Introduction

My name is Lisa Shine and I am in transition year in Muckross Park Secondary School, Donnybrook. As part of my work experience I came here to the Radiation Physics Research Laboratory at UCD for two weeks to see what exactly went on in a research laboratory.

The project I was working on here for my work experience was to determine the amount of total alpha and beta radioactivty in different water samples. I collected water from three different sources. For the determination of background, I took a sample of distilled water which should have no activity in it. I also collected a sample of water from the tap at home, a sample from the tap in the lab here and I went out to collect a sample of water from the lake in UCD. I hoped I wouldn't find any radioactivity in my tap water from home!

This project took almost a week to complete and on this page I have explained how I went about doing this experiment. So please read on to find out "is there radioactivity in our tap water?" and "is there radioactivity in the lake in UCD?"

A rapid method for the determination of total alpha and beta radioactivity in water samples using a low background liquid scintillation counter.

The samples were counted for total alpha and beta radioactivity using a liquid scintillation counter (LSC Quantulus). Sample volumes of 10 ml were placed in a plastic vial and mixed with 8 ml of scintillation cocktail. The mixture was shaken for a minute and left inside the counter for 12 hours before counting to allow decay of chemiluminiscence (light that might interfere with the readings).

Eight different samples of water were measured:

(i) Two samples of deionised water were prepared for counting as described above.

(ii) Two samples of water from the lake here in UCD were prepared.

(iii) Two samples of tap water from the lab here in UCD were prepared.

(iv) Two samples of tap water from my house were prepared.

How I prepared the samples.

Date: 18-1-00

Time: 11.30am - 3.30pm

Apparatus: 2 graduated cylinders, 8 vials. Chemicals: Scintillation cocktail, tap water from home, deionised water, tap water from the lab here in UCD, water from the lake in UCD.

1. I rinsed out a graduated cylinder with tap water from home.

2. I measured 10 ml of the tap water in the graduated cylinder and 8 ml of scintillation cocktail in another graduated cylinder.

3. I poured the water and the scintillation cocktail into a vial - hard to do without spilling any!

4. I shook the vial vigorously so that the water and the scintillation cocktail were mixed well.

5. I labelled the mixture and left it to stand for about half an hour in the liquid scintillation counter while I went off for lunch! It was important to label the mixtures so that I knew which was which.

6. I repeated steps 1 to 5 three more times using deionised water, tap water from the lab and water from the lake here in UCD. Then I prepared a second sample of each.

Results for total radioactivity analysis.

It took 40 hours (almost 2 days) for the quantulus to finish testing the samples. But after two days full of suspense, I was finally able to look at the results. Below is a table of these results.

Conclusion

Here you can see my results on graphs. The first graph shows the beta activity in the water samples (Bq l ^-1). All of the samples had no beta activity in them apart from background. The deionised water, however showed slightly higher beta activity than the other samples. Since I had used it as a background, it should have had little or no radiation in it! The amount it contained though, was very little.

The second graph shows the levels of alpha radioactivity (Bq l ^-1)in the water samples. It is quite clear here that the tap water from UCD and the tap water from home had higher alpha activity in them than the UCD lake sample and the deionised water. It surprised me that the lake water had lower radiation levels in it than the tap water. The level of alpha activity in the water was too small to have any consequences. While the samples of water had some levels of both alpha and beta radiation, the levels are well within the recommended safe levels - Phew!

Gamma radioactivity in a grass sample from UCD.

We have seen how to determine alpha and beta radiation. To learn how to determine the third type of radiation, gamma radiation, I collected 107.58g of grass from the grounds of UCD. I then put this wet grass into an oven to remove any moisture. The grass was left in the oven over night and the next day I weighed the grass again. This time it weighed 21.43g. I took the dried grass and put it in a blender so that it formed a powder. The powdered grass weighed 19.58g. I put the powdered grass into a small container and the placed it in a gamma spectrometer for counting. I got the results the next day - below are the spectra.

This graph shows the ⁷Be peak.

This graph shows the ²¹⁰Pb peak.

This graph shows the ⁴⁰K peak.

As you can see, there is some radioactivity in the grass!! This is, however, due to natural sources. Be-7 and Pb-210 are produced in the atmosphere and brought down to the grass by rain and dust. The K-40 is part of the grass. A proportion of the total potassium is K-40, which is radioactive. As you contain some potassium, this means that you are emitting gamma radiation!!!

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