A-LEVEL PHYSICS 7408/3A Paper 3 Section A Mark scheme June 2019 Version: 1.0 Fina

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Question Answers Additional Comments/Guidelines Mark 01.1 1 idea of maximising distance use tongs / tweezers / handling tool (when handling source to keep as far away from source as possible) ... OR keep at least 2 metres away (if observing) 2 idea of limiting exposure time during experiment remove source from lab / room when not in use / after experiment OR idea of put / replace / keep source in a castle / container when not in use / in the open / after experiment or wtte 3 is about shielding using a named absorber stand behind a lead absorber / screen (when source is in the open) 4 is safe use of source when removed from castle never point (open end of) the source at anyone / at yourself OR do not look directly at / look into the source 5 is about good practice read local rules (about the use of radioactive sources) / OR read / post warning / notice on the door award 1 mark for each valid procedure (unless contradicted) do not award more than 1 mark for safety procedure 1 and do not award more than 1 mark for safety procedure 2 do not credit the same marking point for different do not award more than 1 mark for safety procedure 1 procedures for 1 treat as neutral: ‘keep source at arm’s length / far away’ / ‘use pliers’ / don’t go close (to the source) for 2 do not insist on ‘lead’ treat as neutral: ‘use a lead container’ treat as neutral: ‘limit time of exposure’ / ‘work as quickly as possible’ / ‘don’t keep source out of box for too long’ / ‘keep source sealed’ for 3 accept aluminium or steel for lead; use of lead apron for 4 accept ‘avoid eyes’ treat as neutral: ‘avoid direct contact’ / ‘don’t touch source’ / ‘always point source at ground’ for 5 accept ‘report any damage to a source’ treat as neutral: ‘use safety screen’ / ‘don’t stand in front’ / ‘don’t ingest / swallow’ / ‘wash hands after use’ / ‘wear safety glasses / goggles / gloves / lab coats’/ ‘use film badge’ no credit for procedures that are the responsibility of the teacher / radiation protection adviser, eg ‘obtained signed consent form’ AO2-1g = MAX 2 MARK SCHEME – A-LEVEL PHYSICS – 7408/3A – JUNE 2019 7 01.2 5  correct answer only AO3-1a = 1 01.3 A OR Rn 222 86 / radon 222 / Rn 222  B OR Po 218 84 / polonium 218 / Po 218  I OR Pb 210 82 / lead 210 / Pb 210  if candidates have provided more than 3 responses, each extra error / contradiction negates one correct response; if there are 3 or more errors / contradictions, award no marks answers may be given in any order accept unnamed isotope with correct A, Z eg X 222 86 eg while the suggestion that A, B and I are correct earns 3 the suggestion that A, B, I and J are correct earns 3  1 = 2 AO3-1a = 3 MARK SCHEME – A-LEVEL PHYSICS – 7408/3A – JUNE 2019 8 01.4 suitable procedures to eliminate systematic error are: remove (all radioactive) source(s) (from the room) OR measure Ab before the source is present / in another room OR put source in (lead) castle / (lead) container / behind a (lead) absorber 1 zero / reset the counter AND/OR stopwatch (before use) OR check the counter AND/OR stopwatch has no zero error 2 measure Ab on same day as experiment is carried out 3 measure Ab in same room / location / area as that where experiment is to be carried out 4 suitable procedure to reduce percentage uncertainty in Ab is: use long(er) (integration) time / prolonged time OR (total of) at least 100 s 5 do not insist on references to systematic or random error but give no credit for explicit talk-out eg ‘use a long integration time to eliminate systematic error’ for 1 treat as neutral: ‘measure Ab with no source near’ / ‘check source is shielded / far away / out of range / sealed’ / ‘in another area’ / ‘point detector away from the source (or vice-versa)’ / ‘don’t point source at counter’ / ‘put detector behind source’ for 2 treat as neutral: ‘zero / reset the equipment (before use)’ / ‘zero the detector’ for 3 treat as neutral: ‘measure Ab after experiment to double-check’ for 4 treat as neutral: / ‘keep detector in the same position’ / ‘measure Ab in different positions’ for 5 accept idea of a suggested total time, taking account of repeats, exceeding 100 s (eg 10 repeated 10 s counts and 1 single 100 s single count amount to the same thing) treat as neutral: ‘repeat and average’ ignore anomalous results’ / ‘use room with high background count / record large reading‘ / ‘use more than one detector’ AO2-1g = MAX 3 MARK SCHEME – A-LEVEL PHYSICS – 7408/3A – JUNE 2019 9 01.5 use of 5.5 MeV shown by working on Figure 4 1 minimum thickness MAX 3sf that rounds to 12 mm 23 OR minimum thickness MAX 3sf that rounds to 11 mm / MAX 3sf that rounds to 13 mm 23 for 1 use of 5.5 MeV can be inferred from Figure 4 as a (horizontal) line, a mark on the vertical axis or a mark on the curve / intersection between curve and a vertical line, above 5 MeV and below 6 MeV; a single line / mark between 5 and 6 MeV with no subsequent working can score 1 any line does not have to be ruled or perfectly parallel to the grid line; allow a cross or a small blob as the mark on the curve; do not insist on seeing a vertical line if more than one line is drawn / mark is made then mark as per scheme if a clear decision has been made about which read-off has been used to provide the result for the thickness; allow only one thickness given as final answer 23 OR 23 can be earned without any working on the grid / other intermediate working AO3-1a = 1 AO3-1b = 2 OR use of 7.8 MeV shown by working on Figure 4 AND minimum thickness is 16, 17 or 18 mm 123 for 123 use of of 7.8 MeV can be inferred from Figure 4 as a line, mark on axis / curve etc above 7 MeV and below 8 MeV; accept MAX 3 sf result that rounds to 16, 17 or 18 mm MARK SCHEME – A-LEVEL PHYSICS – 7408/3A – JUNE 2019 10 01.6 e A d  k   1 OR e A d  k   1 OR k d e A   seen 1 states k = gradient OR k = gradient2 2 gradient from d divided by  A 1 with  A 1 ≥ 0.5 3 k minimum 2 sf in range 1.7(0)  105 to 1.9(5)  105 4 unit for k = mm 2 Bq or mm 2 s 1 5 for 1 d must be the subject; allow obvious slips, eg D for d for 2 allow k = m if y = mx + c is quoted so inference that k = gradient is clear; this ma [Show More]

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