Spectrometry of high energy gamma ray

1
Spectrometry of high energy gamma ray
Photons with energies of tenths MeV, GeV and
higher
Creation of electromagnetic shower it is
necessary to determine total energy, direction
and time of initial photon
arrival
Inorganic scintillation detectors are favorable
BGO, BaF2, PbWO4
Combination of convertor (lead, tungsten ..) and
track detectors of electrons and positrons
(spark chambers, silicon )
Electromagnetic shower
E? gtgt 1 MeV photo effect is negligible ? gamma
detection by means of electromagnetic
shower, pair production, bremsstrahlung
radiation ...
Important quantity is radiation length X0 high
energy electron E ? E/e
E? 100 MeV ? detectors with 15 X0 are needed
Width of electromagnetic shower Molier radius R0
cm (about 90 of shower energy is inside, it is
approximately valid R0 0.0265?X0(Z1,2)
2
Particle multiplication increasing number of
electrons, positrons and photons
Deepness expressed with radiation lengths X0
t x/X0
Critical energy EC energy higher it radiation
losses start to dominated over
ionization
For critical energy EC and radiation length X0 is
valid
Energy in EC units ? E/EC
Number of cascade particles increases
geometrically
N(t) 2t
Mean energy of particles e is
Multiplication continues up to critical energy EC
Maximal number of particles NMAX at deepness tMAX
tMAX ln ?
3,2 GeV v Pb maximum tMAX 6, NMAX 400
3
Segmented detectors with long crystals (diameter
is two times Molier radius
Sum of energies from main and surrounding modules
Loss of small contributions from modules, where
Ei is bellow detection threshold
Position of centre is determined
Other types of weights can be used
Example from spectrometer TAPS (BaF2) l 250
mm, d 54-59 mm X0 20,5 mm R0
33,9 mm, EC 12,7 MeV For E? 160 MeV ?E/E
16 only detector with the highest deposited
energy ?E/E 6 - all detectors hit by
electromagnetic shower High energies ?E/E ? 3

Position resolution of TAPS spectrometer
4
Detection of particles using their decay to
photons
Determination of particle invariant mass
Uncertainty of invariant mass determination
Comparision of mean number of affected modules
for photons and neutrons
Typical TOF spectrum (target distance was 4.3 m),
Bi Pb collisions 1 GeV/n
Inclusion of different number Of neighboring
modules