development of micro-pattern gaseous detectors – gem - LMU

1.1. Primary Processes in Gaseous Detectors 15
Photoelectric Effect for 55 Fe
For measurements with the GEM detector later presented, see Ch. 3, a 55 Fe source is used to analyze
certain properties of the chamber.
55 Fe decays by electron capture to excited Manganese 55 Mn which emits photons by returning into
the ground state. Depending on the inner rearrangement of the electrons, two different photon
energies are characteristic. The Kα photon which represents the transition of an electron from the L
to the K shell, carries an energy of Eγ = 5.90keV.
If the 55 Mn atom returns to its ground state via M-to-K transition of an electron, a K β - photon is
emitted with an energy of Eγ = 6.50keV.
The line intensity proportion for Kα and K β is 24.4% and 2.86%, respectively, the Kα process
dominates over K β by approximately 10 : 1 [PDG 10].
When these photons are interacting with the Argon/C02 gaseous filling of the GEM detector, the
photoelectric effect is the dominating process .
The Kα- photon energy is higher than the binding energy or an electron from the K shell of
Argon (EK = 3.22keV). The electron can detach with a remaining kinetic energy of 2.68 keV. The
excited Argon atom then returns to its ground state through emission of one ore more photons that
leave the volume without being detected.
The resulting X-ray spectrum consists of three line with the energies: 2.68 keV, 5.90 and 6.50
keV.
By photoelectric effect the X-ray energy is transferred to an electron in the active volume of the
detector. Recording of this spectrum with the triple GEM detector can be used to state the energy
resolution of the device and is illustrated in Ch. 4.1.
Since excitation dominates over ionization for statistical reasons, the required energy loss for an
ionization is greater than the simple ionization potential. Therefore the total number nt of ionized
particle is:
nt = 1
Wi
dE
dx
(1.27)
where (dE/dx) is the energy loss of the incident particle and Wi represents the average energy required
for the production of an ion-electron pair. For Argon the mean energy for ion-electron-pair creation
is Wi = 26 eV which is nearly a factor two larger than the simple ionization potential of 15.8 eV.
Since the gas used in the GEM detector is not purely mono atomic Argon but a mixture of Ar/CO2
in the ratio 93/7, average values for Wi should be taken. With the given mean energy for ion-electron
creation for Argon and CO2
the gas mixture has a value of:
W gas
i
W Ar
i = 26 eV and W CO2
i = 33 eV ,
Ar
= 0.93 · Wi + 0.07 · W CO2
i = 26.50 eV , (1.28)
as lower limit for ion-electron pair production. Thus the average number of ionized atoms generated
by the 55 Fe photon is: