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Aging Effects in the Central Jet Chamber of the H1 Experiment

Carsten NiebuhrDESY, Notkestr. 85, 22607 Hamburg, Germany

1. The H1 Tracking System

The H1 detector [1] is a nearly hermetic multi-purpose apparatus built to investigate the inelas-tic high-energy interactions of electrons and pro-tons at the HERA ep collider. Closest to the in-teraction point are the central and forward track-ing systems (see figure 1), which are surrounded

Figure 1. Cross section of the H1 track detectors.

by a liquid argon calorimeter consisting of an elec-tromagnetic and a hadronic section. A super-conducting solenoid surrounding both the track-ing system and the calorimeter provides a uni-form magnetic field of 1.15 T parallel to the beamline. The central jet chamber (CJC) consists oftwo concentric rings covering polar angles of 15◦

to 165◦. It is supplemented by two cylindricaldrift chambers at radii of 18 and 47 cm to de-termine the z−coordinate of the tracks along thewire direction. A cylindrical proportional cham-ber is attached to each of those z−drift chambersfor triggering.

1.1. Parameters and Operation of the CJCDetails of the construction and the readout of

the CJC can be found elsewhere [1,2], here onlythose characteristics are summarized which arerelevant for this study.

The inner ring of the CJC, CJC 1, has 24 lay-ers of sense wires strung parallel to the beam axisand covers the radial range 20 < r < 45 cm,while the outer ring, CJC 2, consists of 32 lay-ers covering the range 53 < r < 85 cm. Formost of the running period from 1992 – 1998 anArgon(50%)/Ethane(50%) gas mixture with 0.1%H2O as additive was used (only until May 1999,see section 4 for modifications).

CJC 1 and CJC 2 are connected to one com-mon gas circuit[3]. Gas is circulated through thechambers in parallel at a flow corresponding to 1– 2 chamber volume exchanges per day. Fresh gasis mixed into the system at a rate of 500 l/day.The closed loop circuit is supplied with purifi-cation units for removing oxygen, (water) andlow concentrations of higher organic compounds,which are removed at room temperature by acombination of 1.5 kg 3 Angstroem and 0.5 kg5 Angstroem molecular sieves. Hygrometers andan O2-trace analyzer allow an early detection ofleaks in addition to the complete analysis of thegas composition which is performed by an au-tomatic gas chromatograph station about twiceper day. The sensitivity of this device is about300 ppm.

The chamber is operated at a gas gain of 1 – 2·104. Under normal operating conditions and lowbackground the chamber current in both cham-bers scales almost linearly with the current in theHERA-e ring (positrons or electrons). Typicalvalues in 1997 for a HERA positron current of30 mA were about 25 µA in each of the chambers(see section 4 for deviations from these values for

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Table 1Parameters of CJC 1 and CJC 2

CJC 1 CJC 2nr. of cells 30 60nr. of wires per cell. . . sense wires 24 32. . . potential wires 50 66. . . cathode/field wires 59 75chamber dimensionsinner radius / mm 218.3 544.8outer radius / mm 425.8 823.2active length / mm 2200 2200gas volume / m3 1.14 3.08wire material and diameter. . . sense wires 20 µm gold-plated W (+2% Re) (first and last sense wire: 25 µm). . . potential wires 127 µm Cu (+2% Be). . . cathode/field wires 180 µm Cu (+2% Be)gas system closed common circuit with purifier (molesieve 5A / 3A + Deoxo)gas flow exchange rate ≈ 1 – 2 volumes / daygas tubing ≈ 150 m Cu and stainless steel gas tubinggas mixturestandard ep running 50 % Ar + 50 % C2H6 + 0.1 % H2O (≥ May 1999: → 0.8 % C2H5OH)in other periods 89.5 % Ar + 9.5 % CO2 + 1% CH4

electron running).

2. Wire Corrosion

Already after the first few months of opera-tion at HERA several sense wires of CJC 1 spon-taneously broke during a maintenance operationin the shutdown 1992/1993. The overall HERAschedule did not allow opening of the chamber.Therefore, the broken wires only could be pulledout of CJC 1 to minimize the regions which wereaffected by shorts and the chamber was put backinto operation again. Subsequently, more wiresbroke so that over a period of 7 months in total48 wires broke, randomly distributed in CJC 1,while no damage was observed in CJC 2. Thewires were found broken at the chamber end wallsclose to the point where they are fixed with crimpparts. At the tip of the crimp part a little brass

insert with a 60 µm diameter bore acts as wirefeedthrough. Detailed investigations of both thebroken wires and the crimp parts were performedusing raster electron microscopy with x-ray mi-cro analysis (REM/RMA) and laser mass spec-troscopy (LAMMA [4]), the latter giving sensi-tivity to elements down to hydrogen.

Figure 2a shows that there are lots of impu-rities around the 60 µm bore of the brass insertwith sharp edges causing damage of the gold layeron the wire surface. The impurities are probablyremnants from the gilding process of the crimpparts. The tungsten wire is therefore exposedto complex chemical reactions as can be inferredfrom the LAMMA spectrum in figure 2c of one ofthe wires which shows the presence of a large vari-ety of chemical compounds involving other metalssuch as Na, Ca, and K. Particularly important arecharacteristic peaks between 300 and 400 atomic

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ab

c

Bi

t�ungsten bronzes

Mx WOY

20µm

WCa2Cl

O

CuCl

Cu

Ni

K,CaNaCH

amu

Figure 2. (a) 60 µm bore of wire feedthroughwith remnants from gilding process. (b) end of abroken wire. (c) LAMMA spectrum of wire endindicates the presence of hydrogen and tungstenbronzes.

mass units (amu) which suggest the formation oftungsten bronzes (of the form MXWOY where Mstands for the metal) and a peak at 1 amu whichindicates the occurrence of hydrogen. Tungstenbronzes are known to become brittle when theydissolve sufficient amount of hydrogen. Takingalso into account the observed fracture morphol-ogy (figure 2b) the most likely explanation forthe effect is hydrogen-induced brittle fracture ini-tiated by chemical reactions with remnants fromthe gilding process. However, it remains an openquestion why no such damage was observed inCJC 2 although the manufacturing process forboth chambers was identical.

As a consequence, the whole production andcleaning process as well as the quality control ofthe crimp parts were significantly improved. In

Figure 3. Relative hit efficiency as a function ofwire position. The first and last wires in each cell(25 µm diameter) operate at lower gas gain andare shown with different symbols.

the new design the brass inserts were replaced byjewels as are commonly used in watches. In theshutdown 1993/1994 all sense wires of CJC 1 werereplaced and no further corrosive wire damage hasbeen observed since then.

3. Sense Wire Deposits

In 1996 a small ϕ-dependence of the gas gainwas observed in CJC 2 and to a smaller extentalso in CJC 1. This was interpreted as a result ofthe increased top–bottom temperature gradientinside the tracking volume following an upgradeof the backward region of the H1 detector. Tem-peratures ranging from 30 to 45 degrees Celsiuswere measured with sensors placed at the innerface of the CJC end walls. A compensation ofthe gain differences was made by local HV ad-justments in summer 1996. While the situation inCJC 1 was stable, the top–bottom asymmetry inCJC 2 continuously worsened during 1996/1997although the corresponding integrated charge onwires of CJC 2 was lower than on wires of CJC 1and was estimated to be < 0.01 C/cm. Due tothe larger radial distance to the beampipe andthe larger number of cells the current per wire is

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Figure 4. Wires from three different heightswithin the affected region of CJC 2. The amountof deposits depends on the height of the wire inthe chamber.

typically a factor of 3 smaller in CJC 2 than inCJC 1.

The situation at the end of the 1997 run-ning period is graphically represented in figure 3,which shows the relative hit efficiency for CJC 2wires as a function of their position in the cham-ber. A strong correlation between efficiency andy-coordinate (height) of the affected wire is ob-served. While for wires above y = −30 cm thereis no sign of degradation visible, the wires be-low this level show an almost linear dependenceof efficiency on height reaching relative hit effi-ciencies of about 30 % at the lowest point in thechamber. Wires were removed from the cham-ber and scanned using an electron microscope[5].As an example, pictures of three wires togetherwith a sketch indicating their original position inCJC 2 are shown in figure 4. The wires are coated

C

O

Au

C

O

Au

Figure 5. Energy-dispersive analysis of X-rayspectra (EDAX) taken from two different wires.

with deposits, which get more prominent for wirescloser to the bottom of the chamber. In order toquantify the effect, an energy-dispersive analysisof x-ray spectra was performed. Figure 5 showsthe spectra of two different wires. Normally, justa prominent gold peak is observed while wiresfrom the affected region of CJC 2 show additionalclear peaks of carbon and oxygen. No evidenceof silicon is found on the sense wires, nor are anydeposits seen on the adjacent potential and cath-ode wires. The abundance of carbon and oxy-gen relative to gold was estimated by forming thepeak ratios for a rough quantitative measure. Amonotonous relationship between y-coordinate ofthe wire, hit efficiency and relative abundance ofcarbon and oxygen is observed and is displayedin figure 6. Some amount of carbon and oxygenis already detected on a wire close to the kneein the efficiency plot which does not yet show adegradation of hit efficiency.

Although aging was only observed in the lowerpart of CJC 2, all sense wires of CJC 2 werereplaced in the shutdown 1997/1998 making useof the improved design of the crimp parts devel-

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oped for the repair of CJC 1 in 1993 (see sec-tion 2). The striking dependence of the amountof aging on the height of the wire in the cham-ber strongly suggests an influence of impuritiesin the gas with a gradient in concentration dueto gravitational and/or temperature effects pos-sibly in combination with an insufficient gas flowthrough the chamber. Until the end of 1997, thegas supply to CJC 2 (CJC 1) was done through6(4) inlets at the outer circumference of the cham-ber which were connected to a single gas supplyring. In the same way the gas was taken out onthe other side of the chamber. This setup didnot allow the gas flow through the individual gasinlets/outlets to be controlled, nor did it enableseparate gas analyses of gas coming from differ-ent regions of the chambers. However, the rou-tinely performed gas analyses done with the gaschromatograph, which is part of the gas system,did not show any impurities above the sensitivityof this device of 300 ppm. In order to investi-gate the possibility of an contamination inducedfrom parts of the gas system, a dedicated analysisof gas samples taken during the 1997/1998 shut-down without the chamber in the gas circuit wasdone using a highly sensitive setup available atCERN [6] involving a mass spectrometer (MSD,sensitivity O(ppm)) and an electron capture de-vice (ECD, sensitivity O(ppb)). This analysis didnot yield conclusive results but as a preventivemeasure the gas supply rings were replaced withindividual gas pipes equipped with flow meters inorder to guarantee a more homogenous gas dis-tribution within the entire chamber volume. Adisadvantage of this modification was a further re-duction of the overall gas flow through the cham-ber (for possible consequences see section 5).

4. Malter Effect

Another problem occurred in 1998/1999 whenHERA was operated with electrons instead of thepreviously used positrons leading to significantlyincreased background levels in the H1 interactionregion. The experience from 1992 to 1997 hadshown that under normal conditions the chambercurrent scaled rather well with the positron cur-rent in the HERA-e ring. Therefore, the ratio of

Figure 6. Correlation of hit efficiency with y-coordinate of wires in CJC 2 (a) and with theobserved fraction of carbon or oxygen to gold inthe corresponding x-ray spectra (b). In (a), wireswhich have been investigated with the electronmicroscope are marked with full circles.

the chamber current and the positron (electron)current in HERA is used as a measure of the back-ground conditions. As can be seen in figure 7,where the ratio is plotted as a function of time,the background situation was much worse whenHERA was filled with electrons. As a result, inseveral regions of CJC 1 and CJC 2 sudden stepsin the cell current of the order of 1 – 2 µA wereobserved in 1998. Simultaneously increased cur-rents were also seen in the two adjacent cells onboth sides, which immediately disappeared oncethe central cell was disconnected from high volt-age. This was done to prevent more damage forfurther data taking. Analysis of some data takenwith the increased cell current revealed that in alocalized area in r×ϕ×z no proper chamber sig-nals were recorded. The observations made areconsistent with the Malter effect occuring on thecathode wires. As a practical remedy, before anaccess to the chamber was possible, the 0.1% ad-ditive of H2O was replaced with 0.8% of C2H5OHstarting in May 1999 (no attempt was made toincrease the water content beforehand). Exceptfor the region where the Malter effect showed upfirst and therefore an increased current was drawnfor an extended period before being noticed, allother regions could be put back into operationagain this way. Subsequent visual inspection re-vealed black deposits on an area of a few cm2 ofthe cathode wire planes of the problematic areas.

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Figure 7. CJC 1 chamber current normalised tothe positron- (electron-) current in the HERA-ering as a function of time showing a strong in-crease in background when HERA operation waschanged from e+p to e−p mode.

5. Present Status

At the end of the 2000 data taking period anindication of a possible gain drop was seen againat the bottom of CJC 2. As a preventive measurethe sense wires in the lower part of the chamberas well as all wires in a region around the areaswhere Malter effect had been observed were re-placed. In the future CJC 1 and CJC 2 will beconnected to identical but separated gas circuitsin order to allow an increased gas flow throughboth chambers.

6. Summary

Various kinds of aging effects have been ob-served in the central jet chamber of the H1 exper-iment after moderate integrated charges on thesense wires:

• wire corrosion in CJC 1,

• Malter effect on cathodes of CJC 1 andCJC 2,

• sense wire deposits in lower part of CJC 2.

By modifying sense wire crimp parts, rewiringand changing gas additives the symptoms couldbe cured. However, a number of questions arestill open. Up to now there is no satisfactory ex-planation why the corrosion process was observedonly in CJC 1 or why the sense wire aging only

occurred in the lower part of CJC 2. The obser-vations do, however, underline the importance ofhaving control over as many system parametersas possible from the construction of a chamber toits operation.

REFERENCES

1. H1 collaboration, The H1 Detector at HERA,Nucl. Instr. Meth. A 386 (1997) 310; TheTracking, Calorimeter and Muon Detectors ofthe H1 Experiment, Nucl. Instr. Meth. A 386(1997) 348.

2. J. Burger et al., The Central Jet Chamber ofthe H1 Experiment, Nucl. Instr. Meth. A 279(1989) 217.

3. S. Masson, Entwicklung, Bau und Be-trieb universeller Anlagen zur Gasversorgungfur das innere Spurkammersystem des H1-Detektors, Ph.D. Thesis, PITHA94/50,RWTH Aachen (1994).

4. Courtesy of V.O.M. Verbundzentrum furOberflachen- und MikrobereichsanalyseMunster, Germany.

5. Courtesy of O. Greis, Zentralbereich Elektro-nenmikroskopie TUHH, Hamburg-Harburg,Germany.

6. Courtesy of Mar Capeans, CERN, Geneva,Switzerland.