A 4-PAD MicroMegas system for monitoring purpose at n_TOF facility (CERN) M. Sabaté-Gilarte 1,2 T. Papaevangelou 3, F. Gunsing 3, E. Berthoumieux 3, M. Diakaki 3 and The n_TOF Collaboration 1) European Organization for Nuclear Research (CERN), Geneva, Switzerland 2) Universidad de Sevilla, Sevilla, Spain 3) Commissariat à l’Energie Atomique (CEA/IRFU), Saclay, France

Overview n_TOF facility at CERN Micromegas for monitoring purpose Flux measurement 4-PAD Micromegas at n_TOF for phase 3 Micromegas detectors for neutron induced cross section measurement M. Sabaté-Gilarte, 2nd Special Workshop on Neutron Detection with MPGDs

(*) C Rubbia et al., A High Resolution Spallation Driven Facility at the CERN-PS to measure Neutron Cross Sections in the Interval from 1 eV to 250 MeV, CERN/LHC/98-02(EET) n_TOF (*) is a spallation neutron source based on 20 GeV/c protons from the PS of CERN neutron_Time of Flight (n_TOF) facility at CERN M. Sabaté-Gilarte, 2nd Special Workshop on Neutron Detection with MPGDs

Main features of the n_TOF:  Proton intensity 7x10 12 p/pulse  Proton beam momentum 20 GeV/c  Proton pulse width 6 ns (r.m.s.)  High instantaneous n flux 10 5 n/cm 2 /pulse  Neutrons per protons300  Wide energy spectrum 25 meV<E n <1GeV  Low repetition rate < 0.8 Hz  Good energy resolution  E/E = Neutron beam + state-of-the-art detectors and acquisition systems make n_TOF UNIQUE for: measuring radioactive isotopes, in particular actinides identifying and studying resonances (at energies higher than before) extending energy range for fission (up to 1 GeV !) n_TOF facility M. Sabaté-Gilarte, 2nd Special Workshop on Neutron Detection with MPGDs

4 cm moderator (water/borated water) PS Protons (20 GeV/c) Neutrons (meV to GeV) Pb 1 cm cooling (water) 200 m 20 m tof EAR-1 EAR-2 Experimental Area 1 (EAR 1): High energy resolution: solving resonances in the keV- MeV neutron energy range (  E/E = 4.2e-3 at 1 MeV) Ten years measuring neutron-capture and neutron- induced fission cross-sections of interest in nuclear technology and astrophysics Experimental Area 2 (EAR 2): Expected neutron flux 25 times higher Expected neutron rate 100 times higher than for EAR1 Use of smaller samples and measurements on isotopes with small cross-sections Radioactive sample with very small half-live n_TOF facility

PS Protons (20 GeV/c) EAR-1 EAR-2 Measurements at n_TOF: Measurements: * Neutron flux * Beam profile * Fission: (n,f) * Capture: (n,  ) * Charge particle: (n,cp) Applications: * Astrophysics * Nuclear Technology * Medical Physics * Fundamental Nuclear Physics

time zero sample production target, neutron source reaction product detector flight length L Time of fight method 20 GeV/c protons M. Sabaté-Gilarte, 2nd Special Workshop on Neutron Detection with MPGDs (F. Gunsing, CEA, 2014)

sample production target, neutron source flight length L reaction product detector 20 GeV/c protons M. Sabaté-Gilarte, 2nd Special Workshop on Neutron Detection with MPGDs Time of fight method (F. Gunsing, CEA, 2014)

sample production target, neutron source flight length L reaction product detector 20 GeV/c protons M. Sabaté-Gilarte, 2nd Special Workshop on Neutron Detection with MPGDs Time of fight method (F. Gunsing, CEA, 2014)

sample production target, neutron source flight length L time of flight t reaction product detector 20 GeV/c protons M. Sabaté-Gilarte, 2nd Special Workshop on Neutron Detection with MPGDs Time of fight method (F. Gunsing, CEA, 2014) Kinetic energy of the neutron by time-of-flight

 Megas for monitoring purpose at n_TOF Used at n_TOF since 2001 Low cost Microbulk technology Transparent to neutrons Robust and high radiation resistance Low material in beam with very high radio purity o Minimal beam perturbation o Minimize induced background by the device itself High geometrical efficiency and accuracy than any other monitor systems since sample and detector are both in the beam M. Sabaté-Gilarte, 2nd Special Workshop on Neutron Detection with MPGDs Diameter 60 mm Neutron converter: 10 B, 235 U or 6 Li

Thin and homogeneous samples as neutron converter o Samples are not exposed to the avalanche process since they are shielded by the micromesh o Converters are deposited on the drift electrode o Use very well known reactions to extract the shape of the flux o Cover a wide energy range: M. Sabaté-Gilarte, 2nd Special Workshop on Neutron Detection with MPGDs  Megas for monitoring purpose at n_TOF 10 B(n,  ) standard from eV up to 1MeV 6 Li(n,t) standard from eV up to 1MeV 235 U(n,f) standard at eV and from 0.15 to 200MeV

Thin and homogeneous samples as neutron converter o Samples are not exposed to the avalanche process since they are shielded by the micromesh o Converters are deposited on the drift electrode o Use very well known reactions to extract the shape of the flux o Cover a wide energy range: As monitor: o Relative energy dependence of the flux is searched while its absolute value is obtained from a reference sample as Au for capture measurements or 235 U for fission reaction measurements o It must be placed as close as possible to the experimental setup Micromegas detectors are also used at n_TOF for neutron-induced reaction cross-section measurement M. Sabaté-Gilarte, 2nd Special Workshop on Neutron Detection with MPGDs  Megas for monitoring purpose at n_TOF

2 microbulks: o  = 3.5 cm o Cu(5μm)/Kap(25μm)/Cu(5μm) Windows: o  = 7 cm o polypropylene 4 μm Drift distance: 5 mm Unsealed converters: o 235 U = 1 mg,  = 2 cm o 10 B = 0.6 μm,  = 3.5 cm (S. Andriamonje et al., ND2010 Proceedings ND 1504) Setup up to 2009 (T. Papaevangelou seminar on ESS, Lund, Sweden, 2014)

2 microbulks: o  = 10 cm o Cu(5μm)/Kap(50μm)/Cu(5μm) Windows: o  = 15 cm o Kapton 12.5 μm Drift distance: 5 mm Unsealed converters: o 235 U = 18.5 mg,  = 7 cm o 10 B = 0.6 μm,  = 15 cm NEUTRON FLUX 2010: borated water + demineralized water as coolant & moderator Setup up from 2010 to 2012 n_TOF Collaboration, Facility performance report monitor  MGAS XY -  MGAS (T. Papaevangelou seminar on ESS, Lund, Sweden, 2014)

Setup from 2014 (n_TOF Phase 3) 2 microbulks per chamber o  = 6 cm o Cu(5  m)/kapton(50  m)/Cu(5  m) Anode segmented in 4-PAD Windows: o Aluminized (few nm) Mylar (9  m) Gas mixture: Ar + 10% CF 4 + 2% iC 4 H 10 Fast preamplifier Converters  (cm)m (mg)  m/cm2 EAR 1 6 LiF U (oxide) B XY-  MGAS EAR 2 6 LiF U (oxide) B XY-  MGAS M. Sabaté-Gilarte, 2nd Special Workshop on Neutron Detection with MPGDs MESH -HV DRIFT- HV Drift distance ear2-Li V V7 mm ear2-U -180 V V10 mm ear1-Li V- 400 V7 mm ear1-U -175 V- 400 V10 mm

One chamber per experimental area: 1 XY profiler mMGAS 2 flux monitors (4 pads each) Tested at CEA with X-ray source ( 55 Fe) Flux monitors were taking data during the commissioning of Phase 3 The new flux and profile monitor Status of XY-  MGAS See M. Diakaki presentation (T. Papaevangelou seminar on ESS, Lund, Sweden, 2014)

6 Li 235 U 10 B neutron beam XY -  MGAS 4PAD -  MGAS Setup from 2014 (n_TOF Phase 3) EAR-1 EAR-2 M. Sabaté-Gilarte, 2nd Special Workshop on Neutron Detection with MPGDs

4-PAD MicroMegas PAD electrode was segmented in 4 to reduce the capacitance and hence the counting rate respect to MESH electrode The expected count rate at EAR2 is 100 times higher than at EAR1 Read out from the anode, 4 pads, and the cathode, micromesh Improve S/N ratio when MESH and PAD signals are compared

4-PAD MicroMegas signals Positive ions are collected on the micromesh and they also induce a signal with opposite polarity on the anode. The electronic is not fast enough to see the electrons collected on the anode. Inside the preamplifier, the polarity of the signal is inverted, therefore: * Negative signals: micromesh * Positive signals: anode - 4pads Reflexions after the signal was observed, this is an indication of an impedance mismatch between the detector and the electronic chain  working on-going Induced signals between the pads (is the same electrode): M. Sabaté-Gilarte, 2nd Special Workshop on Neutron Detection with MPGDs FWHM ns signal from micromesh signal from anode FWHM ns induced signal from another PAD Positive signal on one PAD

Signal on one PAD Signal induced on the others M. Sabaté-Gilarte, 2nd Special Workshop on Neutron Detection with MPGDs

Flux measurement at n_TOF (2014) EAR-2 EAR-1 M. Sabaté-Gilarte, 2nd Special Workshop on Neutron Detection with MPGDs

The energy resolution allows separate different type of particles like in 235 U(n,f) measurement: - Good separation between noise + alpha (radioactivity) and fission fragments - The two fission fragment bumps are also well defined Energy deposited in the detector vs time of flight Energy deposited in the detector alphas fission fragments  MGAS detectors for neutron induced cross section measurement

Fission cross-section measurements Setup for fission 10 Microbulk Micromegas:   = 10 cm  Cu(5μm)/Kap(50μm)/Cu(5μm) Windows:   = 15 cm  kapton 25 μm Drift gap: ~ mm Gas:  Ar + (10%)CF 4 + (2%) iC 4 H 10 Samples:  Pu:  = 3 cm each 3.1 mg ( 25.7 MBq) in total  Pu:  = 3 cm each 3.6 mg all ( 0.53 MBq) in total U:  = 3 cm each 3.3 mg M. Sabaté-Gilarte, 2nd Special Workshop on Neutron Detection with MPGDs (T. Papaevangelou seminar on ESS, Lund, Sweden, 2014)

Setup for fission tagging 3 Microbulks :   = 3.5 cm  Cu(5μm)/Kap(50μm)/Cu(5μm) Windows:   = 7 cm  kapton 25 μm Drift gap: ~ mm Gas: H e + (2%) iC 4 H 10 Samples: U:  = 2 cm each 1 mg each (27.3 MBq) The accuracy in the capture cross sections measurement of fissile isotopes is reduced due to the large background contribution from fission reactions Α total absorption calorimeter can be used in order to discriminate capture events using the Q γ A heavy ion detector can be used to discriminate fission events. Specifications: o Low mass o Insensitive to γ’s o High FFs detection efficiency o Good to FF discrimination  Microbulk Micromegas Fission tagging M. Sabaté-Gilarte, 2nd Special Workshop on Neutron Detection with MPGDs (T. Papaevangelou seminar on ESS, Lund, Sweden, 2014)

33 S(n,  ) cross-section measurement Setup for charge particle 14 Microbulk Micromegas:   = 10 cm  Cu(5μm)/Kap(50μm)/Cu(5μm) Windows:   = 15 cm  kapton 25 μm Drift gap: ~ cm Gas:: Ar + (10%)CF 4 + (2%) iC 4 H 10 Samples:  8 33 S:  = 9 cm each 25  g/cm 2 & 70  g/cm 2  2 blank (sample backing):  = 9 cm each  3 10 B 4 C:  = 9 cm each / 0.9  m 1 6 LiF:  = 9 cm each / 8.94mg M. Sabaté-Gilarte, 2nd Special Workshop on Neutron Detection with MPGDs

33 S(n,  ) FF α 242 Pu(n,f) 235 U(n,f) fission tagging

Summary In-beam neutron beam monitor require: o Low mass detector o Minimal beam perturbation o Thin samples (low interaction with neutrons) o Cover wide range of energy o High geometrical efficiency Micromegas is the ideal detector system for monitoring neutron beam. Different setup were used to measure the flux at n_TOF along the time: o In particular, the 4-PAD was design for new EAR2. o Analysis on-going. Micromegas are also used at n_TOF to measure neutron induced cross section: (n,f) and (n,cp).

THANKS FOR YOUR ATTENTION! M. Sabaté-Gilarte, 2nd Special Workshop on Neutron Detection with MPGDs

Neutron detection  neutron to charge converter Solid converter: thin layers deposited on the drift or mesh electrode ( 10 B, 10 B 4 C, 6 Li, 6 LiF, U, actinides…) o Sample availability & handling o Efficiency estimation  Limitation on sample thickness from fragment range  limited efficiency  Not easy to record all fragments Detector gas ( 3 He, BF 3 …) o Record all fragments o No energy loss for fragments  reaction kinematics o No limitation on the size  high efficiency  Gas availability  Handling (highly toxic or radioactive gasses) Neutron elastic scattering  gas (H, He)  solid (paraffin etc.) Availability High energies Efficiency estimation & reaction kinematics Micromegas for neutrons  Highly performing gaseous detector (gain, energy & time resolution, granularity…)  Radiation hardness  Simplicity  Low mass budget  Transparent to neutrons  Cheap & robust Neutron detection with Micromegas (T. Papaevangelou seminar on ESS, Lund, Sweden, 2014)