Neutron diffraction spectrometer
This is the spectrometer which was first used at Ryerson Laboratory, at the University of Chicago, USA, in the scattering of X‐rays by gases for free atom form factor determinations. In 1944 it was taken to Oak Ridge and set up at the face of the Graphite Reactor. There, it was used in 1945 by US physicist Ernest Omar Wollan (1902‐1984) to develop the techniques of neutron diffraction spectroscopy. This technique is used to determine the structure of materials in a similar way to X‐ray diffraction.
The central core of the Altatom which contains all of the atom’s positive charge and most of its mass is known as atomic nucleus. The size of the nucleus is 10,000 times smaller than the size of the atom, but more than 99.9% of the whole mass of the atom is concentrated in the nucleus. Therefore, a nucleus occupies a very small space in the atom consisting of protons and neutrons.
Since neutron is a neutral particle, it has high penetrating power and very low ionising power. Further, electric and magnetic fields have no effect on it. These two constituents of a nucleus (protons and neutrons) are called nucleons. Although the hydrogen nucleus consists of a single proton alone, the nuclei of other elements consist of both neutrons and protons. The different types of nuclei are often called nuclides
Proton is a positively charged particle and has a charge equal to that of an electron. However, the mass of a proton is about 1836 times that of the electron. Charge on proton, e = + 1.6 × 10−19 C and mass of proton, mp = 1.6726 × 10−27 kg = 1.007825 a.m.u.
Neutron is a neutral particle and has no charge. Its mass mn = 1.6750 × 10−27 kg = 1.008665 a.m.u. Atomic mass unit (a.m.u.) is a smaller unit of mass developed to make the study of nuclear masses for convenience. One atomic mass unit is defined as one–twelfth (1/12) of the mass of a atom. According to Avogadro′s hypothesis, there are 6.023 × 1023 atoms in 12g of carbon.
The nucleus is believed to be spherical and hence its size is usually given in terms of radius. It has been found experimentally that the volume of a nucleus is directly proportional to its mass number A. If R is the radius of nucleus, its volume = (4/3) πR3, R = R0A1/3, where R0 is a constant whose value is found to be 1.2 × 10−15 m. The radius of the nucleus depends upon the mass number A and therefore, atomic nuclei of different elements have different sizes.
As the radius of a nucleus is extremely small, is order of 10−15m. In a usual practice express the nuclear radius in a smaller unit of length called fermi, named after the famous physicist Enrico Fermi. (1 Fermi (1fm) = 10−15 m).
Example: Find the nuclear radius of (a) lead and (b) oxygen. R0= 1.2 × 10−15 m
(a) Lead. Mass number, A = 208
∴ Nuclear radius, R = R0 A1/3= 1.2 × 10−15 × (208)1/3 = 7.1 × 10−15 m =7.1 fm
(b) Oxygen. Mass number, A = 16
∴ Nuclear radius, R = R0 A1/3 = 1.2 × 10−15 × (16)1/3 = 3.0 × 10−15 m = 3.0 fm