RADIOACTIVITY AND NUCLEAR ENERGY P.1

Transcription

1 RADIOACTIVITY AND NUCLEAR ENERGY P. HKCEE PAPER I 20 7 It is known that plutonium-238 ( Pu) decays by emitting one particle. (a) Write a nuclear equation for the decay of plutonium-238. Use the symbol Y as the daughter nucleus. (b) A sample of plutonium-238 is put in a cloud chamber. Some tracks are seen. (i) Describe the tracks that are seen. (ii) No tracks can be seen when the sample is covered by a piece of paper. Explain. (c) Plutonium-238 can be used in heater units in spacecrafts for outer space missions. It is known that the power of the heater unit is directly proportional to the activity of plutonium-238 contained. Each heater unit has a power of 2 W when it is newly manufactured. How long can a newly manufactured heater unit last if the minimum power output required is 0.25 W? Given : half-life of plutonium-238 = 87.7 years Radon-222 (Rn-222) has a half-life of 3.8 days and undergoes a radioactive decay series as shown in Figure 8 to become a stable nuclide Lead-206 (Pb-206). Mass number 222 Rn Figure Pb (2) () Bi Po Po 206 Pb Atomic number (a) Estimate the mass of undecayed Rn-222 after 5.2 days if its initial mass is 0-5 g. (b) State the nuclear radiation emitted in the process () indicated in Figure 8. ( mark) (c) Write down the nuclear equation for process (2) indicated in Figure 8. (d) Determine the total number of particles and the total number of particles emitted in the radioactive decay series from Rn-222 to Pb-206. (4 marks) (a) A teacher places a radioactive source cm in front of a Geiger-Muller tube (GM tube) and measures the count rate. When he inserts a piece of paper between the radioactive source and the GM tube, he finds that there is no significant change in the measured count rate. State the conclusion about the type of radiation emitted from the radioactive source. ( mark) The teacher then conducts another experiment to investigate the deflection of radiations inside a magnetic field as shown in Figure 2.3. the GM tube can be rotated from 0 o to 80 o around the magnetic field. Figure 24 shows the count rate recorded at different angles with or without the magnetic field.

2 RADIOACTIVITY AND NUCLEAR ENERGY 80 o P.2 Figure 23 radioactive source in a lead box Recoded count rate / counts per minute 0 o region with magnetic field pointing into paper 90 o GM tube connected to counter 250 without magnetic field with magnetic field 50 0 Figure 24 (b) Estimate the count rate due to the background radiation. ( mark) *(c) Using the result in Figure 24, explain why it can be concluded that the radioactive source emits and rays. (5 marks) (d) Estimate the count rate due to each type of radiation at = 90 o without the magnetic field. Write the answer in Table 3. Type of radiation Count rate / counts per minute / o 0 Table 3 2

3 RADIOACTIVITY AND NUCLEAR ENERGY P In a physics lesson, a teacher uses the apparatus shown in Figure 3 to find the range of particles in the air. Describe the procedures of the experiment. (5 marks) Figure Workers of nuclear plants are required to wear film badges (see Figure 4) to monitor their exposure to radiation. Inside the film badge, an opaque plastic bag is wrapped around a sheet of photographic film. Aluminium and lead sheets are also placed inside the badge (see Figure 5) so that the types of incoming radiation can be distinguished. (a) What type(s) of radiation can be detected by the badge? ( mark) (b) Why is an opaque plastic bag used to wrap the photographic film? ( mark) (c) The film of three workers John, Mary and Ken were developed. Table 3 shows the degrees of blackening on different regions of the films inside the badges which they wore. Regions on the film Degree of blackening (0 5) (0 = not blackened; 5 = most blackened) John Mary Ken Beneath the open window Beneath the mm aluminium sheet Beneath the 3 mm aluminium sheet 5 2 Beneath the 5 mm lead sheet Table 3 *(i) Based on the results in Table 3, explain which type(s) of radiation John and Mary are definitely being exposed to respectively. (4 marks) (ii) Give on reason why different degrees of blackening were recorded on the films of Mary and Ken. ( mark) (d) Suggest one hazard of exposure to ionizing radiations. ( mark) 3

4 RADIOACTIVITY AND NUCLEAR ENERGY P Read the following passage about Iodine-3 therapy and answer the questions that follow. Iodine-3 is a radioisotope which emits and radiations. treatment. It can be used for thyroid cancer A patient suffering from thyroid cancer will first undergo an operation to have the thyroid gland removed. However, some thyroid tissue may remain in the neck of the patient or may be carried in the blood stream to other parts of the body. Iodine-3 is then used to trace and get rid of the remaining thyroid tissue in the body. Iodine-3 therapy consists of two stages. In Stage, the patient will take a low does of Iodine-3 to trace the remaining thyroid tissue. A detector is placed near the patient to monitor the activity of the radiation coming from the patient. In case any remaining thyroid tissue is spotted in State, the patient will then take a higher dose of Iodine-3 in Stage 2. The iodine will be absorbed by the thyroid tissue and the radiation emitted can kill the cancer cells. Special hospital rooms are designed for patients who receive Stage 2 of the therapy. The rooms have metallic shielding in the doors and reinforced walls. Inside the rooms, there are plastic covers on the furniture, doors, handles and switches. Source: Iodine-3 Therapy, The Ohio State University Medical Center, (a) Explain why, in Stage, radiation from the patient cannot be detected by the detector. ( mark) (b) In Stage 2, which kind of radiation is more effective in killing caner cells? Explain you answer. (c) State one special feature of the hospital rooms designed for patients receiving State 2 of the therapy and explain its function Carol performs an experiment to measure the half-life of a radioactive source. She places a Geiger- Muller tube in front of the source and the following results are obtained: Time t / hour Count rate / counts per minute Table (a) Plot a graph of the count rate against time in Figure. (4 marks) (b) Estimate the background count rate. ( mark) (c) Estimate the corrected count rate at t = 0. Hence, or otherwise, estimate the half-life of the source to an alarm circuit slot Figure 4 electrode americium source Figure 5 4

5 RADIOACTIVITY AND NUCLEAR ENERGY P.5 Figure 4 shows a smoke detector. The circuit inside the detector is shown in Figure 5. A small 24 amount of the radioisotope americium-24 ( 95 Am ) is placed between two electrodes. The two electrodes are connected to a battery and an alarm circuit. The detector has slots in it to allow air flow. (a) An americium-24 nucleus decays by emitting an -particle to form a daughter nucleus neptunium (Np), with a half-life of 432 years. (i) Write down an equation for the decay of an americium-24 nucleus. (ii) Find the number of neutrons in the daughter nucleus. ( mark) (b) Under normal conditions, a small current flows in the circuit inside the detector. However, when smoke particles enter the detector, the current drops significantly. This triggers the alarm to sound. *(i) Explain why a current flows between the electrodes under normal conditions. (4 marks) (ii) Suggest one possible reason why the current drops when smoke particles enter the detector. (c) Explain why it is preferable for the radioactive source used in smoke detectors to have a long half-life. 4 (d) Carbon-4 ( 6 C ) is a radioisotope which decays by emitting particles and has a half-life of 5700 years. Explain whether this source is suitable for use in smoke detectors or not. (e) People are concerned about the biological hazards of radiation. If you are the manufacturer of the described smoke detector, how would you explain to the public that using the detector will not pose any health hazard? In April 986, a disastrous nuclear accident happened at the Chernobyl Nuclear Station. A large amount of radioactive substances was released and spread to neighbouring counties. The radiation levels recorded in these countries were much higher than the normal background count rate. (a) State two sources of background radiation. (b) State one way by which the radioactive substances released in the accident were spread to neighbouring countries. ( mark) (c) One of the radioactive isotopes released in the accident was caesium-37 (Cs-37). The following equation shows how Cs-37 is produced: *(d) U 0 n x Cs 37 Rb z 0n (i) If z = 4, find the values of x and y and state their physical meanings. (4 marks) (ii) The half-life of Cs-37 is 30 years. Suppose that a soil sample contaminated by Cs- 37 was found to have an initial activity of Bq (disintegrations per second). A physicist comments that the contaminated sample will affect the environment for more than 300 years. Justify the physicist s claim with calculations. You may assume that the activity of a non-contaminated sample of similar nature is 200 Bq. The development of nuclear energy is a controversial issue. Do you support the development of nuclear energy? State your reasoning to support your point of view. (5 marks) y Figure 5 -detector 3 Iodine ( 53 I ) is a radioisotope which decays by emitting a -particle and rays. It is used in hospitals to test the kidneys of patients. During the test, an iodine-3 solution is injected into the bloodstream of a patient. As the blood passes through the kidney, iodine-3 will be absorbed by the kidney and eventually excreted out of the body with urine. If the kidney is not functioning properly, both the absorption and excretion rates of iodine-3 will decrease. A -detector is placed near the kidneys of the patient to detect the activity of the radiation coming from the kidneys (see Figure 5). 5

6 RADIOACTIVITY AND NUCLEAR ENERGY P.6 (a) Using X to denote the daughter nucleus, write down an equation for the decay of iodine-3 nucleus. (b) Explain why the -particles emitted by iodine-3 fail to reach the detector. ( mark) (c) The half-life of iodine-3 is 8 days. (i) State the meaning of half-life (ii) For safety purposes, the activity of iodine-3 solution in the test should not exceed disintegrations per second. When an iodine-3 solution is prepared, its activity is disintegrations per second. How many days after preparation would the solution be suitable for the test? (iii) Figure 6 shows the variation of the activities of the radiation detected from the right and left kidneys of a patient with time. Activity/disintegrations per second Right kidney Figure 6 Left kidney 0 Time/min 200 *(iv) Which kidney do you think is not functioning properly? Explain your answer. Besides iodine-3, technetium-99m is another radioisotope which is used in the kidney test. Technetium-99m emits radiation only and its half-life is 6 hours. Which of these two sources do you think is more preferable for use in the kidney test? Explain you answer. (5 marks) Battery Lead block S Vertical line Figure 4 Radioactive source A B Q P GM tube 20 cm Figure 4 shows a set-up used to study the radiation from a radioactive source. A Geiger-Muller (GM) tube is placed at position P, which is 20 cm from the source. Two coils A and B connected to a battery and a switch S are placed between the source and the GM tube as shown. Initially, S is open and the variation of the count rate recorded by the GM tube with time is shown in Figure 5. Recorded count rate at P/ counts per min Figure (a) Explain why the count rate shown in Figure 5 is not due to particles, no matter what kind of radiation are emitted by the source. (b) Now switch S is closed. The GM tube is placed at positions P and Q in turn (see Figure 4) and the count rates recorded are shown in Figure 6 and 7 respectively. When the GM tube is placed at any point vertically above P, an average count rate of 00 counts per minute is recorded at each point. Time 6

7 RADIOACTIVITY AND NUCLEAR ENERGY P.7 Recorded count rate at P/ counts per min Recorded count rate at P/ counts per min (i) (ii) *(iii) (iv) (c) 0 Time Figure 6 Figure 7 State the direction of the magnetic field formed between coils A and B. ( mark) What kind of radiation is recorded when the GM tube is held at any point vertically above P? Explain your answer. What conclusion about the radiation emitted by the source can you draw from Figures 6 and 7? Explain your answer. (5 marks) Explain why the sum of the average count rates recorded in Figures 6 and 7 is greater than that recorded in Figure 5. The above experiment cannot determined whether particles are emitted by the source. Suggest a method for finding out the answer. 0 Time 2000 (a) X and Y are two radioactive nuclides with half lives of 2 hours and 2.6 years respectively. Both nuclides decay by emitting particle to form stable product nuclides. (i) After emitting particle, how would the atomic number and mass number of nuclide X be changed (ii) Describe the changes in activity (in disintegrations per second) of a specimen of nuclide X and a specimen of Y after one day. (iii) Comment on the following statement: The mass of a specimen of a nuclide X will be reduced by approximately half in 2 hours. (b) A factory produces aluminium sheets mm in thickness. The thickness of the sheets is monitored by a gauge. (See Figure 6.) A source is used in the gauge. To counter * Figure 6 Aluminium sheet Detector Radioactive source (i) Explain why and sources are not used in the gauge. (ii) Which of the nuclides (X or Y) is more suitable to use as the radioactive source? Explain your answer. *(iii) The count rate recorded should be around 90 counts per second when the thickness of the aluminium sheet is mm. In a certain day when the gauge is operating properly, the following data are recorded: Time/s Recorded count rate/counts per s Table 3 Describe and explain the variation in the readings in Table 3. (5 marks) HKCEE PAPER II Which of the following statements about, and radiations is incorrect? A. Only radiation can travel through a vacuum. B. radiation can be stopped by an aluminium plate of 5 mm thick. C. particles are fast moving electrons. D. radiation can blacken a photographic film. 7

8 RADIOACTIVITY AND NUCLEAR ENERGY P A radioactive source is put in front of a GM tube. The initial count rate is 050 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 (in counts per minute) after 8 hours? A. 50 B. 25 C. 250 D A U nuclide undergoes a certain number of and decays and becomes 82 Pb. Find the number of particles emitted. A. 2 B. 3 C. 4 D st statement: In the production of X-ray, the speed of the X-ray depends on the speed of the electron striking the heavy metal target. 2 nd statement: X-rays are produced when a heavy metal target is struck by fast moving electrons. st statement 2 nd statement A. True True, and is a correct explanation of the st statement. B. True True, and is not a correct explanation of the st statement. C. True False D. False True V X GM tube radioactive source in a lead box Y P Q R The figure above 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 ( ), 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 the paper is applied between X and Y? A. The count rate at P decreases and the count rate at Q remains the same. B. The count rates at P and Q remain the same. C. The count rate at P decreases and the count rates at Q and R increase. D. The count rates at P, Q and R are equal H H 2H e n Which of the following descriptions about the nuclear reaction above is correct? A. It is nuclear fission. B. It is nuclear fusion. C. It is a chain reaction. D. It is a radioactive decay. 0 V 8

9 RADIOACTIVITY AND NUCLEAR ENERGY P α X Y Z The above shows part of a decay series. Which of the following deductions is/are correct? () X and Z are isotopes of the same element. (2) X has two more neutrons than Z. (3) Z has one more proton than Y. A. () only B. (3) only C. () and (2) only D. (2) and (3) only β The half-life of a radioactive sample is 5 hours. The initial count rate recorded is 000 counts per minute. After 5 hours, the count rate recorded becomes 528 counts per minute. What is the background count rate? (Measured in counts per minute.) A. 25 B. 28 C. 50 D Which of the following conditions is/are necessary to sustain the chain reaction in the nuclear fission of uranium-235? () Each fission produces a large amount of energy. (2) At least one neutron is released in each fission. (3) Each fission produces two smaller nuclei. A. () only B. (2) only C. () and (3) only D. (2) and (3) only st statement: particles are emitted from the radioactive nucleus in a decay. 2 nd statement: particles are fast moving electrons. st statement 2 nd statement A. True True, and is a correct explanation of the st statement. B. True True, and is not a correct explanation of the st statement. C. True False D. False True Which of the following descriptions about the half-life of a radioactive substance is correct? A. It is equal to half of the time for all the radioactive nuclei of the substance to decay. B. It is equal to half of the time for a radioactive nucleus of the substance to decay. C. It is equal to the time for the substance to reduce its mass by half. D. It is equal to the time for half of the radioactive nuclei of the substance to decay Which of the following actions will maximize a person s exposure to radiation? A. Using a GM tube and counter to measure the background radiation in laboratory. B. Eating food that has been sterilized by exposure to gamma radiation. C. Listening to radio. D. Going for a flight to a distant place in a high-flying aeroplane Which of the following nuclear reactions is a fission? A. U n Ba Kr 3 n U 0n 94 Pu 2- B. e 0 9

10 RADIOACTIVITY AND NUCLEAR ENERGY P H H2 He 0 0 C. U N Es 4 n D. n Which of the following statements about particles is correct? A. particles carry positive charge. B. particles can be deflected by a magnetic field. C. particles cannot be deflected by an electric field. D. particles can be stopped by a sheet of paper Susan performs an experiment in which a radioactive source is placed closely in front of a GM counter. The graph below shows the variation of count rate with time. Count rate / counts per minute Which is the half-life of the radioactive substance? A. 4 minutes B. 5 minutes C. 8 minutes D. 0 minutes Time / minute In the upper atmosphere, neutrons are produced by the action of cosmic rays. These neutrons interact with nitrogen nuclei as shown in the following reaction: 4 0 n 7 N X H Element X will then emit a particle. The reaction is as follows: 0 X Y - What is the final product Y? 4 A. C 6 3 B. C C. C 3 D. 7 C Some dangers substances are stored in a metal container inside a wooden box as shown in the figure below. What metal should be used for the container and what type of substance is stored? Metal used Type of substance stored A. Iron Radioactive B. Iron Flammable C. Lead Radioactive D. Lead Flammable 0

11 RADIOACTIVITY AND NUCLEAR ENERGY P Some fresh foods are exposed to radiations from radioactive isotopes for a short time so that the micro-organisms in the foods can be killed. Why are the irradiated foods not harmful to people who eat them? A. radiation is an electromagnetic wave. B. radiation has a high penetrating power. C. radiation does not have a high ionizing power. D. radiation does not make the foods radioactive A radioisotope X has a half-life of 2 days while another radioisotope Y has a half-life of day. Initially there are N undecayed atoms of X and 8 N undecayed atoms of Y. After how many days will X and Y have the same number of undecayed atoms? A. 3 days B. 4 days C. 6 days D. 8 days X Y Z P Q R S In the above two decay series, P and Y are two isotopes. Which of the following pairs of nuclides are isotopes to each other? () X and R (2) Y and S (3) Z and Q A. () and (2) only B. () and (3) only C. (2) and (3) only D. (), (2) and (3) source lead castle P Q Geiger-Muller tube A radioactive source is placed in front of a uniform magnetic field pointing into paper as shown above. If a high count rate is recorded at positions P and Q, what kinds of radiation have been detected? P Q A. B. C. D A thorium nucleus ( 90 Th ) decays by emitting a particle to form a daughter nucleus X. Which of the following equations represents this decay? A. Th X B. Th X C. Th X D. Th X

12 RADIOACTIVITY AND NUCLEAR ENERGY P Which of the following nuclear reactions is a nuclear fusion? A. U Ba Kr 2n N n 6 C H H2 He U 90Th B. H C. n D x 4 H H y Z a neutron Find the values of x and y in the above nuclear reaction. x y A. 2 B. 2 2 C. 3 D Different absorbers are placed in turn between a radioactive source and a Geiger-Muller tube. Three readings are taken for each absorber. The following data are obtained: Absorber Count rate / s Paper mm aluminium mm lead mm lead What type(s) of radiation does the source emit? A. only B. only C. and only D., and In order to detect cracks in an underground oil pipe, an engineer proposes adding a radioactive source to the oil. Which of the following sources is most suitable? A. a soruce with a half-life of a few hours B. a source with a half-life of several years C. a source with a half-life of a few hours D. a source with a half-life of several years Which of the following statements about particles and rays is/are correct? () They can both be deflected by a magnetic field. (2) particles have a stronger ionizing power than rays. (3) They are emitted with almost the same speed in radioactive decay. A. () only B. (2) only C. () and (3) only D. (2) and (3) only Activity of P/Bq 800 Activity of Q/Bq Time /min Time /min 2

13 RADIOACTIVITY AND NUCLEAR ENERGY P.3 The figures above show the variation of the activities of two radioactive sources P and Q with time. Find the ratio of the half-life of P to that of Q. A. : B. : 2 C. 2 : D. 4 : Which of the following are essential criteria in choosing radioactive sources as medical tracers in human bodies? () The sources should have a short half-life. (2) The radiation emitted should have a weak ionizing power. (3) The radiation emitted should not be deflected by an electric field. A. () and (2) only B. () and (3) only C. (2) and (3) only D. (), (2) and (3) A radioactive isotope 234 Th undergoes a series of decay processes to form a daughter nuclide Th How many -particles and -particles have been emitted in this decay process? No. of -particles No. of -particles A. 6 7 B. 7 6 C. 7 8 D Which of the following particles cannot be deflected by a magnetic field? A. -particles B. -particles C. neutrons D. protons Which of the following is/are application(s) of radioactiviy? () estimate the age of ancient remains (2) to kill bacteria in food (3) to transmit signals over long distances A. (2) only B. (3) only C. () and (2) only D. () and (3) only Radium ( 88 Ra ) decays by emitting an particle to form a product nucleus X. Which of the following shows the correct equation for this decay? A. Ra X Ra Ra 84 X Ra 86 X Ra 89 X B. X C. D. E The initial activity of a radioactive isotope is 2000 disintegrations per second. After 4 hours, the activity of the isotope drops to 25 disintegrations per second. Find the half-life of the isotope. A. 5 minutes 3

14 RADIOACTIVITY AND NUCLEAR ENERGY P.4 B. 30 minutes C. 48 minutes D. 60 minutes E. 80 minutes P Q R S The above shows part of a radioactive series. Which of the following nuclei are isotopes of the same element? A. P and Q B. P and S C. P and R D. Q and S E. Q, R and S Which of the following statements about particles and rays is correct? A. Both of them carry charges. B. Both of them are transverse waves. C. Both of then can be deflected by a magnetic field. D. Both of them have strong ionizing power. E. Both of them can travel through a vacuum Which of the following is not a safety precaution for handling radioactive sources? A. Radioactive sources should be handled with forceps. B. Users should not eat or drink when handling radioactive sources. C. Users should wash their hands after handling radioactive sources. D. Radioactive sources should not be held close to the eye for visual examination. E. Radioactive sources should be stored in wooden boxes after use st statement: sources are not suitable for injection into human bodies as medical tracers. 2 nd statement: particles carry positive charges and can be deflected by electric fields. st statement 2 nd statement A. Ture True, and is a correct explanation of the st statement. B. True True, and is not a correct explanation of the st statement. C. True False D. False True E. False False HKAL PAPER I (a) A nucleus of radon ( Rn) decays to an isotope of polonium (Po) by emitting an -particle. Given: mass of a radon nucleus = u mass of a polonium nucleus = u mass of an -particle = u u = kg, which is equivalent to 93 MeV charge of an electron e = C (i) Write an equation for the decay and find the energy released, in MeV, in the decay. (ii) The energy released in the decay becomes the kinetic energy of the decay products. Explain quantitatively why the -particle takes most of the energy and find its speed v. Assume that the parent nucleus is at rest initially. (4 marks) (b) (i) -particles with the speed v found in (a)(ii) are directed into an evacuated region with a uniform magnetic field B = 0.5 T normal to the initial velocity of the -particles as shown in Figure 4.. Find the radius of the path described by the -particles in the field region. 4

15 RADIOACTIVITY AND NUCLEAR ENERGY P.5 (ii) Given: the charge to mass ratio of an -particle is C kg Figure 4. uniform magnetic field B pointing into the paper Explain whether the -particles would emerge with a greater speed from the field Carbon-4 dating is used to determine the age of archeological sample by measuring its activity due to the carbon-4 remaining in it. (Given: molar mass of carbon-2 = 2.0 g, half-life of carbon-4 t /2 = 5730 years and year is equivalent to seconds) (a) (i) Calculate the decay constant, in s -, of carbon-4. (ii) It is known that the relative abundance of carbon-4 in living things is that there is only one carbon-4 atom for every atoms of carbon-2. Determine the number of carbon-4 atoms decaying per second for g of carbon in living things. (b) (i) Explain the origin of carbon-4 in the atmosphere and why the abundance of carbon- 4 in living things, such as plants, remains more or less constant. (ii) After corrected for background radiation, an archeology measured an activity of 20 disintegrations per minute from 0 g of carbon in a piece of bone. Use the result in (c) (a) to determine the approximate age of the bone. Give one reason why carbon-4 dating needs correction as there have been open-air nuclear tests since 940. Explain how the result of dating would be affected without such corrections (a) The graph shows the number of ion-pairs produced per mm by a certain type of nuclear radiation versus distance along its track in air. No. of ion-pairs formed per mm -particles of speed v (b) 2000 Distance traveled from source / mm (i) What type of nuclear radiation is represented by the graph? Given one piece of evidence. (ii) Use the graph to estimate (I) the total number of ion-pairs produced by the radiation. (II) the total energy of the radiation in MeV. (Given: average energy required to produce an ion-pair is J electronic charge, e = C) (iii) Explain why there is a peak near the end of the track. ( mark) β, γ β, γ X Y Z The above series shows the decay of a radioactive isotope X to isotope Y and finally to isotope Z. The half-lives of X and Y are 24 days and 72 s respectively. The half-life of Z is much 5

16 RADIOACTIVITY AND NUCLEAR ENERGY P.6 longer than 24 days. The disintegration of both X and Y would each emit a - and - radiation. A sample containing mg of pure X only is prepared initially. (Given: Avogadro constant = mol - Molar mass of X g) (i) Find the total number of nuclear radiations that could be emitted from the sample. (ii) Estimate the time required for the activity of the sample to decrease by 0 % People are often killed in a fire as a result of suffocation. To minimize the loss of lives, smoke detectors can be installed in buildings and a loud sound and a flashing light are triggered when smoke is detected. Figure 5. shows a common ionization smoke detector which has a small radioactive source inside. During normal operation, the radioactive source keeps emitting ionizing particles and a certain ionization current is maintained inside the chamber of the detector. Figure 5. (a) (b) The manufacturer claims that the radioactive source in the smoke detector presents no hazard to health in normal use. Comment on this claim and briefly explain which type of radioactive source should be used in the smoke detector. When the radioactive source in the detector is placed close to a GM-tube, the count rate measure is 2000 s -. The average number of ion-pairs produced by each radiation particle is and each ion carries a charge of.6 0 C. (i) Estimate the maximum ionization current in the smoke detector. Why is the ionization current in the smoke detector significantly less than this maximum value? (ii) The smoke detector should be disposed of when its maximum ionization current 2 drops below 50 A. The manufacturer claims that the life of the detector is 0 years. Estimate the half-life of the radioactive source used in the detector A reaction which takes place in the core of a nuclear reactor is described by the following equation: U 0n 56Ba36Kr 30 n 74.4MeV Mass of one nuclide of Mass of one nuclide of Mass of one nuclide of U = u Ba =4.964 u 9 36 Kr = u 27 u (atomic mass unit).6600 kg, which corresponds to 934 MeV 9 (electronic charge =.6 0 C) (a) (i) Calculate the mass, in atomic mass unit, of a neutron. (ii) 4 The fuel rods in the reactor contain.0 0 kg of U-235 isotope. Calculate the total energy released from the complete fission of all the U-235 nuclei in the fuel rods. (iii) If the mean power output of the reactor is 500 MW and the efficiency of conversion of nuclear energy to electrical energy is 40 %, estimate the time for which the fuel rods can be used. (iv) Explain why the fuel rods are usually replaced well before the time estimated in (a) (iii) has elapsed. (b) In an emergency, explain how the reactor can be shut down immediately. 6

17 RADIOACTIVITY AND NUCLEAR ENERGY P In an experiment to investigate the absorption of and rays by materials, a source emitting and rays is placed at a distance of about 5 cm from a G-M tube as shown in Figure 0. Figure 0. The count rates, N, are measured for different thickness, d, of absorber plates. The results are shown in Figure 0.2, with curve A corresponding to the measurements using aluminium absorber plates while curve B corresponds to those using lead absorber plates. N/s - 60 source 24 days (A) absorbers G-M tube Figure (B) aluminium 20 lead 0 0 d/mm (a) (i) Curve A shows a considerable decreases in count rate up to a thickness of 7 mm; a further increase in d only results in slight decrease in count rates. Explain why this is so. (ii) From the graphs in Figure 0.2, estimate the minimum thickness of lead needed to absorb most of the rays. ( mark) (iii) The source also emits -particles. Explain why their effects ca n be neglected in this experiment. ( mark) A geologists wants to find the age of a sample of rock containing 40 K which decays to give the stable isotope 40 Ar. The activity of the sample is found to be.6 Bq while the original activity of a similar rock having the same mass is 4.8 Bq. The half-life of 40 K is.3 x 0 9 years. (a) (i) Find the decay constant of 40 K. (ii) Give the physical meaning of the decay constant of a radioactive isotope. (b) Find the age of the rock sample. (c) Give two factors that determine the activity of a radioactive source. (a) The decay of 40 K to 40 Ar is spontaneous. How is the magnitude of the binding energy of 40 K compared to that of 40 Ar? (e) Mention a difficulty involved in measuring such a small decay rate of.6 Bq. ( mark) 7

18 Neutron number RADIOACTIVITY AND NUCLEAR ENERGY P.8 AL93-IIB2 235 (a) Figure 2. shows the decay series for 92 U Th Pa U Figure (b) (c) Atomic number (i) Name the particles emitted when Uranium (U) decays to Thorium (Th) and Thorium (Th) decays to Protactinium (Pa). ( mark) U Th : Th Pa: (ii) Given that the half-life of 92U is 7.0 years, what will be the percentage of U left after 0 8 years? The following equation represents a possible nuclear reaction in a fission reactor: Given : the mass of one nuclide of U 0n36Kr 56Ba 30 n u, 235 U 0n.0087u, 9 36Kr u, 42 Ba u, 27 unified atomic mass unit u.660 kg. (i) According to the above equation, what is the mass defect between the reactants and 235 products when one 92U nucleus undergoes fission? (ii) If kg of 92U splits per second, calculate the rate of energy production. Explain how energy can be extracted from the core of a fission reactor for producing electricity In a hospital, radioactive waste produced by iodine-25 is stored in a special store-room before final disposal. Batches of waste with activity 00 Ci are deposited in the store-room regularly at 30-day intervals. It is found that the activity of the waste material in the store builds up gradually at first, but reaches a steady value later, as shown in Figure 5. Figure 5 (a) (i) Explain why the activity of the waste material will not increase further even though new batches are deposited every 30 days. 8

19 RADIOACTIVITY AND NUCLEAR ENERGY P.9 (b) (c) (ii) Suppose the half-life of iodine-25 is 60 days, estimate the activity in the store-room when it reaches a steady value. After five years, this hospital stops using iodine-25. Before the waste material can be disposed of it has to be kept until its activity falls to below 3Ci. Calculate the storage period required. Iodine-25 nuclides emit low energy gamma-ray photons. Suggest two precautions that should be taken in handling this kind of radionuclide. HKAL PAPER II Radioactive nuclides X and Y have half-lives 2 hours and 4 hours respectively. The decay of both nuclides gives stable daughters. Initially samples P and Q contain equal number of atoms of nuclide X and nuclide Y respectively. Which statements are correct? () The initial activity of sample P is higher than that of sample Q. (2) After 8 hours, sample P contains more active nuclei than sample Q. (3) After 8 hours, the chance of a nucleus of X in sample P decaying in the next minute is greater than that of a nucleus of Y in sample Q. A. () and (2) only B. () and (3) only C. (2) and (3) only D. (), (2) and (3) A cancer research project employs a certain radioactive source having a half-life of 5.3 years. The source has been prepared for some time. Now the research treatment requires 0 minutes of irradiation on rats with cancer using the source. If two years later the same source is to be used for this kind of treatment, estimate the irradiation time required in order to have the same radiation dosage. A. 3 minutes B. 5 minutes C. 8 minutes D. It cannot be estimated as the time when the source was freshly prepared is not known In -decay a neutron inside the nucleus changes into a proton and an electron is emitted as a -particle Radioactive nuclide plutonium 94 Pu becomes lead 82 Pb after a series of - and -decays. Throughout the whole process, how many neutrons inside a change? A. 3 B. 6 C. 9 D Pu nucleus have undergone such A radioactive source consists of atoms of nuclide P of half-life 2 days. Another source consists of atoms of nuclide Q of half-life 3 days. After how long will the number of active nuclei in the two sources be equal? (Assume that the daughter nuclides of both P and Q are stable.) A. 6 days B. 9 days C. 2 days D. 8 days Which of the following gives the correct meaning of the decay constant of a radioactive substance? A. It is the rate of disintegrations of the substance. B. It is the number of disintegrations of the substance occurring on one half-life of the substance. C. It is the fraction of the active nuclei present that decay in one second. D. It is equal to the reciprocal of the half-life of the substance. 9

20 RADIOACTIVITY AND NUCLEAR ENERGY P Some typical radiation doses are tabulated as follows: Radiation dose Watch television msv/hr for watching television every day in a year Flying in an aircraft 0.00 msv/hr X-ray check msv each time Arrange the following in ascending order of total radiation dose in one year. () Watching television for 4 hours every day (2) Travelling on an aircraft for 0 hours every month (3) Taking X-ray check every 6 months A. (), (2), (3) B. (2), (), (3) C. (), (3), (2) D. (3), (), (2) The following equation represents a nuclear fission reaction, producing q neutrons U 0n 56 Ba ZKr q0n What are the values of the atomic number Z and the number q? Z q A B C D A detector placed near a source of gamma radiation records a count rate of 960 counts per second. The half-life of the source is 2 hours. A slab of material of thickness 6 cm is then placed between the source and the detector. The `half-value thickness of the material is 2 cm (i.e. the intensity of the radiation would reduce by half after passing through 2 cm of the material). Estimate the count rate recorded by the detector after one day. (Neglect any background radiation.) A. 60 counts per second B. 80 counts per second C. 60 counts per second D. 30 counts per second On which of the following does the activity of a radioactive source depend? () the number of active nuclides in the source (2) the half-life of the source (3) the nature of the nuclear radiation emitted by the source A. () only B. (3) only C. () and (2) only D. (2) and (3) only The activity of a freshly prepared sample of 60 C is.0 x 0 6 Bq. The half-life of 60 C is 5.3 years. Estimate the number of 60 C nuclei in the sample that decay in the first day. A. 5.2 x 0 2 B. 3.2 x 0 8 C. 8.6 x 0 0 D. It cannot be estimated as the initial number of nuclei in the sample is not given The following nuclear reaction represents the two deuterons, 3 isotope, 2 He, with the release of energy H 2 He X energy 2 H, which combine to form a helium 20

21 RADIOACTIVITY AND NUCLEAR ENERGY P.2 Which of the following statements are correct? () This is an example of nuclear fusion. 3 (2) The total mass of 2 He and X is greater than that of the two H (3) X is a neutron. A. () and (3) only B. () and (2) only C. (2) and (3) only D. (), (2) and (3) The sun and stars generate their energy mainly by () radioactive decay. (2) nuclear fission. (3) nuclear fusion. A. () only B. (3) only C. () and (2) only D. (2) and (3) only A nuclide in a radioactive sample has a constant chance of approximate half-life of the sample? A. day B. week C. month D. year 6 0 to decay in one second. What is the The average background count rate, in s -, in Hong Kong is of the order of A B C D The activity of a radioactive sample was 70 Bq at time t = 5 minutes and 49 Bq at t = 0 minutes. Deduce its activity at time t = 0. A. 2 Bq B. 00 Bq C. 95 Bq D. 9 Bq E. 80 Bq The table below gives the corrected count rate (in counts per minute) from three samples of radioactive isotopes at three different times. Isotope 0 min 20 min 40 min X Y Z The above data show that () X produces the most penetrating radiation. (2) Y has the largest decay constant. (3) Z has the longest half-life. A. () only B. (3) only 2

22 RADIOACTIVITY AND NUCLEAR ENERGY P.22 C. () and (2) only D. (2) and (3) only E. (), (2) and (3) A GM counter is placed close to and in front of a radioactivity source which emits both and radiation. The count rate recorded is 500 counts per minute while the background count rate is 50 counts per minute. Three different materials are placed in turn between the source and the counter. The following results are obtained. Material (Nil) Cardboard mm of aluminum 5 mm of lead Recorded count rate / count per minute 500 x y z Which of the following is a suitable set of values of x, y and z? x y z A B C D E The following equations represent some typical nuclear reactions: (I) Be H Li He (II) H H2He0n (III) 92U 0n 57La35Br 30 n Which of the following descriptions of these reactions is/are correct? () Reaction (I) represents a spontaneous -decay. (2) Reaction (II) represents a nuclear fusion. (3) Reaction (III) represents a chain reaction A. () only B. (3) only C. () and (2) only D. (2) and (3) only E. (), (2) and (3) Carbon-4 is radioactive and undergoes -decay with a half-life of about 5600 years. A GM tube together with a scalar are used in an experiment to determine the concentration of carbon-4 in piece of excavated wood. To find an accurate value for the activity of the excavated wood, the experimenter should () prepare a sample of pure carbon-4 from the piece of excavated wood. (2) take a count over as long a period of time as possible. (3) take a second count without the presence of the excavated wood. A. () only B. (3) only C. () and (2) only D. (2) and (3) only E. (), (2) and (3) The activity of a sample of radioactive isotopes decreases to /3 of its initial value in 2 s. How much more time would be required for the activity to decrease to /9 of its initial value? A. 4 s B. 8 s C. 2 s D. 6 s E. 24 s 22

23 RADIOACTIVITY AND NUCLEAR ENERGY P A radioactive sample, initially consists of only nuclide X, decays by the emission of an alpha particle to form a stable daughter nuclide Y. Which of the following quantities will decrease with time? () The rate of decay of nuclide X. (2) The rate of growth of nuclide Y. (3) The rate of emission of alpha particles. A. () only B. (3) only C. () and (2) only D. (2) and (3) only E. (), (2) and (3) When several neutrons and protons come together to form a stable nucleus, which of the following statements is/are correct? () Energy is released. (2) The mass of the nucleus is smaller than the sum of the mass of the individual nucleons. (3) In the nucleus, the electrostatic repulsion between two protons is overcome by the nuclear force between them. A. () only B. (3) only C. () and (2) only D. (2) and (3) only E. (), (2) and (3) Ra is one of the nuclides in the uranium decay series. If the stable end-product of this series is Pb, the number of -particles emitted between the 88Ra stage and the end of the series is A. 4 B. 6 C. 0 D. 4 E A radioactive source is test as follows: Absorber placed between Effect on count rate Source and GM tube () Thin aluminum foil (0.2 mm) (2) Thin lead sheet (2 mm) (3) Thick lead sheet (20 mm) What type(s) of radiation does the source emit? A. only B. only C. only D. and only E. and only Fall appreciably. No significant difference with (). Falls below that in () A counter is placed near a very weak radioactive source which has a half-life of hour. The counter registers 00 counts/min at noon and 80 counts /min at p.m. The expected count rate, in counts/min, at 3 p.m. on the same day is A. 40 B. 50 C. 55 D. 60 E

24 RADIOACTIVITY AND NUCLEAR ENERGY P Rn decays to 86 Rn with a half-life of 600 years. Which of the following statements is/are correct? () particle is produced in the decays. (2) 222 All 88 Rn has decayed after years. (3) 226 The half-life of 88 Rn can be shortened by heating. A. () only B. (3) only C. () and (2) only D. (2) and (3) only E. (), (2) and (3) A detector is used for monitoring an source and a reading of 20 units is observed. After a time equal to the half-life of the source, the reading has fallen to 64 units. If a 5 mm thick lead sheet is inserted between the source and the detector, the reading would probably be A. 0 unit B. 4 units C. 8 units D. 6 units E. 32 units A stationary uranium-238 nucleus undergoes -decay. What is the ratio of the kinetic energy of the daughter nucleus to that of the -particle? A. 238:4 B. 4:238 C. 234:4 D. 4:234 E. : The diagram shows an alpha-particle colliding head-on with an atomic nucleus. At P, the alphaparticle is at the closest distance r o from the nucleus. Which of the following statements is/are correct? () At P, the electric potential energy of the system is maximum (2) r o is of the order 0-4 m (3) r o gives an upper limit for the sum of the radii of the alpha-particle and the nucleus. A. () only B. (3) only particle r 0 C. () and (2) only + atomic nucleus D. (2) and (3) only P E. (), (2) and (3) The number of radioactive nuclides in two different samples P and Q are initially 4N and N respectively. If the half-life of P is t and that of Q is 2t, the number of radioactive nuclides in P will be the same as the number of radioactive nuclides in Q after a time of A. t/2 B. t C. 2t D. 4t E. 8t A radioactive source consists of a mixture of two radioisotopes P and Q. P has a half-life of hour and Q has a half-life of 2 hours. Both P and Q have stable daughter nuclei. The initial activity recorded by a counter is 600/min. After 4 hours the counter registers an activity of 60 counts per min. What was the contribution of P to the initial count rate (in counts per min.)? 24

25 RADIOACTIVITY AND NUCLEAR ENERGY P.25 A. 20 B. 200 C. 360 D. 400 E N7 alpha particle proton X In the above nuclear reaction, X is A. B. C. D. E O. F. N. C. 8 F A radioactive source is placed in front of a GM tube connected to a counter. Various absorbers are placed between the source and the GM tube and the count-rate recorded. The following results were obtained: Absorber Counts per minute No absorber 7 A sheet of paper mm thick aluminum mm thick lead block 28 It can be deduced from these results that the radiation(s) emitted by the source is/are A. and rays only B. and rays only C. rays only D. rays only E. rays only A neutral particle decays in a short time into a proton and a negative particle. The initial and subsequent paths of the particles in a magnetic field acting into the plane of the paper are shown above. Which of the following gives the respective paths for the neutral particle, the proton and the negative particle? Neutral particle The proton Negative particle A. R S T B. R T S C. S R T D. S T R E. T R S A helium atom, a hydrogen atom and a neutron have masses of u,.008 u and.009 u (unified atomic mass units) respectively. Assuming that hydrogen atoms and neutrons can fuse to form helium, the binding energy of a helium nucleus is A u. B u C..07 u. D u. E u. S R T 25

26 RADIOACTIVITY AND NUCLEAR ENERGY P Proactinium extract from a solution of uranyl nitrate decays with a half-life of 72 s. The value of the decay constant is 3 A s. 3 B s -. C s. D s. E s An alpha particle collides with a stationary helium nucleus ( 2 He ) in a cloud chamber. Which of the following diagrams represents the most probable set of tracks? A. B. C. He He D. E. He A radioactive source of gamma rays has a half-life of 2 days. A geiger counter placed 3 m from the source initially has a count-rate of 440 per minute. After 6 days the counter is moved back to a distance of 9 m from the source, and its count-rate, in counts per minute, is then A. 20 B. 60 C. 80 D. 320 E A radioactive source with half-life t / 2 initially contains N atoms of the radioactive element. The energy released in each disintegration is E. What is the total energy relaesed in time 2 t / 2? A. NE 4 B. NE 2 3 C. NE 4 D. NEt / 2 2 He 26

27 RADIOACTIVITY AND NUCLEAR ENERGY P A stationary radioactive nucleus of mass N units emits an alpha particle of mass 4 units, leaving a residual nucleus of mass ( N 4 ) units. The ratio of the kinetic energy of the alpha particle to the kinetic energy of the residual nucleus is N 4 A. 4 B. C. D. E. N 2 ( N 4) ( N 4) N 4 2 ( N 4) 4 2 ( N 4) Which of the graphs below correctly represents the variation of the activity X of a radioactive sample with the number N of undecayed nuclei in the sample? A. B. C. D. E. X X X X X N N N N N 27