Question 38 Answer and Explanation _ACT December 2013 Form 71E Science Section

Passage VII

The 3 primary interactions between photons (particles of light) and matter are：

· the Compton effect: A: photon collides with, and gives some of its energy to, a free electron (e^-). The photon and e^- are scattered away from the collision site.

· the photoelectric effect: A photon is absorbed by, and gives all of its energy to, an e^- bound to an atom. The e^- is scattered away from the atom.

· pair production: A photon passes close to an atom and is transformed into an e^- and a positron (e⁺, a particle with the same mass as an e^-, but the opposite charge), converting energy to mass. The e^- and e⁺ are scattered away from the nearby atom.

Study 1

A slab of carbon (C) surrounded by particle detectors was bombarded by photons that each had an energy between 0. 01 million electron volts (MeV) and 100 MeV. When an incident photon interacted with the slab and scattered 1 or more particles into the detectors, a successful event was registered. When the photon passed through the slab without interacting, an unsuccessful event was registered. In each successful event, the energy of the incident photon and the type(s) of scattered particle(s) were recorded. When 500,000 successful events had occurred, data from these events were then used to calculate the relative probability (P) for each known type of photon-matter interaction to occur at various incident photon energies. Plots of P versus incident photon energy for the 3 primary interactions are shown in Figure 1.

Study 2

The produce of Study 1 was repeated, except a lead (Pb) slab was bombarded. Plots of P versus incident photon energy for the 3 primary interactions are shown in Figure 2.

38. Which of the following questions CANNOT be answered by an analysis of Figure 2 ?

F. What is P for the photoelectric effect in Pb at an incident photon energy of 1 MeV ?

G. What is the probability of an event being successful in Pb at an incident photon energy of 1 MeV ?

H. Above what incident photon energy is P for pair production higher than 50% in Pb ?

J. At what incident photon energy are P for the photoelectric effect and P for pair production equal in Pb?

Answer and Explanation

Your Answer is

Correct Answer is G

Explanation

First of all, for option F, you only need to find the abscissa as 1 in figure 2 and read the ordinate;

For option H, you can see it in figure 2, probably when When the abscissa is greater than 5, the P of pair production will be greater than 50%;

Finally, for option J, just find the intersection point of photoelectric effect and pair production in figure 2.