1.3 Rutherford’s First Artificial Nuclear Transmutation
5
1.3 Rutherford’s First Artificial Nuclear Transmutation
The discovery that nitrogen could be transformed into oxygen by alpha-particle
bombardment marked the first time that a nuclear transmutation was deliberately
achieved (Rutherford 1919). This work had its beginnings in experiments conducted
by Ernest Marsden in 1915.
In Marsden and Rutherford’s experiment, alpha particles emitted by radium
bombard nitrogen, producing hydrogen and oxygen via the reaction:
4
2 He +
14
7 N →
1
1 H +
17
8 O.
(1.16)
The hydrogen nuclei (protons) were detected via the scintillations they produced
upon striking a fluorescent screen. The -values for this reaction are:
⎧
⎪ ⎪ ⎨
⎪ ⎪ ⎩
4
2 He
= 2.425
14
7 N
= 2.863
1
1 H
= 7.289
17
8 O
= −0.809.
(1.17)
The Q-value of this reaction is –1.19 MeV. That Q is negative means that this
process has a threshold of 1.19 MeV, that is, the bombarding alpha must possess at
least this much kinetic energy to cause the reaction to happen. This energy is available
from the spontaneous decay of radium which gives rise to the alphas; refer to the
preceding section where it was shown that decay of
226 Ra liberates some 4.87 MeV
of energy, more than enough to power the nitrogen-bombardment reaction.
In reality, for reactions with Q < 0 the threshold energy is actually greater than |Q|
because both energy and momentum have to be conserved; for the above reaction
the threshold energy is about 1.53 MeV if the incoming alpha strikes the nitrogen
nucleus head-on. The conditions of energy and momentum conservation relevant to
head-on “two body” reactions of the general form A + B → C + D are detailed in
Appendix C. A companion spreadsheet, TwoBody.xls, allows a user to input nucleon
numbers and -values for all four nuclides, along with an input kinetic energy for
reactant A; nucleus B is presumed to be stationary when struck by A. The spreadsheet
then computes and displays the Q-value for the reaction, the threshold energy (if
appropriate), and the post-reaction kinetic energies and momenta for the products
C and D. Of course, most reactions will not be head-on, but the point here is to get
some sense of the numbers and to be able to make a judgment as to whether or not
a transformation is possible in principle. Many nuclear physics textbooks examine
the physics of non-head-on collisions, an important aspect of analyzing scattering
experiments.
Independent of the Q-value being positive or negative, a related issue in these
transmutation reactions that needs to be kept in mind is whether or not the incoming
particle has enough kinetic energy to overcome the Coulomb repulsion of the target
5
1.3 Rutherford’s First Artificial Nuclear Transmutation
The discovery that nitrogen could be transformed into oxygen by alpha-particle
bombardment marked the first time that a nuclear transmutation was deliberately
achieved (Rutherford 1919). This work had its beginnings in experiments conducted
by Ernest Marsden in 1915.
In Marsden and Rutherford’s experiment, alpha particles emitted by radium
bombard nitrogen, producing hydrogen and oxygen via the reaction:
4
2 He +
14
7 N →
1
1 H +
17
8 O.
(1.16)
The hydrogen nuclei (protons) were detected via the scintillations they produced
upon striking a fluorescent screen. The -values for this reaction are:
⎧
⎪ ⎪ ⎨
⎪ ⎪ ⎩
4
2 He
= 2.425
14
7 N
= 2.863
1
1 H
= 7.289
17
8 O
= −0.809.
(1.17)
The Q-value of this reaction is –1.19 MeV. That Q is negative means that this
process has a threshold of 1.19 MeV, that is, the bombarding alpha must possess at
least this much kinetic energy to cause the reaction to happen. This energy is available
from the spontaneous decay of radium which gives rise to the alphas; refer to the
preceding section where it was shown that decay of
226 Ra liberates some 4.87 MeV
of energy, more than enough to power the nitrogen-bombardment reaction.
In reality, for reactions with Q < 0 the threshold energy is actually greater than |Q|
because both energy and momentum have to be conserved; for the above reaction
the threshold energy is about 1.53 MeV if the incoming alpha strikes the nitrogen
nucleus head-on. The conditions of energy and momentum conservation relevant to
head-on “two body” reactions of the general form A + B → C + D are detailed in
Appendix C. A companion spreadsheet, TwoBody.xls, allows a user to input nucleon
numbers and -values for all four nuclides, along with an input kinetic energy for
reactant A; nucleus B is presumed to be stationary when struck by A. The spreadsheet
then computes and displays the Q-value for the reaction, the threshold energy (if
appropriate), and the post-reaction kinetic energies and momenta for the products
C and D. Of course, most reactions will not be head-on, but the point here is to get
some sense of the numbers and to be able to make a judgment as to whether or not
a transformation is possible in principle. Many nuclear physics textbooks examine
the physics of non-head-on collisions, an important aspect of analyzing scattering
experiments.
Independent of the Q-value being positive or negative, a related issue in these
transmutation reactions that needs to be kept in mind is whether or not the incoming
particle has enough kinetic energy to overcome the Coulomb repulsion of the target