Lab Exercise - Questions for Introduction to Radiochemistry

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 investigate 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 material.

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.

Question 1:
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.

Question 2:
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?

Question 3:
Show that the amount of ²³⁸U and ²³⁴U is in agreement with the equilibrium definition λ₁N₁=λ₄N₄. (Assume the abundance of the two isotopes is equal to the abundance in natural uranium.)

Question 4:

H₁ / H₄ = N₁ / N₄     Here H₁ symbolizes the atomic abundance of a ²³⁸U and H₄ symbolize the atomic abundance of ²³⁴U . Compare this with

T_{1/2 (1)} / T_{1/2 (4)}    Does this agree with the claim that the two isotopes are in equilibrium?

Question 5:
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 disintegration yielding low-energy betas will not be measured since we shield the source with 7 layers of tape - low-energy betas will not get through).

Background

Note down the values from the background measurement. The longer the measurement the better. Preferably it should be started the day before.

Counting number:   N_bck = Counts

Counting Time: t_bck = sec

Standard deviation: (√N_bck ) / t_bck = cps

Counting Efficiency

Do a one minute count on every shelf that is in the detector. Use these measurements to calculate the counting -efficiency of the GM-detector in %. The activity of the sample can be calculated from the amount of UO₃ used.

A = IR

where A is the true activity I is the efficiency of the detector and R is the observed counting number.

Twenty measurements with constant distance from the source

Do twenty measurements lasting for one minute. Keep the source fixed to one position. Calculate the standard deviation and complete the table