1. Gamma Rays Detection LNF – 14.2.18 | INSPYRE school Danilo Domenici

2. It consists of photons of the highest energy (> 1 MeV)
What are the Gamma Rays?
Gamma Rays (or γ rays) are the most energetic electromagnetic radiation
It consists of photons of the highest energy (> 1 MeV)

3. A bit of history
γ rays have been discovered in 1900 in the Radium element
In 1903 Ernest Rutherford dubbed the 3 radioactive decays as
α rays, β rays and γ rays
a neutron of the nucleus becomes a proton and a β particle (electron) is emitted together with a neutrino
an α particle (2 neutrons and 2 protons) is emitted by the nucleus
a γ photon is emitted by the nucleus
the atomic number is decreased by 2 and the mass number is decreased by 4
the atomic number is decreased by 1 and the mass number is the same
the atomic number and the mass number are the same

4. Some exemples
sometimes the 3 decays happens in a sequence called «radiocative chain»

5. One more example: Cesium 137
the energy of the emitted radiation is equal to the difference of mass between initial and final nuclei according to
E = mc2

6. How we detect gamma rays?
gamma is an ionizing radiation interacting in matter in 3 possible ways
low energy (< 100 keV)
photoelectric effect
medium energy (100 keV – 1022 keV)
Compton scattering
high energy (> 1022 keV)
pair production
all these processes increase with the Atomic Number Z of the material

7. Then how we detect free electrons?
there are a lot of particle detectors: based on gas (Geiger counter), on semiconductors (Silicon detectors)
We will use the scintillators

8. Scintillator materials
Scintillators are materials having the property of emitting visible light (mostly blue) when hit by ionizing particles through the fluorescence process
There are several materials with this property
(and others are being invented)
NaI(Tl) - CsI(Tl) - BaF2 - BGO
PbWO4 - LYSO - YAP – Polistirene

9. Scintillator detectors
In order to build a scintillator detector we need to assemble:
a scintillator block
a photosensor
incoming gamma ray
scintillator
gamma ray absorbed
light emitted
photosensor detect light
electric signal output

10. A complete detection set-up

11. Silicon Photomultipliers or SiPM
invented at the beginning of 2000 to replace the old vacuum Photomultipliers Tubes (PMT)
SiPM are solid-state photon sensitive micro-device on a Silicon substrate
typical 5x5 mm2 sensor is made of a matrix of thousands of single photo-diodes with micrometric dimensions
an avalanche inside the Silicon amplifies the signal