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A level: Radioactivity Questions

Q20. An isotope of potassium is used to date rocks.

(a) The isotope decays into an isotope of argon (Ar) mainly by electron capture.

The decay is represented by this equation:

40 and 18 on the Argon (in correct place)

electron neutrino (if there was a negative superscript on neutrino the mark was not awarded)

(i) Complete the equation to show the decay by filling in the gaps.

[2 marks]

(ii) Explain which fundamental interaction is responsible for the decay.

The interaction is the weak interaction/weak nuclear force because it involves leptons and hadrons or because quark character/flavour/identity/type changed

[2 marks]

(b) One decay mechanism for the decay of results in the argon nucleus having an excess energy of 1.46 MeV.

It loses this energy by emitting a single gamma photon.

Calculate the wavelength of the photon released by the argon nucleus.

E = 1.46 x 106 eV = 1.46 x 106 x 1.6 x 10-19 J

E = hf

(1.46 x 106 x 1.6 x 10-19) = 6.63 x 10-34 x f

f = (1.46 x 106 x 1.6 x 10-19) /(6.63 x 10-34)

f = 3.52 x 1020 Hz

c = fλ

λ = c/f

λ = 3.00 x 108/(3.52 x 1020)

= 8.51 x 10-13 m

[3 marks]

(c) The potassium isotope can also decay by a second decay process to form a

calcium-40 nuclide - .

Suggest how the emissions from a nucleus of decaying potassium can be used to confirm which decay process is occurring.

Writing the equation got you two of the three marks - only leaving you to explain how the electron could be identified.

The mechanism will be beta-emission

and therefore

an electron will be released (from nucleus) which may be identified by deflection in a magnetic or electric field in a cloud chamber and/or absorption.

an antineutrino will also be released, but no photon.

 

[3 marks]

(Total 10 marks)