A-LevelPhysicsQuantum physics9702/42 · May/June 2021 · Q129 Marks
A stationary nucleus of samarium-157 (¹⁵⁷₆₂Sm) emits a gamma-ray (γ-ray) photon of energy 0.57 MeV.
- (a)State what is meant by a photon.[2]
- (b)(i)its wavelength[2]
- (b)(ii)its momentum.[2]
- (c)(i)Using your answer to (b)(ii), determine the speed of the samarium-157 nucleus after emission of the photon.[2]
- (c)(ii)By reference to your answer in (c)(i), explain quantitatively why the speed of the samarium-157 nucleus may be assumed to be negligible compared with the speed of the photon.[1]
Mark scheme
Marking points from the official mark scheme (May/June 2021). The mark or mark type is shown in brackets.
- [1] quantum of energy
- [1] of electromagnetic radiation
- [1] energy = hc / λ OR energy = hf and f= c/λ
- [1] λ = 2.2 × 10⁻¹² m
- [1] p = h/λ OR p = E/c
- [1] = 3.0 × 10⁻²² N s
- [1] mass (of Sm-157 nucleus) = 157 × 1.66 × 10⁻²⁷ OR mass (of Sm-157 nucleus) = 0.157 / (6.02 x 10²³)
- [1] = 1.2 x 10³ m s⁻¹
- [1] (1.2 ×) 10³ m s⁻¹ is much less than (3.0 ×) 10⁸ m s⁻¹
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