1. MINOS: a new vertex tracker for in-flight γ-ray spectroscopy
Laurent AUDIRAC
CEA-Saclay, DSM/IRFU/SPhN
MPGD Workshop, CEA-Saclay, 6-8 December 2011

2. Outline The MINOS project: Physical motivations General description
Readout electronics
Simulations performed
Details of the procedure
Readout electronics in the simulation
The realistic case of 53K(p,2p)52Ar
Test of a prototype
Conclusion and perspectives

3. The MINOS project:
Physical motivations
General description
Readout electronics
Simulations performed
Details of the procedure
Readout electronics in the simulation
The realistic case of 53K(p,2p)52Ar
Test of a prototype
Conclusion and perspectives

4. Physical motivations (1)
Shell gaps are well established for nuclei near the valley of stability.
For exotic nuclei, new shell closures and shell reordering are predicted.
This question is of prior importance for neutron-rich nuclei for a better understanding of the r-process.
Nuclear shell gaps can be characterized by the energy of the first 2+ excited state.

5. Physical motivations (2)
Exotic nuclei are studied by prompt γ spectroscopy from secondary reactions of exotic beams produced by fragmentation at intermediate energy: 300 MeV/nucleon at RIKEN (Japan), FAIR-GSI (Germany).
Nucleon knockout reactions coupled to a γ-detection array are efficient to populate the most exotic species and perform their γ spectroscopy. A
A-1
A Doppler correction is needed since the γ-decay occurs in-flight.
This Doppler correction is limited by the uncertainty over the angle of emission θ and the velocity β of the ejectile.
Up to now, one has no access to the velocity and thin targets of Be, C are used.

6. The MINOS project
A new project, MINOS (MagIc Numbers Off Stability), supported by the European Research Council (ERC) for the period , is being developed to study the most neutron-rich exotic nuclei by nucleon-removal reactions thanks to a better Doppler correction and a statistics improvement.
Protons of 100 MeV, about 60 keV lost in the TPC
A resolution better than 3 mm at FWHM over the vertex position has to be reached.

7. Description of the setup
The target:
liquid-H2 target (IRFU/SACM)
diameter: 56 mm
length: 150 mm
delimited by a Mylar window of 150 µm
The TPC:
filled with a gas mixture of Ar82(CF4)15iso3
42 mm < radius < 92 mm
length: 300 mm
Vcath < V
use of a Micromegas detector
pad size ≈ 2 mm²
a few 104 electrons per pad after amplification (gain 1500)
≈ 5000 electrons per time bin (10 ns)
no magnetic field

8. Readout electronics
About 5000 channels with about 500 sampling points are needed.
A few kHz for the data acquisition rate is attempted.
The readout electronics is based on the GET collaboration (NSCL,CENBG,GANIL,CEA Saclay).
The AGET chips will be used.
20 front-end cards with 256-channels (4 AGET chips), connected via a set of six 900-contact to the TPC

9. The challenges The main features to achieve this challenge:
1 pps, 300 MeV/u, 1 week of beam time
The main features to achieve this challenge:
Use of a TPC-Micromegas to reconstruct the proton tracks
Determination of drift times from the electronics readout
Measure of the reaction vertex

10. The MINOS project:
Physical motivations
General description
Readout electronics
Simulations performed
Details of the procedure
Readout electronics in the simulation
The realistic case of 53K(p,2p)52Ar
Test of a prototype
Conclusion and perspectives

11. Simulations: procedure
What do we want?
We want to calculate the vertex resolution for a concrete reaction of nucleon-removal, in our case for a (p,2p) reaction on 53K: 53K(p,2p)52Ar at 250 MeV/A and 0.5 pps.
Four main steps to achieve the resolution:
Reaction process
Energy deposited in the TPC
Drift of electrons towards the detection plane
Vertex position
We get the energy loss by the charged particles in the TPC at each step: data set of x (mm), y (mm), z (mm), e (eV).
Geant4 simulation

12. Drift of electrons Number of ionization electrons:
following a Gaussian distribution
Drift of individual electron: transversal and longitudinal diffusions with coefficients σT,L(z) = σT,L√(z(cm)).
Pad geometry and time sampling.
The number of electrons for the pad and time bin is incremented.
Parameters
Value
Ionisation threshold
25 eV
Longitudinal diffusion
0.186 mm/√cm
Transverse diffusion
0.195 mm/√cm
Drift speed
0.066 mm/ns
Mean gain for Polya
1500
θ for Polya
0.3
Noise
900 e-
Detection threshold
4000 e-
Time binning
10 ns
Number of rings 20
Number of segments
256
105
electrons number
For each pad and each time sampling:
- Gain1: Polya distribution
- Noise: 900 electrons RMS
- detection threshold 1

13. Electronics readout
We want to include the MINOS electronics readout in the simulations for the time measurement.
INPUT
OUTPUT
electronics
processing
Question: how extracting the time information from the output signal?
Time barycenter of the charge distribution
Fluctuations of the difference between the time value at half-maximum and the time barycenter
Resolution of 1 ns (taken at 5 ns in the simulations)  to be discussed
F. Druilliole for the signal processing

14. The case of 53K(p,2p)52Ar: kinematics
Reaction yield: 37.6%
- for (p,2p): 0.4%
Detection efficiency: 97%
- 2p detected: 74%
- 1p detected: 23%
Two protons detected: at least one proton in the region where R < 3 mm.
One proton detected: major part of the protons (70%) where R > 3 mm.

15. The case of 53K(p,2p)52Ar: resolution
Use of one proton track (those for which the best resolution is achieved) and the beam trajectory.
Determination of the beam trajectory:
Use of the beam detectors (PPAC 1&2) with a position resolution of 1 mm
DSSD detector at about 20 cm after the target with a position resolution of 1 mm.
Resolution at FWHM: 2.85 mm

16. The MINOS project:
Physical motivations
General description
Readout electronics
Simulations performed
Details of the procedure
Readout electronics in the simulation
The realistic case of 53K(p,2p)52Ar
Test of a prototype
Conclusion and perspectives

17. Test of a prototype Use of MIMAC TPC (need adaptation)
Micromegas plane with 4 different geometries of pads
T2K electronics
Laser source for timing
α source for tracking
Test of the readout electronics
Test of flat cables
Verification of the results from the simulations

18. Conclusion and perspectives
A new target-detector setup is being developed for the in-flight γ-spectroscopy of the most neutron-rich nuclei.
It consists in a liqui-H2 target coupled to a tracker detector in order to increase the statistics of production when achieving a measure of the vertex position for a better Doppler correction.
Thank you…
Simulations of the setup have been performed in a realistic case for the study of 52Ar produced by (p,2p) reactions.
A resolution of 2.85 mm over the vertex position is achieved, which is below the 3 mm required.
The future:
Tests of a prototype in order to validate the results of the simulations and test the electronics readout
Production of the final instrument and test at Saclay
In-beam tests
First experiments