100 Hyperfine structure and isotope shift
The proton has ag-factor ofgI=5.6, so forn=1=Zwe have
∆EHFS
h=1.42 GHz.This is very close to the measured value of 1 420 405 751. 7667 ± 0 .0009 Hz.
This frequency has been measured to such extremely high precision be-
cause it forms the basis of the hydrogen maser, described below.^6(^6) This hyperfine transition in hydrogen
is the one detected in radio astronomy,
where it is commonly referred to by its
wavelength as the 21 cm line.
6.1.2 Hydrogen maser
Maser stands for microwave amplification by stimulated emission of radi-
ation and such devices were the precursors of lasers, although nowadays
they are less widespread than the devices using light. Actually, lasers are
usually operated as oscillators rather than amplifiers but the acronym
with ‘o’ instead of ‘a’ is not so good. A schematic of a hydrogen maser
is shown in Fig. 6.4; it operates in the following way.- Molecular hydrogen is dissociated in an electrical discharge.
- Atoms effuse from the source to form a beam in an evacuated chamber.
- The atoms pass through a region with a strong magnetic field gradient
(from a hexapole magnet) that focuses atoms in the upper hyperfine
level (F= 1) into a glass bulb. The atomic beam contains atoms in
both hyperfine levels but the state selection by the magnet^7 leads to a
(^7) This works on the same basic princi-
ple as the Stern–Gerlach experiment in
Section 6.4.
Fig. 6.4The hydrogen maser. The
principle of operation is described in
the text. A magnetic shield excludes
external fields and the solenoid cre-
ates a small stable magnetic field. In
this way the frequency shift produced
by the Zeeman effect on the hyperfine
structure is controlled (in the same way
as in the atomic clock described in Sec-
tion 6.4.2).
H source
State
selector
Magnetic
shield
Microwave
cavity
Microwave
output
Solenoid
Storage
bulb
Atoms in states
and