NA 62 – straw detector

NA 62 – straw detector Answers Straw review 20/11/2009

Answers 1. Specifications: - What should be the spatial resolution of the tracker: ~100m, <100m, <60m ? Answer:~130m for 1 straw and ~70m for 4-coordinate straw chamber - What is the expected maximum particle flux and as a result the maximum expected channel occupancy for the 10 mm straws? Answer: ~500 kHz/straw occupancy = 0,15 - What is the time structure of the spills – this is a necessary input to decide on the drift velocity and thus on the possible counting gases. Answer: The assumption is LHC bunching beam structure and we need additional information about spill structure of SPS extracted beam. However, in the baseline duty cycle with a flat top of 4.8s we have assumed 3s uniform profile, which gives a 500kHz in the hottest straws. - As the hermeticity is crucial for the experiment the current planned 200 moverlap between the straws might impose to stringent requirements on the straw straightness. Could the overlap not be slightly enlarged to 300 or 400 m ? Answer: The measurement on the NEW 64 straws prototype will show if we need to change the distance and with what amount. The first results will be presented in the next review meeting. For track measurement it is not important; for usage of straw detector as a charged particle veto it is very important. Each 200 m gives ~4% additional material in detector. Straw review 20/11/2009

- For the reconstruction of the single track signal events ( ) cross talk in the neighboring channels might lead to unwanted reconstruction arte-facts and wrong track parameters. Is there an upper limit which digital cross talk level is acceptable for the track reconstruction? Answer: Typical cross-talk level <= 3% is ok! - Which noise level can be tolerated? Answer: Thermal noise sigma is about 1 fC (Th=3-4 fC). An acceptable level is up to 2.5 fC (Th=10 fC) and gas gain G=10**5 - It should be foreseen to “software” align the position of every anode wire. Answer: yes What is the maximum dos2. Strawtubeoperation point: As counting gas a quiet unconventional mixture of CO2/isoC4H10/CF4 (80/10/10) has been chosen for most of the tests performed so far. What is the motivation to select this gas which, because of his iso-butane content, is prone to aging, and which, because of the CF4 content could lead to etching of the metal coating on the cathode and of the wire. Moreover the CF4 has a large attachment coefficient, which significantly degrades the resolution, in particular for small radii (near the anode wire). Answer: To achieve good spatial resolution tha gas mixture CO2/isoC4H10/CF4 with a low electron drift velocity was proposed. Iso-butane is good quenching admixture and CF4 – for cleaning cathode and anode from aging products. Why is a more common choice for the counting gas such as Ar/CO2 (80/20) not considered? Are the large drift velocities of the CO2/isoC4H10/CF4 mixture really needed (see the question on the time structure of the expected spill/events). Answer: 1) CO2/isoC4H10/CF4 mixture has a LOW electron drift velocity Straw review 20/11/2009

2) Garfield simulation results shown bad spatial resolution near anode wire up to R=2 mm at nominal gas gain (G=4x104). High gas gain (~105) for this gas mixture is impossible to obtain due to bad quenching properties CO2 for Ar. Nevertheless, Ar/CO2 mixture will be studied in parallel. The operations conditions presented in the meeting have been optimized to give the best resolution and the highest possible efficiency: as a result gas gains larger than 105 have been chosen. While the efficiencies and resolutions for this gas gains have been shown no number for the cross talk (which should be very high at these gains) have been mentioned. Answer: Spatial resolution from HV dependence is shown that a minimal gas gain is G=2x104 If there is no timing reason to choose the CO2/isoC4H10/CF4 mixture, it probably would be much easier and less risky to use a simple Ar/CO2 mixture. Why was this not considered. Answer: Garfield simulation results shown bad spatial resolution near anode wire up to R=2 mm at nominal gas gain (G=4x104) . Given the concern Ar/CO2 will be studied in parallel. The choice of the counting gas should also consider the properties of the final electronics. Answer: Ok. We will study this in the 64-straw prototype. Straw review 20/11/2009

3. Aging: What is the maximum dose in the “hottest” detector area up to which the detector has to operate (see also above: max. particle flux). Answer: Q=(ionization Ne=100e-)x(gas gain G=10**5)x(Time of ion drift T=10 msec)x(Flux per straw N=0.5 MHz)x(time data taken T=0.7x3.15x10**7 sec) = 0.02 C per year 5 years: Q=0.1 C Max: Q= 0.1 C per 10 cm anode wire per 5 years (near beam pipe) Anode wire length with max flux L=30 cm (beam halo) The aging tests performed so far focused on the classical anode aging due to impurities or gas admixtures such as iso-butane. Equally worrisome is the high-rate behavior of the cathode: due to the (electrically) very inhomogeneous cathode surface (the metal coating is removed in the ultra-sonic welding process) a charge-up of the cathode under higher irradiation cannot be excluded. As a result sustained discharge currents (Malter effect) could build up. A large area irradiation at high intensities should be performed. The anode currents should be carefully monitored (w/ and also w/o irradiation). In case CF4 is used as a gas admixture the long-term behavior of the wire coating and the cathode coating should be studied (in particular in case aluminum is used as cathode coating) in a large area irradiation test. Answer: It need to be studied when the first prototype becomes operational. Possible test in GIF++ or elsewhere, using the 64-straw prototype, is envisaged. This includes a study of the etching of the cathode and possible Malter effect. Straw review 20/11/2009

4. Straw ultra-sonic welding procedure The current welding gives at least for the majority of the straws already astonishing good welding results. Currently, the origin of the few bad welds is not systematically understood. The PHYSICS of the welding process must be understood. Only then, one shall proceed to specification and approval procedures. - How can one ascertain the intense rupture of the metallization, needed for a good, diffuse melting zone, with a "normal" formation and spatial density of chain entanglements (essential to toughness) ? Would it be safer to remove (by abrasion ?) the metallization locally prior to welding ? - What is the importance of the "acoustic impedance" of the mandrel ? (If not negligible, then, for example, check regularly for fatigue cracks !) - Where, on the frequency axis, are the peaks of the dielectric (*) tan(delta) (at the relaxation eigenfrequencies of course, but which are the freq. values ?) ? Shall one tune the sonotrode to one of these peaks ? Answer: A request was send to ULTRASONIC co. (Moscow) and Metal-Polymer Research Institute (Gomel, Belarus) Optimization and Monitoring: - How have the parameters of the welding process been optimized (feed-forward speed, compression, power)? - To which extent these parameters are continuously monitored and are reproducible? Answer: - Continuously monitored - Ultrasonic generator frequency and amplitude Straw review 20/11/2009

and at a given interval verify the: - compression of the seam. - feed-forward speed fixed: V=2.4 mm/sec - friction with the head: F<= 100 gram When is a weld good? Answer: - weld width h=(0.4-0.8) mm - weld shape – uniform (by video control) - tensile test – will be specified - How will the quality of the welding process be controlled in the future? Is the optical inspection enough? Should a regular tensile test be included in the quality monitoring? Answer:: - optical inspection of each straw during it manufacture - regular tensile test of each straw end (start and finish samples) - overpressure test at dP=3 bar per T=10 min - gas leak measurement, leak les than 0.002 cc/min - long term test (dP=1.5 bar at min T=1-3 month) Breaking straws: In overpressure testing, final breakdown systematically occurs on the longitudinal seam. More systematical studies are needed to relate results from the optical inspections to the breaking points of bad straws. A failure analysis of the bad straws should be performed. A specification and approval of the welding procedure including welding procedure tests tailored for the final application should be foreseen. Alternative: In order to avoid the local absence of DC-conductivity as a result of the welding, the possibility of sourcing graphite-loaded Mylar should be investigated. Answer: The search for graphite-loaded Mylar producer has started (Moscow, “Polimer”). This is a new material for us! Straw review 20/11/2009

5. Permeation The current gas permeation of the straws extrapolated to the full detector is probably to high. Do we understand why the coating of the straws does not give the expected improvement (only a factor ~ w/r to non-coated straws). Are surface damages the reason of the high permeation of coated straws? What is the preferred coating: 2aluminum, 1Cu+Au ? Answer: We will study one coating and the preferred coating is Cu/Au. Optimization of the thickness is now needed. What is the influence of the welding seam on the gas permeation? Answer: The plan is to measure this once we get the new material and can make new straws. - Since it appears that a metallic coating is by no means a perfect barrier, much depending on environmental conditions during deposit, feedstock for measurements should be sourced from VARIOUS suppliers. One of the significant applications of coated PETP foils is permeation barrier. Answer: We are looking for different suppliers of straw material. - Measurements should be done on bare feedstock samples directly, avoiding confusing influences of coating damage (scratches) and welds. Answer: This is planned once we have made straws with the new material (Cu/Au) - It is understood that a post-metallization of the assembled straw would be very painstaking. However, in view of the severity of the problem at hand, such possibilty should be addressed seriously. Answer: Discussion has started Straw review 20/11/2009

6. Front-end electronics As the CARIOCA chip is the baseline choice for the front-end electronics the detector performance (resolution, efficiency, cross talks) has to be determined with the CARIOCA on a realistic chamber prototype including straws, feed-throughs, flexible cables, and using the preferred counting gas mixtures. The initial gas gain should not be higher than 5104. With this setup define a minimum threshold setting at which the electronics can be operated. Do a HV scan and measure resolution, efficiency and cross talk. For a gas gain of 5104 do a threshold scan and measure resolution, efficiency and cross talk. Answer: This will will be done during October beam test What is the maximum tolerable (non-linearities) excursion (signal level) that CARIOCA can have at its input ? From there on, an upper limit on the allowable intrinsic noise from the (resistive)straw can be set. It will also tell you something (more) on "gas_gains_that_make_sense". Electro-dynamical properties of the straws: as the metal coating of the straws is probably thinner than the skin depths for the interesting frequencies the amplitude cross talk of the straws might be very high. Should be studied. Answer: This will be done before the October beam test. The straw and the wire is a transmission line and it needs to be treated as one unit. Straw review 23/11/2009

7. Relaxation and straw traction The deformation of polymers, at at given temperature, is an essential function of time. The decrease of modulus, typically as result of a glass transition (a "relaxation mode"), with temperature, for a given rate, will also be seen by increasing the test time duration, at a given temperature. This is often wrongly called "creep". It is wrong, because relaxation is a linear effect, so playing a role for the smallest stresses already. True creep, as non-linear effect, would be an additional problem setting in at higher loads. According to a published systematic study of polyethylene terepthalate the effective (relaxation) modulus after 5 years would be less than half the initial modulus. In a chamber design of frozen elongation, one would thus loose more than half of the traction force. The tests done so far are not clear-cut enough to contradict the above results, and even if they were, then still it should be understood why such discrepancy is observed. 8. Gluing of end plugs Guaranteeing long-time reliability of room-temperature-glued interfaces is a non-trivial thing. These interfaces have to be gas-tight, and are constantly subjected to the straw traction. Surface preparations may be different between PETP and coated PETP. The need or benefit of a primer has to be addressed. High temperature curing to enable chain entanglement has to be studied? The issue needs expert attention. J. M. Hawthorne, Stress relaxation behavior of biaxially oriented poly(ethyleneterephthalate), Journal of applied polymer science, Vol.26, 3317-3324 (1981). Straw review 20/11/2009

From Neil Straw review 20/11/2009

Spares Straw review 20/11/2009

NA 62 – straw detector