Status of PNPI R&D for choice of the MUCH tracking base detector

(this work is supported by INTAS)

■ Introduction

■ MICROMEGAS

■ GEM

■ MICROMEGAS+GEM

■ Future plans

Introduction

CBM requirements :

● rate up to 107 1/cm2∙s events

• in time <100 ns detector should be ready for next event

• high occupancy – thin granularity (especially in central region)

● not too high spatial resolution – σ~500μ

● low discharge probability

● radiation resistance

As candidates to fulfill CBM requirements are considered MM/GEM base tracking detectors. Both types of detectors are working well in real experiments.

Main goals of R&D:

- getting some experience in working with MM/GEM

- finding practical technological solutions and requirements in building the detectors based on MM/GEM

- choosing the working gas

- estimating the descharge probability

- estimating the efficiency

- estimating the radiation rigidity

- getting a competence for designing the prototypes for beam test

MICROMEGAS

75μ

3 mm

cathode

mesh

Pillars made by chemical etching from photo-resistant layer

4mm between pillars, diameter of each pillar - 300μ, height - 75μ

PCB (5x5 cm2)

PA

First steps in MICROMEGAS study were made for 200μ and 100μ mesh-PCB gap. There was not pillars and mesh before gluing to the frame was strained. Results of that were shown at last CBM workshop.

Main reason to move to the gap of 75μ was time width of the signal - full width was ~650 ns for 200μ gap and ~300 ns for 100μ (in Ar/CO2 gas mixture), and have working regime at lower HV (lower energy in discharge).

Following work was made with gap of 75μ.

Metallic meshStainless steel woven mesh (wire - 30μ, cell - 50μ) taken from CERN was rolled and placed on the pillars without any stretch

Ar/CO2 (90%/10%), 55Fe

Gas gain calculated with MAGBOLTZ

Gas gain vs. voltage applied to the mesh, cathode voltage is constant =50V

Estimated energy resolution ~ 15%

Combined metallic-plastic mesh

Gas gain vs. cathode voltage,

mesh voltage is constant =500V

Gas gain vs. mesh voltage,

cathode voltage is constant =1000 V

Example of the spectrum (Fe55 )

Ar/CO2 (90%/10%) Woven mesh (stainless steel/nylon wire – 30 μ, cell - 50 μ) taken from CERN did not have any stretch

Energy resolution ~18%

Ar/CO2 (90%/10%)

GEM (5x5 cm2) was produced by CERN

GEM

Gas gain vs. voltage applied to GEM, cathode voltage is constant =1000V

Example of the spectrum (55Fe)

Energy resolution ~ 12%

Combined GEM+MICROMEGAS (metallic mesh)

1) Uc=1200V, Ut=900 V, Ub=500 V,

Um - variable

2) Uc=1200 V, Ub=500 V, Um=400 V, Ut –Ub - variable

3) Uc=1200 V, Ut – Ub=450 V,

Um - variable

Ar/CO2 (90%/10%)

Combined GEM+MICROMEGAS (metallic/plastic mesh)

Spectra (55Fe) for two different voltages at the mesh

From CERN

Ar/CO2 (90%/10%)

Gas gain vs. voltage applied to the GEM, Umesh= 450 V, Ucathode=1550 V

α- source 241Am (5.5 MeV) response (gas gain ~ 6∙104)

At the following stage for Micromegas we used rolled mesh of Russian production – stainless steel (wire - 32 μ in diameter and cell - 64μ).

We saw the difference in gas gain of ~4 times for the same voltage applied in comparing to previous case (wire - 30 μ in diameter and cell – 50 μ). Qualitatively it looks reasonable. But quantitative estimations we will get later in special measurements for set of different mesh dimensions.

Ar/CO2 (90%/10%)

MM+GEM, low voltage at MM (350 V)

GEM alone

MM+GEMGas Gain vs. voltage applied to GEM

MM Voltage is low (350 V)Note, GG at 440 V at GEM in case of MM+GEM is 40 times more then for single GEM

Ar/CO2 (90%/10%)

GG vs. Voltage applied to the cathode.The modest GG Value (Um=350 V, Ugem=390 V)

The same as previous butvariable Voltage betweenGEM and Mesh

Working points

Ar/CO2 (90%/10%)

He/CO2 (90%/10%) gas mixture

Comparison between Ar/CO2 and He/CO2

Signals (55Fe) measured by the scope

Full width (Ar/CO2) ~ 180 ns

Full width (He/CO2) ~ 100 ns

Changing distance between GEM and MICROMEGAS

to 1-1.5 mm and using HE/CF4 should decrease the collecting time to ~ 50 ns

Comparison between Ar/CO2 and He/CO2

He/CO2 (90%/10%)

Gas gain vs. voltage in the mesh-gem region

Gas gain vs. voltage in the gem-cathode region

Working point

Working point

He/CO2 (90%/10%)

Example of the spectra (55Fe)

Gas gain vs. voltage applied to the mesh

Voltages at the cathode and both side of the Gem were equal

Spectra in HE/CO2 mixture

Parameters of interaction X-Ray Fe55 (E=5.9 keV) He comparative Ar ArPhotoabsorption cross section σ ph =280.2 cm^2/gCompton cross section σ c =0.0688 cm^2/gK1 =3.206 keVMean Energy for ion pair production wi=26 eV HePhotoabsorption cross section σ ph =0.1498 cm^2/gCompton cross section σ c =0.1246 cm^2/gK1=24.6 eVMean Energy for ion pair production wi=41 eV

So for He compton scattering probability is comparable with that for photoionisation and full absorption peak is not so pronounced as in Ar.

Example of the spectra (55Fe)

Gas gain vs. voltage applied to the Gem.Voltages at the mesh was equal 400 V

He/CO2 (90%/10%)

Gas gain is not a problem and we can have it as high as we want.

Use of 3 component gas mixture with small portion (~ 5%) of isobutane gives about two order for gas gain in He at the same voltage.

Using isobutane in working gas mixture should considerably put down working voltage to make lower discharge energy

Scheme for work with β-source (90Sr)

200 μ of FR4+18 μ of Cu

Stainless steel mesh of 60 μ cell And 30 μ wire

400 μ of FR4

Plastic scintillators

Result of GEANT simulation

Efficiency estimation

Real value of efficiency will be received with beam test

Discharge probability estimation He/CO2 (90%/10%)

55Fe (5∙103 counts/s) +

β-source 90Sr (3∙104 counts/s)

Spark probability was estimated as ratio of spark number ( count of the signals laying above some high threshold) to number of total counts

Summary

● It was assembled prototypes of MICROMEGAS, GEM, and MICROMEGAS+GEM detectors

● Prototypes were tested with radioactive sources 55Fe and 90Sr using Ar/CO2 and He/CO2 gas mixtures

● Gas gain, efficiency and discharge probability estimations were obtained

● From our point combined MICROMEGAS+GEM version is good candidate for MUCH base detector

● Some practical technological requirements and approaches for designing and assembling the beam test prototypes were found

Plans for next year

● Variety of mesh geometry

● Three component gas mixture

● Designing and assembling two prototypes for beam test – MICROMEGAS only, MICROMEGAS+GEM

● Preparing to the beam test