Answer to Question #5220 Submitted to "Ask the Experts"

I am interested in using ⁴⁹V, an x-ray emitter (5 keV), in some life science research and would like to quantify the x-ray emissions of samples. Do scintillation cocktails respond to soft x rays?

Yes, liquid scintillation (LS) cocktails are often used very effectively to detect and quantify low-energy x-ray emitters. Perhaps one of the most common radionuclides so measured is ¹²⁵I, which emits x rays at about 28 keV. These are low energy but still considerably higher than the 5 keV photons to which you refer. The x rays of any significant yield, emitted by ⁴⁹V, range from about 4.5 keV to 5 keV with a combined abundance of about 20%. The attenuation coefficients for the photons at these energies are very high and virtually all of the photon energy will be absorbed in the scintillation fluid following decay events that occur a millimeter or two or more removed from the vial wall.

In addition to the x rays, ⁴⁹V also emits some Auger electrons at about 4 keV with a combined yield of about 70%. These may well contribute more to the count rate than will the x rays in the LS solution. The x-ray and electron energies are just slightly lower than the average beta energy of tritium and, as I expect you know, tritium can be measured with a high efficiency in LS systems, and you should be able to achieve an acceptable counting efficiency for the vanadium.

If your scintillation counter presently is set up with a counting window for tritium, the vanadium pulses should all fall within that window. Because the energies of the x rays and the electrons are so low the light pulses will be small, however, and it is important to try to minimize color and chemical quenching in the LS samples in order to maintain as many pulses as possible above the threshold for the system. Time and care spent in sample preparation to minimize quench effects will be worthwhile. Good luck.

George Chabot, PhD, CHP