Radiation: Theory

Definitions - Gamma Radiation

A Co-60 source is the agreed standard for the Total Ionising Dose characterisation of electronic devices (see specification 22900 ).

Co-60 is produced at reactor sites where the neutrons are used to irradiate stable Co-59. The irradiated nucleus absorbs one neutron to create Co-60 which has a half-life of 5.27 years. A totally irradiated Co-60 source can have a specific activity as high as 1000Ci / gram, though in practice this activity is normally much lower. In the case of the Estec source 100 grams produce an activity of 2000Ci. (ie about 20Ci / gram).

Co-60 decays by beta (electron) emission and two associated gamma rays (photons) with energies of 1.1732MeV and 1.3325MeV. Gamma rays interact with matter in three principal ways :

• Photoelectric Effect (for gamma energies < 200KeV)
• Compton Effect (200KeV < E < 1.5MeV)
• Pair Production (E > 1.5MeV)

 

The activity of a Co-60 source on a given date is determined from the familiar radioactive decay law which applies to the exponential decrease in the dose rate over time :

D(t)=D(0)(1/2)(t/t0.5)

                                                            where D0= the initial (measured) dose at t=0
t _0.5 = half-life for Co-60 (5.271 years)

Typically such dose measurements are made in air and to calculate the dose equivalent in other materials the energy of the incident radiation and the 'affinity' of the material to absorb radiation must be taken into account.

D _mat = 0.877 X (u_en/ rho) _mat / (u_air/ rho) _air

where D _mat = Material dose in rads
X = exposure in Roentgens
u_en/rho = mass energy absorption coefficient in cm²/gram

Hence the dose in air maybe equated to the dose in any other material by :

D _mat = Dair(u_en/ rho) _mat / (u_en/ rho) _air

The mass energy absorption coefficient u_en for common materials with respect to an average gamma ray energy of 1.25MeV (for Co-60) is given :

Material	Mass Energy Absorption Coefficient
Air	0.02666
Silicon	0.02652
Copper	0.02428
Water	0.02965
Gallium	0.02371
Arsenic	0.02355

The dosimetry at the Estec facility is referenced to water, therefore for a silicon device the dose ratio is :
D _Si / D _water = 0.02652/0.02965 = 0.898.

For GaAs the mass absorption coefficient is calculated by taking into account the ratio of the atomic mass of the constituents, ie A_Ga=69.723 and A_As=74.92 This results in a mass absorption coefficient (at 1.25MeV) of 0.02362 and a dose ratio to water of

D _GaAs / D _water = 0.02362/0.02965 = 0.796.