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                      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
             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.


                                          pos time=1min h#     CPM %error CPM Lumex% Elapsed Time
standard          255300dpm on 28 Jan 85    1          1 -38.5 62365   0.8 5587      0        1.47
background                                  2          1 -9.7     17 48.51    8    0.4        3.26
3H                5microliters toluene+     3          1 -10.6 23471  1.31 720       0        5.11
                  50 microliters tritium
3H+asphalt        5microliters toluene      4          1 16.8 21523   1.36 215       0         6.9
                  +50 microliters tritium
                  + 20microliters asphalt
                                             pos time=1min h#     CPM       %error CPM Lumex% Elapsed Time
Oil                                            1          1   6.9    33      34.82   20   3.53         1.74
Phosphogypsum                                  2          1   -10    26      39.22   14   0.18         3.46
H20                                            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.