On 4/30, Ken Fox asked how the mass of neutrons is measured. After the
replies that have been given, and puzzling over the question for a while, I
consulted with a colleague and some nuclear physics texts on our shelves,
and have come up with the following:
1. Originally (1932), Chadwick determined an estimate of the neutron mass
by firing them into an ionization chamber alternately filled with hydrogen and
nitrogen gas. By measuring the recoil energies of the target nuclei (H-1
and N-14), the ratio of (M-target + M-neutron) for the two cases led to
separating the neutron mass out. The result was, of course, that Mn is
approximately 1 u. [Ref. K. N. Mukhin, Experimental Nuclear Physics, Vol.
1, Mir Publishers, Moscow--translated from the Russian--c. 1987]
2. A more accurate value of the neutron mass comes from measuring the
deuteron binding energy, as mentioned by Jim Pengra. Semat and Albright
(Intro. to Atomic and Nuclear Physics, 5e, 1972) describe it thus: "Since
the masses of the proton and deuteron are known very accurately from
measurements with the mass spectrograph, the photodisintegration of the
deuteron affords the most accurate means of determining the mass of the
neutron. The value so determined is 1.008665 u." (p. 491)
Photodisintegration is the reaction:
gamma + D => p + n
This requires gamma rays of energy greater than 2.22 MeV, which you
must get from nuclear decays (e.g., Tl-208 at 2.62 MeV). After taking the
recoil of the p and n into account, and knowing the gamma energy, the
mass of the neutron comes out.
3. The reverse of photodisintegration is combining protons and slow
neutrons, like in water:
p + n => D + gamma
Measuring the energy of the gamma ray emitted, after correcting for the
recoil of D, gives the neutron mass. (Ref., Krane, Introductory Nuclear
Physics, 1987, p. 81).
4. An additional slow (thermal) neutron process mentioned in the Mukhin
book [see 1, above] is the process that produces carbon-14:
N-14 + n => p + C-14
Measuring the kinetic energies of the reaction products (p and C-14), and
calculating the mass balance, one gets the neutron mass. Other slow
neutron processes could be used, as well, one would suppose.
I found this little research project to be very interesting and informative.
Hope it helps Ken and others.
Rondo Jeffery
Physics Department
Weber State University
Ogden, Utah
RJeffery@weber.edu