NS 7991 MEDICAL PHYSICS, HEALTH PHYSICS AND NUCLEAR
ENGINEERING LABRATORY
27 Feb 00
Jeffery Orr 433-53-5704
Will Hill
Lab 4: Liquid Scintillation Counting
Theory: In liquid scintillation counting theory, a solution of a fluorescent substance
dissolved in toluene or other suitable solvent is the sensitive volume of the detector. This
solution is called a Liquid Scintillator (LS). The radioactive substance is dissolved
directly in the liquid scintillator or, if the sample is insoluble in the solvent used, it may
sometimes be suspended as a fine dispersion. Charged particles, emitted from radioactive
atoms, interact with the liquid scintillator to produce very small flashes of light
(scintillations) too faint to be detected by the unaided eye. A photosensitive multiplier
system is used to detect the flashes of light. However, because the flashes of light are so
faint, even for detection by a sensitive photo multiplier tube, certain refinements of the
method are required over that used for citing other kinds of radiation.
Purpose: This experiment is to introduce the student to the basic principles of measuring
radiation by the scintillation process, to familiarize students with LS instruments, and to
measure several isotopes one might encounter when doing environmental radiological
evaluations. During this experiment the students will compare the effects of color
quenching with toluene-tritium and asphalt-tritium solutions and compare the amounts
relative to those in the tritium standard, phosphogypsum wash, Pipe scale wash and Baton
Rouge tap water.
Materials: pipette dispenser
tritium stock solution
asphalt (color quencher)
liquid scintillation vials
toluene stock solution
liquid scintillation instrument
Baton Rouge tap water
Pipe scale wash
3
H measured at 255300dpm on 28 Jan 85 (~ 0.2Ci)
phosphogypsum wash
Method: A standard solution must first be obtained. The standard used in this
experiment was 3H measured at 255300dpm on 28 Jan 85 (~ 0.2Ci). Once the standard
solution was obtained then we proceeded to create the tritium-toluene solution. This was
done by mixing 5ml of toluene with 50l of tritium. Then these materials were placed in
the LS machine in the following order: 1) standard 2) empty vial for background 3)
toluene-tritium 4) asphalt-tritium. (See attachment for results) Last the results of the
experimental vials were compared to the standard to prove the effects of color quenching.
For the second part of the experiment solutions of phosphogypsum, pipe scale, and Baton
Rouge tap water were obtained. The phosphogypsum came from uranium extracting site,
the pipe scale from old oil pipes, and the tap water from an unused and settled tap in the
lab. A second LS scan was made with the new materials in the following order: 1) oil 2)
phosphogypsum 3) tap water. (See attachment for results) These two parts of the lab are
only related by the process of LS and their findings are independent of one another.
Data:
Tritium
standard
background
3H
3H+asphalt
255300dpm on 28 Jan 85
~.2microcuries
5microliters toluene+
50 microliters tritium
5microliters toluene
+50 microliters tritium
+ 20microliters asphalt
pos time=1min h#
CPM %error CPM Lumex% Elapsed Time
1
1 -38.5 62365
0.8 5587
0
1.47
2
3
1 -9.7
17
1 -10.6 23471
4
1
16.8 21523
48.51
1.31
8
720
0.4
0
3.26
5.11
1.36
215
0
6.9
Ra-222
Oil
Phosphogypsum
H20
pos time=1min h#
CPM %error CPM Lumex% Elapsed Time
1
1
6.9
33 34.82
20
3.53
1.74
2
1
-10
26 39.22
14
0.18
3.46
3
1 -11.7
24 40.82
8
0.19
5.28
( See Appendix B )
Analysis/Results: The equations of this experiment as well as the full analysis are in
appendix A. Assume that all counts present came from either Ra or 3H or background
for each part of the experiment. All the counts present were then lumped into one
channel. Uncertainty of this total is the uncertainty sum of the different channels present
in each part of the experiment. For the tritium uncertainty and efficiency were calculated.
In the matter of Color quenching, the asphalt accounted for an approximate 5 percent
difference in the efficiency and a 10 percent difference in the total number of counts. The
efficiency of the standard was 65 percent, the toluene/tritium was 48 percent, and the
asphalt sample was 41 percent. For the second part the activity of the radon was
calculated. Then the amount of radium present was calculated, assuming that all of the
radon was dissolved in the sample’s water. The radon the pipe scale came in first with
11.26 pCi/L, the tap water second with 9.9 pCi/L, and the phospogypsum third with 4.5
pCi/L. No efficiency calculation was possible because there was no standard.
Conclusion: There was error produced by the faulty or improperly calibrated equipment.
The machine was unable to resolve energy as shown by the skewed graph in Fig 1. The
LS machine was unable to properly place the counts from the different sources into their
respected channels. Since the samples were either water based or contained only tritium
then it was reasonably certain to rule out interference by any other radioactive elements.
The uncertainties were so great and the counts were so low, almost all the counts could be
background for the radon. This lab might be improved by increasing the count time to
that of the half-life of the radium which is four hours. This could produce a
distinguishable difference in the sample counts and the background.