
> indicated a dose rate almost four times the expexted value.  I would image
> that the Am 241 Be source dose rate would be within 10% of the expected
> value.  Is the Snoopy monitor calibration way out or am I missing something?

There is variance from source to source of efficiency and variance from
the nominal Ci content.  We have 17 such sources - but I have only
calibrated and characterized 3 of them.



> 
> Any comments will be welcolme.
> 
> Best Regards,
> Mike Malaxos.
> Radiation Safety Services
> Mike@radiationsafety.com.au
> 69 Robinson Avenue Belmont Western Australia 6104
> Phone 61 (0)8 94750011 FAX  61 (0)8 94750011
> 
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