輻射與放射性

Which of the following statements about $\alpha$, $\beta$ and $\gamma$ radiations is incorrect ?

A radioactive source is put in front of a GM tube. The initial count rate is $1050$ counts per minute. It is known that the half-life of the source is $4$ hours and the background count rate is $50$ counts per minute. What is the most likely count rate after $8$ hours ?

The figure shows a radioactive source placed near two parallel metal plates $X$ and $Y$ that are connected to a power supply. When a GM tube is moved along the dotted line ($- \cdot - \cdot -$), the count rate shows a significant increase at $P$ and $Q$ respectively. Which of the following statements is correct when a magnetic field pointing out of paper is applied between $X$ and $Y$ ?

The activity of a radioisotope is $960$ Bq. Its activity drops to $240$ Bq in $2$ minutes. How much more time would be required for its activity to become $30$ Bq ?

In the figure above, two charged metal balls $P$ and $Q$ are hung by insulating threads. $P$ is positively charged while $Q$ is negatively charged. An $\alpha$ source is put near the balls without touching them. Which of the following figures shows the situation after a period of time ?

In the figure, a $\beta$ particle enters a region with a magnetic field pointing into paper and an electric field of unknown direction. The $\beta$ particle has no deflection. What is the direction of the electric field ?

The half-life of a radioactive sample is $15$ hours. The initial count rate recorded is $1000$ counts per minute. After $15$ hours, the count rate recorded becomes $528$ counts per minute. What is the background count rate ? (Measured in counts per minute.)

Which of the following descriptions about the half-life of a radioactive substance in a sample is correct ?

Which of the following actions will maximise a person's exposure to radiation ?

Which of the following statements about $\beta$ particles is correct ?