1. A new-concept Absorption Calorimeter for the ELI-NP facility
Michela Lenzi
INFN – Sezione di Firenze - ITALY
On behalf of the
Gamma Beam Characterization Team
OUTLOOK
The ELI-NP Gamma Beam
The Beam Characterization System
The Absorption Calorimeter
14th Topical Seminar
on Innovative Particle and Radiation Detectors Siena, 3 – 6 October 2016

2. ELI-NP Gamma Beam Extreme Light Infrastructure
Beam parameters
Eg (2-lines)
0.2 – 19.5 MeV
DEg/Eg
≤ 0.5%
Ibw
 8.3∙108 ph/s
Peak Brillance
>1019 ph/s·mm2· mrad2 0.1%
Inverse Compton Scattering
1 macro-pulse
32 micro-pulses
~1ps
496 ns
10 ms
16 ns
Pulsed electron beam
Laser beam
monochromatic
g beam
Collimator

3. g Beam characterization system
Compton Spectrometer (Firenze) energy distribution
Nuclear Resonant Scattering Spectrometer (Catania) absolute energy calibration
Profile Imager (Ferrara)  spatial distribution
Absorption Calorimeter (Firenze)  average energy and intensity
Absorption Calorimeter
Nuclear Resonant
Scattering System
Profile
Imager
Compton
Spectrometer
g-beam

4. Compton Spectrometer: concept
Task: measure and monitor the photon energy spectrum
Working Principle: reconstruct the energy of the interacting photon by:
sampling Compton interactions of single gamma in a ultra-thin target
measuring the energy and position of the scattered electron
Energy detector
Position
detector
Gamma beam
Collimator
Electron
Micrometric
target e-
Fast
Gamma
detector g
Detection of recoil gamma in coincidence for background suppression and micro-pulse identification

5. Compton Spectrometer: design
High resolution electron detectors required  HPGe (energy) + Si-strip (angle)
HPGe Detector
collimator
HPGe cooling system
Target wheel
g beam
Silicon Detector
Matrix of 4x4 BaF2 crystals
Hamamatsu multianode
Gamma Detector

6. Nuclear Resonant Scattering System
Task: perform an absolute energy calibration for the other detectors
Working principle: detection of gamma decays of properly selected nuclear levels of a given nucleus when resonant condition with the beam energy is achieved
Design: gamma counter (BaF2 crystals) for fast energy scan to provide a prompt information on the established resonant condition; Gamma spectrometer (Lyso crystal) to precisely identify resonant excited levels
g beam
Gamma
Counters
(BaF2)
Spectrometer
(LYSO)
target
26Al: MeV; MeV 12C: MeV; MeV 9Be: MeV
6Li: MeV

7. Gamma Profile Imager CCD camera Target holder g beam
Task: check the alignment and measure the spatial distribution of the beam
Design: scintillator targets intercept the gamma beam at 45°. The light emitted by the scintillator is acquired using a mirror and lens system to focus onto a CCD camera.
g beam
Target holder
CCD camera

8. Absorption Calorimeter: concept
PET
Lead
Total attenuation (cm2/g)
Photon Energy (MeV)
Task: provide a measurement of the total intensity and average energy of the gamma beam within a macro-pulse during the commissioning phase (moveable platform)
Working principle:
low-Z absorber  longitudinal profile of the energy deposition depends on the average beam energy
low bandwidth of the beam  the beam intensity can be inferred from the measured total energy release
high intensity of the beam  low statistical error
Design: blocks of 22 Polyethylene (PET) absorber interleaved with layers of silicon detectors

9. GEANT4 Simulations
Gamma beam energy (1-20 MeV)
Fraction of released energy
The expected longitudinal profile of the energy deposition is parametrized by detailed Monte Carlo simulations.
The average energy of the beam can be measured by fitting the measured longitudinal profile against the MC parameterized distributions

10. Expected performance
Best possible statistical accuracy for a single micro-pulse is a few %  in a few seconds of beam operation s(Eg)/Eg < 0.1%
Achievable Energy resolution for a single photon micro-pulse
GEANT4 simulation

11. Calorimeter Layer Front End electronics
Si-strip technology chosen for:
fast response time
radiation hardness
response linearity
A prototype of the front-end board has been produced and equipped with 7 silicon devices
7 Silicon devices
Front End electronics
channel output
8.0 cm
1.0 cm
Hamamatsu devices:
128 strips bonded together
300 um thick
Bulk capacitance ~ 250 pF
Depletion voltage ~ 200 V
Operation Voltage = 600 V
(to saturate the drift velocity)

12. Laser test setup EOT fast photodiode for Laser monitoring
PicoQuant fast laser diode:
Wavelength: 1060nm
Frequency: up to 80MHz
Pulse FWHM: < 100ps
Max Average power: 21 mW
External trigger and burst mode operation provide series of 32 pulses (16 ns) every 10 ms
Data acquisition: Caen V1742 Switched Capacitor Digitizer (1024 samples at a frequency up to 5 GHz)
X axis stage for horizontal scan
Z axis stage for longitudinal scan

13. Silicon Signal 16ns Sampling frequency: 5 GHz
1% contribution from previous pulse
Sampling frequency: 1 GHz

14. 32 micro-pulses signal
32 micro-pulse train: the signal of each pulse is affected by the previous pulses
 A fit procedure is needed for single pulse deconvolution
single pulse data is used as template function fT(t)
Fit function:
fit parameters: t0 , pi
Raw data (pedestal subtracted)
Fit Result
Fit procedure has been validated with simulated signals: achieved precision is ~ 0.1%

15. Absolute Energy Calibration
Silicon Detercors tested at DEFEL (ELectrostatic DEFlector), the chopped beam line of the Tandem accelerator at INFN-LABEC in Florence
Pulsed bunches of: Protons
Energy: 3 MeV/p
Adjustable proton multiplicity
l = 2
l = 4
l = 5

16. Device Linearity
A feasibility test to verify silicon and electronics linearity has been performed  to be repeated up to the maximum expected energy deposition per layer (400 MeV for low energy line and 1.4 GeV for high energy line)
Residuals

17. Summary
A new concept Absorption Calorimeter has been designed to suit the unprecedented specifications of the ELI-NP Gamma Beam
The calorimeter will measure the average energy and the total intensity of the beam by exploiting the energy dependence of the gamma absorption cross-section
The design of the sampling calorimeter, made of blocks of low-Z absorber interleaved with layers of silicon detectors, has been validated by GEANT4 simulations
A first prototype of the Front-End board for silicon devices has been produced and tested (fine tuning of electrical parameters is in progress)
Several tests performed with an infrared pulsed laser have shown the capability of the device to disentangle pulses separated by 16 ns
The feasibily of an absolute energy calibration and of a linearity measurement of the device has been evaluated by exposing a prototype to a proton beam at the Labec facility, in Florence.