Question 4
Part 1 Chadwick and the discovery of the neutron.
(a) In 1936 Chadwick carried out an experiment in which he bombarded beryllium (9Be) with a-particles. He found that a very penetrating radiation was produced and also another element. He believed that this penetrating radiation consisted of energetic neutrons. Assuming that neutrons are produced in this interaction, write down the equation for the interaction of an a-particle with a nucleus of beryllium(9) and identify the other element produced.
9Be + 4He --> 12C + 1n The reaction asked for is the one above. Chadwick determined a more precise value for the mass of the neutron from the similar reaction involving boron11 as the starting material since a more precise value for the mass of B11 was known at the time.
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(b) To determine the mass of the particles he carried out two further experiments.
(i) In the first of these Chadwick bombarded a slab of paraffin wax with the radiation produced in the above reaction. The radiation ejected protons from the wax. Describe briefly the principle by which Chadwick measured the velocity of these ejected protons.
IB allowed answers that mentioned deflecting the protons with electric or magnetic fields and they allowed the statement that Chadwick used the thickness of Aluminum through which the protons would penetrate. What he actually used was the relationship between the thickness of aluminum and the percentage penetration.
The editor considers the question to be outside the normal range of IB questions and to be unfair because students would not be expected to know the details of Chadwick's work.
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(ii) In the second experiment Chadwick allowed the unknown radiation to enter a Wilson cloud chamber containing nitrogen(14). He observed tracks that he assumed were produced by ionized nitrogen atoms being "knocked on" by collision with the neutrons.
Describe briefly how Chadwick determined the velocity of the nitrogen atoms (ions).
The energy of charged particles in the cloud chamber was estimated from the path length (in air).
.... normal practice but the editor doubts that many candidates scored well on this part.
[3]
(c) Chadwick now applied the ideas of conservation of momentum and energy to the data he obtained from the recoil experiments. The diagram below shows a neutron mass m moving with speed v about to collide head on with another particle of mass M.
The collision takes place along a line joining the two particles. After the collision the neutron rebounds with a speed v' and the other particle moves off with a speed V as shown in the diagram below. The collision is perfectly elastic.
Write down an equation in terms of the respective masses and velocities of the two particles that expresses the ...
(i) ... conservation of momentum.
mv = MV - mv'... (be careful with the negative sign)
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(ii) ... conservation of mechanical energy.
mv2 = MV2 + mv'2[2]
(d) It can be shown that in such a collision ...
In the experiments with protons and nitrogen in 1936 Chadwick obtained the following data:
Maximum speed of protons ejected from paraffin wax = 3.3x107 m/s
Maximum speed of nitrogen ions in a cloud chamber = 4.7x106 m/sIf the mass of a nitrogen atom is 14 mp where mp is the mass of a proton, show, using the equation above and this data, that the mass of the neutron mn is determined as....
mn =1.16 mp The values are taken from Chadwick's 1932 paper - the answer is given as help and it is worth 5 marks. Take care and think carefully. It could be a little tricky.
For the neutron/proton collision ....
3.3x107 = 2mn/(mn + mp)v ... where v is the neutron velocity
For the neutron /nitrogen collision ....
4.7x106 = 2mn/(mn + 14mp)v Eliminating v gives ...
3.3x107/4.7x106 = (mn + 14mp)/(mn + mp) (1 + mp/mn )7.0 = 1 + 14mp/mn ..... 7 + 7mp/mn = 1 + 14mp/mn ...... 6 = 7mp/mn mn = 1.17mp [5]
A difficult question to score well on if the candidate was unfamiliar with the details of Chadwick's paper. It is believed that very few attempted it and fewer scored well.
