NaI Crystal

The function of an inorganic crystal like NaI is to convert the energy deposited by a high energy gamma ray into a large number of lower energy photons. In order for this process to be successful, the crystal must have certain important parameters:

1. high Z element - increases the probability that an incoming gamma ray will interact with the crystal.
2. high density - provides more detector atoms for the incoming gamma ray to interact with.
3. large number of conversion photons - improves energy resolution
4. transparent to the conversion photons - improves energy resolution
5. thermally stable and able to be machined

NaI crystals can be grown in large sizes with few defects. It is fairly rugged for a crystal, easy to machine and has good density and light output. The Iodine in the crystal provides it with a fairly high detection efficiency since photoelectric cross sections go as Z⁵. BGO detectors have better efficiency than NaI for a given size due to the higher atomic number of Bi compared to I and BGO detectors are used in some applications. However, BGO is more difficult to machine and can not be grown in as large a size as NaI so NaI detectors remain the primary scintillator detector used. An inorganic scintillator crystal is usually doped with a small amount of an activator impurity (in the case of NaI this is thallium) to create activator excited and ground states inside the forbidden region between  the valence and conduction band of the crystal as shown below.

These sites improve the emission efficiency and moves the emission spectra into the visible region which is more efficient for coupling the crystal to the photocathode. It also moves the emission spectra to a longer wavelength than the absorption spectra which reduces self absorption in the crystal and makes the crystal transparent. One disadvantage of NaI is that it is hydroscopic. The crystal must be hermetically sealed inside a dry box when a company builds a detector.  The diagram below shows the parts of a standard commercial NaI detector.

The crystal is usually painted with an opaque paint and wrapped with a reflecting material (shiny foil) or covered with dry MgO powder on all sides excepts where the crystal will be attached to the photocathode. This insures that the low energy photons emitted by the crystal during de-excitation can only leave the crystal by striking the photocathode. An optical gel (high viscosity silicon oil) or wave guide is used to connect the crystal to the photocathode for impedance matching (i.e. improve photon transmission) since the materials have different indices of refraction. The importance of these elements of a detector to the detector's energy resolution can be understood in terms of simple statistics. Assume that a  gamma ray of 1 MeV strikes a crystal causing on average 100 low energy photons to be emitted to the photocathode. If the statistics obey a normal distribution then the relative uncertainty is 10% (100% divided by the square root of the number of photons emitted). If bad coupling or reflective material, poor crystal light output, or other factors reduce the number of photons striking the photocathode to 25 photons then the relative uncertainty will be 20%. The numbers used above are very close to those of a real crystal used in gamma detection. A quality NaI crystal will generally produce around 300 photons for a 0.667 MeV gamma ray. Although other parts of the detector system may introduce additional uncertainties, the scintillator crystal properties dominate these detector systems. The energy resolution of these detector systems is specified in terms of percent resolution by  dividing the full width of a full energy gamma peak as measured at the half height to the energy of the peak. Since the value depends on the energy of the gamma, nuclear physicists have chosen to use a 0.667 MeV photon from Cs¹³⁷ as the standard. One of the advantages of using Cs¹³⁷ is that the source produces gamma rays of only one energy. For systems based upon NaI, the detectioon system energy resolution usually ranges between  6-10% depending on the size and quality of the crystal and the proper fine tuning of the other parts of the detection system. From our past discussion, you should now understand why the detector resolution (peak width/peak height) improves (i.e. decreases) for higher energy gamma rays. The gamma ray deposits more energy in the crystal so the crystal supplies more low energy photons to the photocathode. This also explains why solid state detectors like high purity germanium (HpGe) have much better energy resolution than scintillator detectors since HpGe detectors create tens of thousands of electron-hole pairs to send the energy information instead of a few hundred of low energy photons. Thus, HpGe detectors have energy resolutions of a 2 to 4 keV for a 0.667 MeV photon instead of something like 60 keV for a scintillator based detection system. Unfortunately, Ge is a low Z and low density material and it is difficult to build large crystals. Thus, HpGe detectors have much poorer detection efficiency than scintillators and cost at least an order of magnitude more. Although great strides have been made over the past two decades in solid state detector technology, scintillators remain the detector of choice for almost all experiments except radiation safety where energy resolution isn't needed and issues like cost, ruggedness, and portability are of concern (Geiger and proportional counters and plastic scintillators) or nuclear energy spectroscopy studies where energy resolution is essential (solid state detectors).