Introductory Questions and Preparation of the Sample (Introduction to Radiochemistry)
Lab Exercise and Rapports
In this exercise you are going to make a calibration source of uranium. You will use this source to calibrate and get some experience with using a gas-filled detector. You will then use the source to investiage uncertainty in radioactive measurements.
Your supervisor will demonstrate and explain the various equipment you are going to use. Never just "try it out" but ensure that you know exactly what to do and how before you do any operations with radioactive materil.
Before you can start in the lab you must calculate the exact amount of uranium needed for calibration source. Therefore you should answer the following questions before you arrive at the lab. Write the answers down in your laboratory journal and ask your supervisor to check before you start any work in the lab.
- Draw the disintegration schematic of the uranium series from 238U to 230Th. Write down the half-lives and the type of disintegration, including the energy of emitted particles.
- Assume that the sample of uranium that you have is one year old. (The uranium is chemically separated from the other elements). Why is 234Th, 234Pa and 234U in equilibrium with 238U, while 230Th is not?
- Show that the amount of 238U and 234U is in agreement with the equilibrium defenition λ1N1=λ4N4. (Assume the abundance of the two isotopes is equal to the abundance in natural uranium.)
Here H symbolises the aboundance of atoms. Compare this with
Does this agree with the claim that the two isotopes are in equilibrium?
- What is the mass of UO3 that must be weighted in to get the right amount of the radioactive calibration source you are going to make?. Assume that the the counting speed R should be equal to 100 cps and that the counting efficiency is ε = 15%. Your counter will measure high-energy betas (the disintegrations yilding low-energy betas will not be measured since we shield the source with 7 layers of tape - low-energy betas will not get through).
Preparation of the Sample
- A filter-holder with vacuum suction will be used to make the uranium calibration source. The setup will be demonstrated by your supervisor. Before you start with the uranium, test the setup thoroughly for leaks and other problems (with water). All equipment and radioactive material should be in a tray with absorbing paper at the bottom. Non-active equipment and materials should be kept separately. Keep your workspace tidy.
- On the high-precission scale (demonstrated by your supervisor) measure the exact weight of your weighing ship.
- Transfer the calculated amount of uranimoxide to your weighing ship using the laboratory scale (low precision) to control the amount.
- When you transport your UO3 (or any other radioactive material) from one place to another (e.g. from the scale to your work space) always use a tray, preferentially with a lid, such that any spills/contamination will be in the tray and not on the floor.
- Measure the exact amount of uraniumoxide you have on the high-precission scale by weighing the ship with the oxide. Calculate the exact amount of uraniumoxide you have.
- Transfer the UO3 into the filter-holder setup by flushing out the ship with water. Make sure everything is transfered and try to do it in such a way that the oxide is evenly distributed on the filter.
- Apply vacuum to remove the water and leave it on until the uraniumoxide on the filter is reasonably dry.
- A sample holder card (cardboard with a punch-out hole slightly larger than the filterpaper you are going to use) is prepared for holding and sealing the uraniumoxide: Put seven layers of tape on one side such that the hole is completely and securly covered.The seven layers of tape will ensure that low-energy betas will not pass through (why is this important?).
On the left side is a prepared counting paper with a filter with uraniumoxide taped on. On the right side there is a unprepared filter.
- Disconnect the vacuum suction and "catch" the filter with the tape on the sample holder card. Remove it very carefully, turn it and securly seal the filter/uraniumoxide with tape.
- Write your initials, date, and amount of UO3 on the sample card.
- Check the sample card holder for surface contamination by using a swipe (your supervisor will explain/demonstrate).
- Clean up your workplace and make sure that all radioactive waste is disposed of properly in appropriate waste containers. Update the waste accumulation logs.