RE: X-ray dose.

[Mayes Dave E <DEMAYES@dera.gov.uk>]

On the subject of previous correspondence I am surprised for a Brit. to
be using non-SI units.  However, to return to the question....

Rough calculations using British Standard 4094 confirm the doserate to
about 170 Rads (sic) at 17 cm.  However this is for a constant potential
tube.  If you are using a half wave rectified set you may expect about a
half or perhaps even one third of this.  How old is your X-ray tube?
Old tubes with worn or pitted anodes may exhibit the "heel" effect.  A
marked variation in intensity across the field along the anode/cathode
axis.  Additionally, a "normal" personal dosimetry TLD will under-read
by approx 20% under the superficial dose area.  A combination of these
factors may get you down to somewhere around the figure you have.

Bear in mind the data you are using, and even BS 4094, date from the
1950s.  Developments in the last 40 years mean tube outputs will have
increased, if you are using a modern ceramic tube you could expect the
output to be much higher than 170 Rads.

Please let me know if you come up with a positive answer.

Dave Mayes
Senior Health Physicist
DRPS,
Crescent Rd
Alverstoke
UK

Tel:   023 92 768140
Fax:  023 92 768150

	-----Original Message-----
	From:	D J Hornsey [SMTP:bssdjh@bath.ac.uk]
	Sent:	23 September 1999 12:27
	To:	demayes@mail.dera.gov.uk
	Subject:	X-ray dose


	  Dear Radsafers,
	              I wonder if some of you could suggest an
explanation for the
	following problem I have. It concerns the considerable
differences I am
	finding between the  expected and measured dose in the main beam
of an
	X-ray source.
	              The tube is running at 60 kV with a tube current
of 3 mA.
	The filtration is 1 mm Be. According to my calculations, at 17cm
from
	the window, the dose should be 170 cGy (170 rads) for a 10 sec
exposure.
	A TLD in the main beam consistently records a factor of about
10x less
	i.e. 140 mGy (14 rads). A series of measurements under different
	conditions reproduces this factor of a 10x inconsistency. Would
this be
	expected? Should I not be using a TLD for this type of
measurement and if
	not, what should I be using?
	     Any suggestions will be most welcome.

	                 David Hornsey


	        

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