TPG tracking for MICE

~~A time projection chamber with GEM readout (TPG) is being developed as an alternative to scintillating fibers. This type of device can produce a large number of points on each track~~ ~~and a minimal amount of material, at the expense of a longer integration time.~~ ~~It is also inexpensive~~.

The reasons for a TPC with GEM amplification as a tracking device for MICE can be summarized in a few points:

· A TPC presents a light and uniform material along and across the beam phase-space~~line~~, yet producing a large number of points on each track.

· The GEMs, manufactured by standard photolithographic processes, are less expensive than the traditional electron multiplication for TPCs (usually wire chambers); at the same time, the GEMs are able to minimize the ion feedback into the drift volume, so reducing the risk of uncontrolled field distortions.

· By choosing an appropriate gas mixture, it is possible to make the detector transparent to the X-ray background emerging from the RF cavity. This allows to counterweight the disadvantage of the long integration time.

2. Operational mode

A sketch of the envisaged device is shown in figure 1 for the downstream spectrometer. An identical one will be installed in the upstream spectrometer. The operational principles of the device are as follows. The sensitive volume of 1m long and 30 centimeters diameter is situated in the homogeneous magnetic field region of the spectrometer solenoids. The electric field provided by a field cage surrounding the sensitive volume is parallel to the guiding magnetic field. The charges produced by ionization of muons are collected on the far side of the chamber with respect to the cooling section, so as to ensure a minimum of material. The charges are amplified by GEM foils, and read-out on a plane of pads from which the signal is shaped in preamplifiers and digitized by flash-ADCs~~fast ADCs~~. The total length of the chamber corresponds to about 120 ~~100~~ samplings at a drift velocity of 1.7 cm/ms, so that the device provides for each track 120 ~~100~~ 2D points times three coordinates. The chamber will be filled with low mass gas (a Helium dominated mixture) thus reducing multiple scattering and offering very small conversion probability for X-rays resulting from the cavities dark current.

Fig. 1 View of the downstream spectrometer of MICE with a TPG as tracking device.

32. Characteristics and performance of the chamber

A complete definition of the operational parameters of the chamber will only be possible after a full scale test to be performed in spring-summer 2003 using the HARP TPC magnet, field cage, digitizing electronics and gas system at CERN. The proponents of the project have been previously involved in the conception, construction and operation of the HARP TPC, and can provide a large fraction of the electronics.

Operation of drift chambers with a helium-based mixture is quite customary, and performance figures will be given here for a 90%He, 10% iso-butane mix at atmospheric pressure. A drift voltage of 500 V/cm provides a drift velocity of about 1.72cm/ms. The maximal potential of -50 kV is situated in the plane of the field window and needs to be degraded in less than 5cm of insulator in the outward direction of the solenoid,, (this can be done with solid insulator such as Teflon,) and in less than about 50 cm in the direction of the liquid hydrogen absorbers , (this can be done with a suitable gas, e.g. low pressure N₂, or with vacuum).

The most probable number of primary ionization electrons along a minimum ionizing muon is calculated to be 12 per cm. These electrons will drift toward the GEMs with a transverse diffusion of 1.4 mm.Öz[m], and a similar longitudinal diffusion. The GEMs are made of 50- micron thick foils with holes of 70 microns diameter at a pitch of 150 microns, as shown in Fig.2. . They introduce ~~very little~~a small additional diffusion;, ~~on the primary electrons~~ ~~(this is not fully true: one~~each primary electron will produce a “spray” of about 0.5 mm to 1mm transverse in diameter on the readout plane. This parameter is not well known and will be measured in the forthcoming tests. ~~This is anyway good, since it gives a sort of “pad response function”)~~. The read-out board described below has a pitch of 450 microns, contributing also very little to the resolution. If the exposure time of MICE during the RF pulse is of the order of 500 microseconds, the available ADCs allow digitization in 1024 time slots of 500 ns second each. Each track is then sampled in 120~~100~~ time slots of 500 ns, 0.851cm long, containing 102 primary electrons each, giving 120~~100~~ points with a spatial resolution, in the worst case, of the order of the pitch of the readout plane~~500~~ .Ö~~z[m]~~ ~~microns each~~. In a perfect chamber this would give a transverse momentum resolution of about 50 KeV/c. ~~? 0.2 MeV/c ?.(to be verified, action Mario)~~

Fig. 2 Photograph of the GEM showing the 70 microns diameter holes at a pitch of 150 microns, and description of the electric field lines in the Gas Electron Multiplier (GEM) foils.

Fig 3: The TPG read-out: left: the 3 GEM foils providing amplification to the hexaboard. Right: the hexaboard structure with a third of the pads (blue) connected in strips at 30^o, one third at 120^o(red), and one third at 90^o(green).

3.1 Read-out

The amplification in each GEM will depend on the high voltage, but it is planed to work at an amplification level of 20-50 per GEM, giving a total signal of 10⁴ to 10⁵ electrons on the pad plane per primary electron. These will be read-out by a hexaboard as described in Fig.3. The signal is distributed among several individual hexagons. The hexagons could in principle be readout individually, but for cost reasons the hexagons will be connected to form strips in three orientations Each primary electron will thus give signals in at least three projections. The strip signals, will be collected from the pad plane and send over flat cables of 16 channels each to the preamp boards. In the present design, each preamp board will collect 48 channels. The signal will be shaped to a length commensurate to the sampling frequency of 500 ns, and then sent to the FADCs. The FADCs from the HARP TPC will be used. .

Prototype preamp boards in “HARP” style and adapted to MICE geometry have been produced and delivered. The channel count is 667 per coordinate, a total of 4000 for two TPGs.

Fig 4. Description of the elements of the TPG.

Fig 5. Simulation of 100 microseconds of data taking with the upstream MICE TPG. Vertical axis is the strip number in each of the three projections (u,v,w at 30, 90 and 150 degrees). Horizontal axis is the time slot number. Each time slot is 500 ns. The muon rate is assumed to be 0.1 per ISIS proton bunch, i.e. 3 per microsecond.

4. Sensitivity to background

Two kinds of backgrounds can be detrimental on the performances of the TPG.

The first kind is the pollution by RF noise, both in the experimental area and in the beam pipe.

Thanks to the mechanical construction, most parts of the detector are screened by thick aluminum parts, which will be duly grounded. The only parts that cannot be enclosed in massive metallic pieces are the GEMs: RF noise coming from the cavity may, in principle, reach the readout plane and produce fake hits. However, even if the GEMs are very light, the thin Cu layer deposited on them is already an effective screen for 200MHz EM waves, since its thickness is equivalent to the skin depth.

Even if we are confident in the immunity of the detector to RF noise, specific tests will be performed with a small prototype chamber to make sure that all the details of the RF immunity, from the readout plane to the FADCs, will be understood.

The second kind of noise will come from the X-ray emission from the cavity. Conversion of X-rays may produce fake hits and, ultimately, endanger the ability to reconstruct a clean set of tracks.

As we have seen, one time-slice corresponds – with the present indication of gas mixture and field cage potential – to about 0.85cm. In terms of conversion probability in the gas, this amounts to about 10^-4 for X-ray energy of 10KeV.

If one makes the hypothesis of a primary X-ray rate of 1GHz 10KeV photons over the whole diameter of the detector, each 0.85cm slice will integrate a few conversions. This is perfectly tolerable, since each slice is read out in 1800 coordinates. Softer X-ray spectra may of course be more dangerous, but we estimate that the rate may still be tolerable at 5KeV.

More precise understanding of the influence of the X-ray background will be possible when an X-ray emission spectrum will be measured.

5. Preliminary timescale for tests in 2003

The tests of a TPC with GEMs read-out will be carried with several aims:

First, the capacity to shield the detector against RF electromagnetic radiation will be tested on a small chamber built in Frascati, equipped with electronics similar to that which will be used for the final detector. This will be done at CERN with the help of the RF group who will provide a tunable antenna radiating at 200 MHz.

Then the exact performance of the read-out system, diffusion properties in the gas and especially in the GEMs will be tested in a 0.7 T magnet with the HARP solenoid and field cage. This will be a test of a full size readout board, in which an already sizeable amount of electronic channels will be involved (600). If the test is successful the readout board could be part of one the final MICE trackers. Fig 6 shows the layout of the test in the HARP solenoid.

Fig 6. Schematic of the test of the TPG read-out in the HARP TPG.

At the same time the response of the TPG and in particular of the GEMs to RF field emission should be tested. The effect of photon conversions in the TPG gas is easily calculable, their effect on the GEMs themselves is not. The 88 MHz test cavity at CERN will be powered at the beginning of 2003. Alternatively, one could bring a small chamber to the labG at Fermilab. This may be a critical issue and should proceed quickly.

~~The goal of the test is to build a real-scale prototype of the readout plane and GEM amplification. They will be tested in the existing HARP infrastructure, located in the T9 beam line at the CERN PS.~~

The exact schedule for construction of the final detectors is being worked out.

52. Costs

From preliminary estimates based on the construction for the tests in 2003, one can estimate the cost of the readout and amplification system to be less that 100K$ per tracking station.

Based on the cost of the HARP field cage, whose design is, for the moment, being considered for the MICE TPGs, we estimate building the two field cages may account for 200K$.

Ancillaries like the gas and HV system may account for another 100K$.

6. Links

The Cern Gas Detector Development page:

http://gdd.web.cern.ch/GDD/

The MICE web page:

http://hep04.phys.iit.edu/cooldemo/

The CERN muon storage rings web page

http://muonstoragerings.web.cern.ch/muonstoragerings/

The MICE LOI submitted to PSI and RAL can be found here:

http://proj-bdl-nice.web.cern.ch/proj-bdl-nice/cool/loi-final-ral.pdf

One can see there that the tracking devices that were considered in November 2001 were either scintillating fiber trackers or silicon trackers. A sketch of MICE with these trackers is here:

http://proj-bdl-nice.web.cern.ch/proj-bdl-nice/cool/micesketchold.pdf

One of the problems of these proposals was the expected noise from photons and electrons generated by the RF cavities of the cooling section. In particular, the combination of inefficiencies in the scintillating fibers and noise made it feared that reconstruction would be seriously hampered by ambiguities. G. Barr discussed this in this note:

http://proj-bdl-nice.web.cern.ch/proj-bdl-nice/cool/barrnoisenote.pdf

In February 2002 Ugo Gastaldi suggested that one could use a TPC with GEM readout for the trackers of MICE. The first ideas and sketches together with a possible scheme for the readout can be found in this talk:

http://hep04.phys.iit.edu/cooldemo/detectors/gastaldi_32002.pdf

or in the draft note:

http://proj-bdl-nice.web.cern.ch/proj-bdl-nice/cool/gastalditpgdraft.pdf

which was presented in one of the MICE detector working groups meetings

http://hep04.phys.iit.edu/cooldemo/detectors/detectors.html

in March 2002

The readout has been discussed by Emilio Radiccioni

http://proj-bdl-nice.web.cern.ch/proj-bdl-nice/cool/tpgemiliomarch2002.pdf

who also presented a program of tests:

http://hep04.phys.iit.edu/cooldemo/cm/cm4/talks/radicioni_tpg.pdf

The track reconstruction and resolution was studied by Mario Campanelli

http://hep04.phys.iit.edu/cooldemo/cm/cm4/talks/campanelli_tpg.ps

This note describes the proposal of a Time Projection chamber for MICE with GEM read out. It is understood as a beginner’s introduction with the sake of providing refernces for those interested in the project.

~~The MICE LOI submitted to PSI and RAL can be found here:~~

~~http://proj-bdl-nice.web.cern.ch/proj-bdl-nice/cool/loi-final-ral.pdf~~

~~One can see there that the tracking devices that were considered in November 2001 were either scintillating fiber trackers or silicon trackers. A sketch of MICE with these trackers is here:~~ ~~http://proj-bdl-nice.web.cern.ch/proj-bdl-nice/cool/micesketchold.pdf~~

~~http://proj-bdl-nice.web.cern.ch/proj-bdl-nice/cool/barrnoisenote.pdf~~

~~http://proj-bdl-nice.web.cern.ch/proj-bdl-nice/talks/ugo.pdf~~

~~which was presented in one of the MICE detector working groups meetings~~

~~http://hep04.phys.iit.edu/cooldemo/detectors/detectors.html~~

~~in March, see~~

~~http://hep04.phys.iit.edu/cooldemo/detectors/detectormeetingMar2002.html~~

This having a rather imprecise timing information (100 ns time slots at least) it would have to be complemented with at least one plane of scintillating fibers and a segmented time-of-flight system. Nevertheless, the fact that it could provide hundreds of space points along a track with potentially very small multiple scattering (for a He-isobutene filled TPG) allied with very small mass for photon conversions, make it a very interesting option. The space resolution with Helium filling needs to be evaluated.

~~The readout has been discussed by Emilio Radiccioni and seems to be quite OK:~~

~~http://proj-bdl-nice.web.cern.ch/proj-bdl-nice/cool/tpgemiliomarch2002.pdf~~

Finally the issue of pulling out the signals will have to be investigated in detail, for reasons of noise and of the relationships with the other detectors. A possible sketch of how to do this for the downstream spectrometer can be found here.

~~http://proj-bdl-nice.web.cern.ch/proj-bdl-nice/cool/micesketch.pdf~~

~~What needs to be done?~~

Ahe first and most urgent thing to do in my opinion is to begin the hardware test of the readout cleanliness in the vicinity of a mega-monster: the RF cavities are a mere three meters away in the MICE set up and at the time when the muons pass by there are being powered by an instantaneous power of 4 MW! this is bound to require very careful shielding of the readout system, if it can work at all.

~~Also needed is a simulation of the performance of this device plugging in first the space resolution as function of drift time, then the possible effect of noise.~~