Friday, March 29, 2013

1932: A Watershed Year in Nuclear Physics

I should have posted this article last year, to mark the 80th anniversary of the annus mirabilis of nuclear physics. Unfortunately, I didn’t think of it until I read 1932: A Watershed Year in Nuclear Physics by Joseph Reader and Charles Clark, which appeared in the March 2013 issue of Physics Today. The article describes four major discoveries that changed nuclear physics forever.

The first landmark result was published by Harold Urey on January 1, 1932, in which he reported his discovery of deuterium, the isotope 2H. Russ Hobbie and I mention deuterium in Homework Problem 45 of Chapter 4 in the 4th edition of Intermediate Physics for Medicine and Biology.
Problem 45 Using the definitions in Problem 44, write the diffusion constant in terms of λ and vrms. By how much do you expect the diffusion constant for “heavy water” (water in which the two hydrogen atoms are deuterium, 2H) to differ from the diffusion constant for water? Assume the mean free path is independent of mass.
Unlike many elements, for which the various stable isotopes differ in mass be only a few percent, deuterium has twice the mass of normal hydrogen. Even when deuterium is incorporated into water, the H2O molecule’s mass increases by a significant 11%. Heavy water has been used as a non-radioactive biological tracer.

A second advance of 1932, and in my opinion the most important, is the discovery of the neutron by James Chadwick. Of course, the idea of a neutron is central to nuclear physics, and you cannot make sense of isotopes without it (I wonder how Urey interpreted the deuterium before the neutron was discovered). Russ and I discuss neutrons throughout Chapter 17 on Nuclear Medicine, and in particular we describe boron neutron capture therapy in Chapter 16
“Boron neutron capture therapy (BNCT) is based on a nuclear reaction which occurs when the stable isotope 10B is irradiated with neutrons, leading to the nuclear reaction (in the notation of Chapter 17)
105B + 10n → 42α + 73Li
... Both the alpha particle and lithium are heavily ionizing and travel only about one cell diameter. BNCT has been tried since the 1950s; success requires boron-containing drugs that accumulate in the tumor.”
Discovery number three is the positron, the first example of antimatter. Carl Anderson found evidence of this positive electron in cosmic ray tracks in cloud chambers. Positrons appear in IPMB in two key places. In Chapter 15 about the “Interaction of Photons and Charged Particles with Matter,” positrons are key to pair production.
“A photon with energy above 1.02 MeV can produce a particle–antiparticle pair: a negative electron and a positive electron or positron…. Since the rest energy (mec2) of an electron or positron is 0.51 MeV, pair production is energetically impossible for photons below 2mec2 = 1.02 MeV.”
The positron appears again in our discussion of β+ decay in Chapter 17.
“Two modes of decay allow a nucleus to approach the stable line. In beta or electron) decay, a neutron is converted into a proton. This keeps A [mass number] constant, lowering N [neutron number] by one and raising Z [atomic number] by one. In positron (β+) decay, a proton is converted into a neutron. Again A remains unchanged, Z decreases and N increases by 1. We find β+ decay for nuclei above the line of stability and β- decay for nuclei below the line.”
Isotopes that undergo β+ decay are used in positron emission tomography.
“If a positron emitter is used as the radionuclide, the positron comes to rest and annihilates an electron, emitting two annihilation photons back to back. In positron emission tomography (PET) these are detected in coincidence….

PET can provide a functional image with information about metabolic activity. A very common positron agent is 18F fluorodeoxyglucoseglucose in which a hydroxyl group has been replaced with 18F. The PET signal is largest in those cells that have taken up the 18F because they are actively metabolizing glucose. PET has become particularly important in studies of brain function, where active neurons are detected by an increase in their metabolism, and in locating metastatic cancer.”
The last of the four great developments of 1932 is the first use of accelerators to study nuclear reactions. John Cockcroft and Ernest Walton built an accelerator to produce high energy protons, which smashed into 7Li to produce two alpha particles. Their work was soon followed by the invention of the cyclotron by Ernest Lawrence, which is now the main tool for producing the unstable isotopes used in PET. Russ and I explain that
“Positron emitters are short-lived, and it is necessary to have a cyclotron for producing them in or near the hospital. This is proving to be less of a problem than initially imagined. Commercial cyclotron facilities deliver isotopes to a number of nearby hospitals.”
Soon after the miraculous year of 1932 Hitler came to power in Germany, and nuclear physics became much more than a scientific curiosity. The story of how the discoveries of Urey, Chadwick, Anderson, Cockcroft and Walton led relentlessly to the Manhattan Project is told masterfully in Richard Rhodes' book The Making of the Atomic Bomb.

I have a few personal connections to this watershed year. My father Ron Roth, now retired and living in Lenexa Kansas, was born in 1932, proving that we are not so far removed from that historic time. In addition, my academic genealogy goes back to Richard Chadwick and Ernest Rutherford (whose lab Cockcroft and Walton worked in). Finally, Carl Anderson worked under the supervision of Robert Millikan, who was born in Morrison, Illinois, the small town I grew up in.