Half-life

A source contains a radioactive material. When the radioactive source is not present, a detector measures a background count rate of $20$ counts per minute. The source is then placed in a fixed position close to the detector. At first, a count rate of $520$ per minute is measured. What count rate would be measured after a time of two half-lives of the radioactive source?

May/June 2015

The half-life of a radioactive isotope is $24\,\text{hours}$. A sample of this isotope gives an initial count rate of $720$ counts per second. How long will it take for the count rate to decrease to $90$ counts per second?

May/June 2015

Which statement is correct about the half-life of a radioactive isotope?

May/June 2016

One radioactive isotope of hydrogen has a half-life of $12\text{ years}$. A sample contains $40$ million atoms of this isotope. How many atoms of this isotope remain after $24\text{ years}$?

May/June 2018

The count rate detected from a radioactive source reduces from $4000$ counts per minute to $500$ counts per minute in $72$ minutes. What is the half-life of the source?

May/June 2021

A radioactive source has its count rate decrease from $4000$ counts per minute to $500$ counts per minute over $72\,\text{minutes}$. What is the half-life of the source?

May/June 2021

A tritium sample has a mass of $32\,\text{mg}$. The half-life of tritium is $12$ years. What mass of the tritium sample decays over $36$ years?

May/June 2023

The graph presents the findings from an experiment used to determine the count rate of a radioactive isotope. What is the half-life of the isotope?

May/June 2025

An isotope’s half-life is $6000$ years. How long does it take for the count rate from a sample of the isotope to fall to $\frac{1}{16}$ of its starting value?

May/June 2025

A radioactive isotope $^{14}\text{C}$ emits beta-particles. A sample of this isotope is kept for a time equal to its half-life. Which two quantities fall to half of their starting values during this interval?

Oct/Nov 2015

A newly prepared sample of radioactive material has a count rate of 8000 counts per minute. After 20 days, its count rate is 500 counts per minute. What is the half-life of the material?

Oct/Nov 2016

A newly prepared sample of radioactive material has a count rate of $8000$ counts per minute. After $20$ days, its count rate falls to $500$ counts per minute. What is the half-life of the material?

Oct/Nov 2016

A sample includes atoms of an isotope whose half-life is $7.2\,\text{years}$. Which quantity is reduced by half every $7.2\,\text{years}$?

Oct/Nov 2017

Isotope P is radioactive, and its half-life is $7.0$ years. Initially, a sample has $0.016\,\text{kg}$ of P. How long will it take before the sample has $0.0020\,\text{kg}$ of P?

Oct/Nov 2020

What does the term half-life of a radioactive element mean?

Oct/Nov 2020

What does the term half-life of a radioactive substance mean?

Oct/Nov 2022

A wooden item is thought to be 6000 years old. Which isotope, present in the item, is used to find its age?

Oct/Nov 2022

The isotope radon-222 has a half-life of $3.8$ days. Which statement is correct for a sample containing $1.0\,\text{mg}$ of radon-222?

Oct/Nov 2023

The half-life of a radioactive isotope is $2.5$ minutes. This isotope is the only radioactive isotope present in a radioactive sample. From time $t = 0$ to $t = 7.5$ minutes, the mass of this isotope in the sample falls by $70\,\mu\text{g}$. What is the mass of this isotope at $t = 0$?

Oct/Nov 2023