P7 Radioactivity. Student Book answers. P7.1 Atoms and radiation. Question Answer Marks Guidance

Transcription

1 P7. Atoms and radiation a radiation from U consists = particles, radiation from lamp = electromagnetic waves, radiation from U is ionising, radiation from lamp is non-ionising b radioactive atoms have unstable nuclei whereas atoms in lamp filament do not, decay of radioactive atom cannot be stopped whereas atoms in lamp filament stop emitting radiation when filament current switched off a i alpha a ii beta or gamma b gamma 3 atoms have unstable nuclei, these nuclei become stable by emitting radiation 4 a substance emits (ionising) radiation so radioactive 4 b paper stopped most radiation from substance reaching Geiger counter, paper absorbed radiation, so must be alpha radiation Oxford University Press 07 This resource sheet may have been changed from the original.

2 P7. The discovery of the nucleus nucleus much smaller than atom, nucleus positively charged, mass of atom concentrated in nucleus all positive charge of atom concentrated in nucleus a B b A: attracted by nucleus C: unaffected by nucleus D: repelled in wrong direction by nucleus 3 a i atoms not indivisible, atoms contain negatively charged electrons 3 a ii any two from: nuclear: all positive charge concentrated in nucleus much smaller than atom, plum pudding: positive charge spread out throughout atom, nuclear: most mass concentrated in nucleus, plum pudding: mass spread out throughout atom nuclear: most atom empty space, plum pudding: no empty space 3 b nuclear model explains why some alpha particles scattered through large angles, in plum pudding model such large-angle scattering should not be observed 4 a similarity: proton and neutron have about same mass (or both found in nucleus) difference: proton is charged whereas neutron has no charge Oxford University Press 07 This resource sheet may have been changed from the original.

3 4 b He nucleus contains 4 (neutrons + protons) whereas H nucleus only contains one = a single proton, protons particles in He nucleus because He nucleus has twice as much charge as H nucleus, other particles in He nucleus are neutrons Oxford University Press 07 This resource sheet may have been changed from the original. 3

4 P7.3 Changes in the nucleus a 6 p + 6 n b 7 p + 33 n c 9 p + 43 n d 4 p 0 n a 9 p + 46 n b 90 p + 44 n c 9 p + 43 n 3 a 35 3 U Th α Cu Zn Bi 84 Po b β 4 β Oxford University Press 07 This resource sheet may have been changed from the original. 4

5 P7.4 More about alpha, beta, and gamma radiation a stops irradiation of nearby people or objects b alpha c α, β a i gamma a ii alpha a iii beta b i gamma b ii alpha 3 a can knock electrons from atoms, this ionisation damages cell (or kills cell or affects genes in cell which can be passed on if cell generates more cells) 3 b (place Geiger tube in a holder so it can be moved horizontally,) move tube so end close to source and Geiger counter detects radiation from source, move tube gradually away from source until count rate decreases significantly, distance from end of tube to source is range of α radiation from source 4 very little γ radiation absorbed by foil, it would all pass straight through so thickness of foil would not affect detector reading Oxford University Press 07 This resource sheet may have been changed from the original. 5

6 P7.5 Activity and half-life a average time for no. nuclei in sample of isotope to halve b 90 cpm a i 4 milligrams a ii milligram b 5% of 8 mg = 0.4 mg so mass < 0.5 mg after 4 half-lives, time taken just over 4 half-lives about 65 hours 3 a i 60 million atoms 3 a ii 3 a iii remaining = = 0 million atoms 3 b after 4 half-lives, count rate = initial count rate of = < 37.5 cpm so time taken to drop to 40 cpm from start < 80 minutes (4 half-lives) 4 after half-lives count rate due to wood = 5% of initial count rate, the wood is 00 years old (= yrs) Oxford University Press 07 This resource sheet may have been changed from the original. 6

7 P7.6 Nuclear radiation in medicine a beta or gamma, can be detected outside body b gamma, radioactive source injected into patient to enter organ to be imaged so needs to do least damage whilst in body, gamma radiation passes through body tissue and detected using gamma camera any two from: food, drink, radon 3 a small seeds of radioactive isotope placed in tumour, radiation from isotope destroys cancer cells, use isotope with half-lives not long enough to damage normal cells surrounding tumour, half-life not too short or unstable nuclei decay before radiation destroys tumour 3 b beta or gamma 4 a too short: radioactive isotope decays too much before scan completed, too long: patient exposed to ionising radiation unnecessarily 4 b too long: after scan radioisotope needs to be stored for a long time until radioactivity insignificant, too short: radioactive isotope decays too much before scan completed Oxford University Press 07 This resource sheet may have been changed from the original. 7

8 5 a any three from: 3 emits radiation detectable outside body (e.g., gamma), non-toxic, short half-life ( 4 hours), decays into stable isotope 5 b stable isotope in body (or elsewhere) not dangerous whereas unstable isotope harmful as it emits ionising radiation harmful to body Oxford University Press 07 This resource sheet may have been changed from the original. 8

9 P7.7 Nuclear fission a nucleus splits into two fragments, releases energy and several neutrons b nucleus absorbs neutron without undergoing fission, forms unstable nucleus which decays (in order) B, A, C, D, B 3 a to absorb fission neutrons, and keep chain reaction under control by maintaining even rate of fission 3 b more fission neutrons absorbed so of fission neutrons in reactor core decreases, rate of release of energy due to fission decreases 3 c thick steel withstands very high temperature and pressure in core, thick concrete walls absorb ionising radiation that escapes through steel walls 4 a A and D 4 b undergone fission and released neutrons and energy 4 c C and E Oxford University Press 07 This resource sheet may have been changed from the original. 9

10 P7.8 Nuclear fusion a formation of nucleus when two smaller nuclei collide and fuse together b 3 forms He nucleus (with protons and single neutron) a so enough KE to overcome force of repulsion between nuclei so they fuse b energy output < energy input so does not produce any energy overall 3 advantages any two from: nuclear fusion fuel easily available fusion products non-radioactive (or less radioactive than) fission products fusion stops if plasma out of control, disadvantages any two from: very large current needed to heat plasma to start fusion plasma difficult to control, at present, fission reactors produce far more power than fusion reactors 4 a p and n 4 b 3 H p He 4 c 3 3 He + He He + p p 4 + Oxford University Press 07 This resource sheet may have been changed from the original. 0

11 P7.9 Nuclear issues a i hazardous and a danger to people and animals if it escapes a ii contains radioactive isotopes with long half-lives b absorbed by surrounding tissues and could damage or kill cells in body or cause cancer outside less dangerous as α radiation has no penetrating power a may be more concentrated than outdoors and people could breathe it in, lungs then exposed to α radiation, ionising effect of α particles in tissue damages cells (or kills cells or causes cancer) b install pipes under house and pump radon gas out of ground before it seeps into house, top of outlet pipe from pump needs to be high up outside house 3 benefits: any two from: no greenhouse gas emissions, reliable and secure electricity supplies, large-scale generation from small sites compared with renewable supplies that take up much larger areas (or other valid points) drawbacks: any two from: long-term storage of nuclear waste, possible escape of radioactive substances into environment, impracticality of fusion reactors, (or other valid points) Oxford University Press 07 This resource sheet may have been changed from the original.

12 4 any five from: total annual dose 400 units/year so risk of death about in /year, some such as cosmic radiation unavoidable, measures reducing total dose by < 0 units/year (e.g. avoiding air travel) have negligible effect, reducing food and drink unlikely to be effective, may be counterproductive due to adverse effects, e.g., cutting food intake by more than half only reduces annual 5 dose by about 3% ( 0.5 ) so reduce radiation risk very slightly but would harm human health in most cases, could reduce risk from medical X-rays where possible by restricting use of X-rays and using MRI instead Oxford University Press 07 This resource sheet may have been changed from the original.