Answer to Question #2199 Submitted to "Ask the Experts"

Q

Is it possible to reliably measure a dose rate from 15 keV x radiation with a pancake G-M tube? In my situation, I have a Ludlum 44-9 pancake tube (3300 cpm/mrem/hr with, I believe, ¹³⁷Cs gamma) that reads about 150 kcpm due to x radiation emitted from a high-vacuum/high-voltage device. Am I correct in thinking that the tube is more efficient for this energy than for ¹³⁷Cs? Does anyone have experience or have a chart of efficiency vs. energy for such a pancake tube?

A

For the U.S. Navy military spec “8767” pancake G-M tube measuring a 15 keV photon field, I’d say the dose to air in cps per µGy/hr, relative response to a 662 keV ¹³⁷Cs gamma field, is about a factor of 1.7 overresponse to that of ¹³⁷Cs. Back in the mid-1980s I worked with colleagues in the United Kingdom and here in the States to type test most of the popular-style G-M tubes and probes. (See my paper and Figure 7 in the proceedings of the 1987 Health Physics Society Midyear meeting, noting energy response from 10 keV to 1.25 MeV average ⁶⁰Co gamma energies.) As will be noted, the pancake G-M tube has a peak overresponse thru the window, at about 40 keV, of about a factor of 4.8 relative to ¹³⁷Cs. This work was then extended with a ¹⁶N source, into the 6 MeV gamma energy range, and presented at the 1992 IRPA meeting. In this 6 MeV field, the pancake tube and probe both overresponded by a factor of 1.9 relative to ¹³⁷Cs.

David J. Allard, CHP