Nuclear Instruments and Methods in Physics Research A325 (1993) 326-334North-Holland

The use of multiwire proportional counters in a cosmic rayair shower experiment

Laurence Horton, Juris Ulrichs and Murray WinnFalkner High Energy Physics Department, School of Physics, University of Sydney, NSW2006, Australia

Received 22 June 1992

We discuss the design and performance of multiwire proportional counters working as "proportional mode counters". An arrayof 511 of these counters was used to record the densities of particles near to the core of PeV air showers

1. Introduction

Multiwire proportional counters (MWPCs) are nor-mally used in a mode where each individual particle istracked through the detector . Many wires are used,each one fed to one channel of the recording system .The pulse amplitude from each wire is generally not

recorded, only whether there is a pulse or not. In thispaper we examine the use of MWPCs in the propor-tional mode, where a MWPC defines a small sensitivearea (10 em x 10 cm). Onewishes to know the numberof particles passing through this area in each showerand this was obtained from the amplitude of the pulsegiven by the MWPC at the time of the event. It was notimportant for our work, to know the coordinates ofevery particle traversing a counter.

512 of the MWPCs were assembled into an arraycovering an area of about 4 m x 4 m and of these 511

were used to record the particles in cosmic ray airshowers (see figs . 1 and 2) . This array was continuously

active, the recording of data being triggered by an eightscintillator cosmic ray air shower array described else-where [1]. The scintillator array was used to detect ashower and, from the scintillator records, determine:(a) the arrival direction of the shower (the azimuth

and zenith angles of its axis),(b) the core position of the shower (the coordinates

(X, Y) of the point where the shower axis passesthrough the array plane) and

(c) The size, N, of the shower (an estimate of the totalnumber of particles in it).

The two array system was built to obtain information

on the particle density patterns at the centres of cosmicray air showers caused by primary cosmic rays of ener-gies in the PeV region . We hope that a detailed studyof these patterns will lead to better understanding ofthe particle physics in air showers and that this, in turn

will ultimately lead to better understanding of thecomposition of the primary cosmic ray beam at PeVenergies .We chose to use MWPCs because they can be made

thin (a few mg cm -2) and, if the detector gas is at nearatmospheric pressure, the counters can have very thinentrance windows. As a result, the density of showerparticles is not expected to be significantly changed bythe presence of the detector ; neither reduced by parti-cle absorption nor increased by local showers startedby particle interactions in the cell . This is distinct fromscintillators ; even though it is normal to use organicscintillators with average atomic numbers close to that

0

0168-9002/93/$06 00 © 1993 - Elsevier Science Publishers B.V All rights reserved

NUCLEARINSTRUMENTS& METHODSIN PHYSICSRESEARCH

Section A

0

Fig. 1 Semtillator array general layout (see ref . [11) . Four fasttiming scintillators (larger open squares), each of 0.64 m2were placed at the corners of a 25 m square Within thissquare was a 3.9 mX 3.9 m square of 0.16 m2 triggeringscintillators (smaller open squares) with the MWPC array

nested inside it. (Further data given in ref. [11.)

Fig. 2. The MWPC array (small black squares) together withthe four triggering scintillators (open squares) are shown. Asdescribed m ref. [1], a three fold coincidence from thesescintillators initiated recording of the data from both thescintillator and MWPC arrays . A box of eight MWPCs (smallopen squares) at the centre of the MWPC array had anotherbox of eight counters installed below it for comparison pur-poses. Also shown as a small open square is counter number23 which was continuously exposed to a gamma activatedsilver secondary X-ray source in order to test the counter gas

composition (see appendix) .

for air, the scintillator is solid and thus has a densityhundreds of times greater than its surrounding medium,leading to a greater chance of interactions and incor-rect estimates of particle densities [2].

2. Design of the multiwire proportional counters

The specifications of the MWPC array and its coun-ters are given in tables 1 and 2.

of each counterNumber of wiresWireTopwindowTop cathodeBottom cathodeBottom of gas boxGas mixtureGas pressureOperating voltage

L. Horton et al. / MWPCs in a cosmic ray air shower experiment

" " "MEMMEMEMNE

10 cm x10 cmx2cm thick60 020 mm diam . gold plated tungsten7 mg cm- Z aluminised plastic film9 mg cm -2 aluminium1.6 mm aluminium1.6 mm steelAt 94% CO, 6% continuous flowatmospheric1520 V

Table 2MWPC array specifications

glass epoxy printed circuit material, spaced 12 cm

apart, were mounted on a perspex/ plexiglas frame 12

cm wide . Six equally spaced anode wires ran from a

copper track on one transverse epoxy strip to thecorresponding track on the next strip and were at-tached to the strips by soldering; the wires in eachcounter were connected electrically in parallel .

The choice of six wires within the 12 cm width wasmade after a series of tests where the number of wireswas progressively increased, first two, then three etc.

up to six wires. Pulse height spectra were taken using a

collimated beam (2.3 mm wide) of X-rays from a sec-

ondary X-ray source (zirconium excited by [3s from

147Pm). The beam was scanned across the counters in adirection at right angles to the wires. There was an

unacceptably large variation in response for two andthree wires and a marginal result for four wires; the

result for six wires was judged satisfactory (see fig. 3).The six wire counter was further tested by moving

the X-ray source in the longitudinal direction. Theestimated effective sensitive area, 100 cm2 as given in

m io

0

327

Fig. 3. The variation in the response of the counters is shownas a function of the number of wires in the 10 cm width of atest counter. This counter was scanned perpendicularly to thewires with a narrow beam of X-rays from a zirconium sec-ondary X-ray source . The ordinate is the fractional standard

deviation.

i

The MWPCs were housed, eight at a time andarranged in line, in each of 64 boxes made of galva-nized steel sheet. In each box, nine transverse strips of

so

ôao

Table 1a

30m

MWPC specifications`m~+ 20

Active volume0

Altitude 30 m above sea level"Latitude 33°.89 southLongitude 151°.19 eastHut roof 0.2 g cm-2 Al+ 1.4 g cm-2 strawRoof thickness 5 cm

""""" Distance from bottomof roof to top

"""""""""" o """""""""""""""" cathode of MWPCs 18 cmNumber of counter rows 25Number of countercolumns 23

Row separation 15 cm approx .Column separation 18 cm approx .

328

table 1, was determined, to an accuracy of better than5%, from the results of the longitudinal and transversescans.

3. Electronics and data handling

Pulses from each counter were preamplified at thecounter and then transmitted by coaxial cable to themain amplifier rack. Each preamplifier had two out-puts for pulses at different gain levels in order to coverthe wide dynamic range of pulse heights required . Thepreamplifiers also accommodated part of the simula-tion pulse generating system which produced simula-tion events, called sims . These consisted of artificialpulses, all of the same size, injected at the anodes ofthe MWPCs. Four different sizes of pulse were used,corresponding respectively to the passage of 1, 5, 12and 50 particles. Such events were produced automati-cally in a cycle spanning 4 h. Responses outside thenormal ranges were noted in print by the array's oper-ating program. The sim event data helped to identifyfaulty electronics ; except for the sim-Os described be-low, they were not used for calibration purposes .A fifth sim, called sim-0, was generated automati-

cally every ten days by triggering the array at anarbitrary time, i.e. not in coincidence with a masterpulse from the scintillator array. Except for accidentalresponses, sim-Os should give "pulse heights" dis-tributed in amplitude, for a given MWPC, according tothe electrical noise distribution of the electronics .Analysis of sim-Os shows that the rms value of thenoise distribution was on average equivalent to 0.033particles in the more sensitive of the two gain channels .A "de offset" or "zero error" was also observed . Thiswas due to the nonzero offset voltages in the cabledriver ICs of the preamplifiers . It had a mean over theMWPC array, again equivalent to 0.033 particles. Indi-vidual values were stable over months and most werewithin -0.1 and +0.1 particles. The response of coun-ters to showers was corrected for these zero errors .

The pulses received at the main amplifiers rangedup to 1 V. After amplification, these were fed to trackand hold circuits . The transition from tracking to hold-ing modes was controlled by a trigger pulse from thescintillator array which was delayed to "catch" theMWPC pulses at their peak . Checks using a storageoscilloscope showed that the pulse heights recordedthis way were within 1-2% of their maxima . The sam-pled signals were digitized and stored .

Data from the scintillator array was analyzed andshowers were accepted if their axes passed within thesquare of fast timing scintillators and their sizes weregreater than 30 000 particles. (The size cutoff was givenas less than 40000 in ref. [1] but reduced later.) Withthese cuts we recorded showers at a rate of about one

L. Horton et al. / MWPCs in a cosmic ray air shower experiment

every 6 min. It was not practical to record data fromthe MWPC array for all the showers which trigger thescintillator array. Thus we recorded the MWPC datafor showers where more than 35 counters had a signalcorresponding to more than 2.5 particles. This oc-curred about 12 times per day.

Whilst waiting for events, the track and hold circuitswere in the tracking mode and were thus "transparent"in the sense that the signal from each MWPC wascontinuously available for pulse height analysis . A sin-gle pulse height analyzer sampled the counters in turn,for 3 h each, to measure the spectra due to single"straight through" particles. Using the least squaresmethod, parabolae were fitted to the single particlepeaks and the results stored for calibration purposes asdescribed in the next section .

The next stage of analysis was performed "off line" .The event data and calibration data were combinedand the counter response in "particle numbers" calcu-lated . This calculation needed a measure of the aver-age signal per particle. We now describe the variousways in which we determined and monitored this quan-tity .

4. The response of multiwire proportional counters toindividual particles

Each MWPC produces an electrical pulse when airshower particles pass through it . As explained above,the routine calibration is to analyze the pulse heightspectrum for an individual counter for a period of 3 h.Fig. 4a shows such a spectrum (recorded for 160 hrather than the normal 3 h) .

Each shower particle passing, at normal incidencethrough the 2 cm (3 .7 mgcm -Z) of gas mixture in thecounter, releases on average 7.5 keV of ionizationenergy . This is comparable to the response from photo-electrons produced by low energy X-rays from orbitalelectron capture in uranium and thorium series ele-ments. These elements exist in the environment (floors,etc.) at significant levels and their X-rays would con-tribute to the large low energy response shown belowthe peak (in fig . 4a).

Because of its low threshold, the counter will re-spond to low energy shower particles travelling at largeangles to the shower axis . To include these in thecalibration and to exclude X-rays we performed severaltests where counters were gated with a cylindricalplastic scintillator, 50 mm diam and 25 mm high,placed immediately below the MWPC being tested . Anexample is shown in fig. 4b . The form of the gatedpulse height distribution results from a combination ofthe energy lost in the MWPC due to variations inparticle energies and directions, the Landau distribu-tion, gas amplification and circuit noise. In large detec-

lKv)

MV)

Response N,fromPolssonian term(particles)

Fig . 4 . (a) Shows a pulse height spectrum taken without gatingand (b) was taken using a 50 mm diam . 25 mm high gatingscintillator immediately below the MWPC. A fitted distribu-tion is shown and the mode and mean shown by verticaldotted lines . The mean to mode ratio (R) in this case was

1 .94 .

tors the multiplicity distribution is also important; thereis a nonzero probability of two and more particlesarriving together .

Probably the most important parameter in the dis-tribution is the mean to mode ratio, R; it provides alink between the mode (determined from fitting a

L . Horton et al. / MWPCs in a cosmic ray air shower experiment

pulse height, v

pulse height, v

parabola to the peak of the spectra) and the conversionof pulse height to particle number in showers.

To evaluate R, we fitted several distributions, in-cluding lognormal and gamma distributions, to thegated spectra. In all cases R was near to 2.0. It turnsout that the R is not very sensitive to the angulardistribution since by replacing the 50 mm diametergating scintillator with a scintillator 40 cm X 40 cm X 10cm thick mounted 76 cm below the MWPC under test,the same value of R was obtained .

5. The response of multiwire proportional counters toair showers

In the period August 1989 to January 1991, 123 825showers were recorded . 1567 events had their corepositions, as determined by the scintillator array alone,greater than 10 m from the centre of the MWPC array.This subset of "distant showers" provided furthermeans of evaluating the response of the MWPCs to airshower particles .

First, we were able to check on the value of themean to mode ratio, R, in two different ways . For thelow density, distant, showers we calculated the averageparticle number (Na) for each even using R = 2 andcorrecting for the "zero errors" mentioned above. Wecompared this estimate with the corresponding quan-tity (NP ) obtained for the number of counters showingzero and nonzero MWPC responses and assuming aPoissonian distribution . Fig. 5a shows that apart fromthe expected fluctuations, there was a linear relationbetween the two estimates up to the two particle level;

(b)m

NmÛrô.dc 2

d

3

329

I

I

~I~1

2

3

~

I

I

I �

MWPC responses Na (particles)

0

2

3MWPC response N a (particles)

Fig. 5 . (a) Response of the MWPC array to "distant showers" scatter diagram of response estimated from proportion of zeroMWPC responses (ordinate) as a function of the average MWPC response (abscissa). The solid line, slope 1 .0, shows the idealrelationship . Not shown is the line of best fit through the origin for responses less than 2 particles (small dots), which has a slope of0.930. (b) Scatter diagram as in (a) except that the ordinate is now obtained from the average response of the four triggeringscintillators immediately surrounding the MWPC array. The solid line (slope 1 .0), shows the ideal relationship . The line of best fit

through the origin (dotted) has a slope 1.10833 .

330

a least squares fit (assumed to pass through the originhad a slope of 0.93 . That is

Np =cNa =0 .93Na .

The lower Np estimated for higher densities may bedue to the non-Poissonian nature of the particle num-ber fluctuations which become more important athigher densities .

Second, we compared the average MWPC responsewith the average response of the four triggering scmtil-lators adjacent to the MWPC array. Allowing for thescintillators' greater area there was excellent agree-ment (fig . 5b). A least squares fit through the origingave a result similar to the earlier one with c = 1 .09 .Individual ratios of scintillator to MWPC responsewere distributed about this value with a standard devi-ation of 0.25 . This is in agreement with previous work[3] where a ratio just greater than unity was foundwhen comparing scintillator and Geiger counter re-sponses at > 10 m from the cores of showers with 10 5to 10 6 particles.

Further detailed information on MWPC response toshowers was found by examining spectra of responsesfor each event in the subset . We allowed for the effectof inclined particle directions through the MWPCs,multiplying the responses by the cosine of the zenithangle of the shower as determined by the scintillatorarray. Finally we corrected for the gradient of particledensity across the MWPC array, assuming the coreposition calculated by the scintillator array and that thedensity was inversely proportional to the distance ofthe MWPCs from the core position .

To each shower we fitted a lognormal distributionto the spectrum of nonzero responses. This gave usvalues for the two distribution parameters (w, and (7)for each shower . An attempt was then made to findhow these parameters depended on the average re-sponse of the MWPC array.

It was found that the mean [a = exp(, - (Q,2/2))]

of the lognormal distributions, modified to account forzero responses from the counters, was distributed abouta mean value Na. The modification was to multiply aby the probability of a nonzero response, Pnz = 1 -exp(-Na ) . It seemed reasonable to let the variance ofthe modified distribution be proportional to Na. Wethus arrive at formulae for Q and A which may then beused in a lognormal distribution when fitting showers.

where k was adjusted to fit the subset and the best fitwas for k = 3 .3 particles (compared with k = 1 for

L . Norton et al. / MWPCs in a cosmic ray air shower experiment

Fig . 6 Log normal fit to the distribution of MWPC responsesm individual "distant showers" . The log normal parameters p,and o,, fitted to each shower are plotted against averageMWPC response in that shower The curves are :

I kU=

UIogfN+11[1-exp(-N,)]

1

tr = log

10

20

30

40Average particle number in MWPCs N,

Na

1-exp(-Na)] 2

with k = 3 3 particles

/logr k

Q = UIN+ll[1- exp(- Na) ],

Na

J

Q z

k = 3.3 particles .

(a)sepal number 167886

(b)serial number 159472

o-

0

50

Fig. 7. The distributions of MWPC responses m two "distantshowers" showing the lognormal distributions as calculatedusing the parameters p and o, as given by the two formulae .

k0'= log +Na

1 1[ 1 - exp( -NJ ] , (1)

r Na l a- 2

~, = log l -l ,-exp(-Na) J

Z (2)

Poissonian case). This dependence together with theindividual data points is shown in fig . 6. Figs . 7a and 7bshow histograms of MWPC responses for two eventstogether with the lognormal distributions using o, andw as given by eqs. (1) and (2) above.

6. The analysis of events, some preliminary results

The lognormal distribution was specified above, wasincorporated into a program to analyze the records ofshowers obtained with the MWPC array. For this pre-liminary study we assumed that each shower produceda density of particles (p) which fell off with radicaldistance (r) from the shower axis with a dependence

p =M(r+ 0.075)'-Z ,

where M is a constant which has to be fitted in eachshower, r is the perpendicular distance, in m, from adetector to the shower axis . The constant 0.075 m,allows for the averaging effect over the finite sizedMWPC' and is comparable with the linear dimensionof a counter.

L . Horton et al. / NIWPCs in a cosmic ray air shower experiment 33 1

The expression we used is an approximation to theso-called NKG formula [4] giving the density of showerparticles as a function of perpendicular distance fromthe shower axis ;

PNKG -M~rr

\1 + rtThe s� , called the age, is a measure of the develop-ment of the shower ; r, (called the Moli6re length,equal to 79 m at sea level) is the natural scale lengthfor shower development in air . Our approximation isadequate for r « r l , and our use of the constant 0.075m conveniently avoids divergence problems at r = 0which arise because sn is less than 2. For simplicity wewill from now on use the term age when referring tothe parameter s in eq . (3) .

During 523 days of operation 123 825 showers wererecorded ; for 6210 of these MWPC data was alsorecorded . We further selected those showers for whichthe core position, as determined by the scintillatorsalone, was within 3 m of the centre of the MWPCarray. To these showers we fitted the function (eq. (3))to the MWPC data alone using the maximum likeli-

r l s^ -

2 j

r \s,-45

a The core position of shower 240996 was mislocated by the scintillator array because the four inner scintillators were saturated .After inspection of the MWPC responses the core position was placed in the middle of the block of saturated MWPC' (high flatsection at the back of the plot). The shower size shown in the table was recalculated from the outer scintillator signals. The agewas estimated from the nonsaturated MWPC' surrounding the saturated block.

Figure

Fig. 8 .

Serialnumber

Perspective plots showing

Maximumresponse[particles]

the response of the MWPC

Showersize

array to four showers .

Zenithangle[deg]

Fittedage[s]

(a) 173470 93 0.77 x 10 6 14 1 .13(b) 179042 20 0.074 x 10 6 13 0 .97(c) 238930 149 1 .43 x 10 6 16 1 .22(d) 240996 a 243 2 .7 x 10 6 41 1 .33

33 2

hood method . The parameter s was stepped over arange of values . For each value of s the central point(r =0) was stepped in the x and y directions of thehorizontal plane. For each s, x and y the M value wasset to make the sum of the expected MWPC responsesequal to the sum of observed responses. At this prelim-inary stage it was assumed that the MWPC responseswere described by a Poisson distribution rather thanthe modified lognormal one.A satisfactory fit was found for 1407 showers. To

these were added six large showers where inspectionclearly showed that the scintillator core had been in-correctly placed more than 3 m from the MWPC array.

Each of the 1413 showers was now examined subjec-tively . Note was taken of best fitting location of thecentral point and how this position varied over therange of ages s. A large number of events were dis-carded because of the large variations of the centralpoint with the age. Accepted events often had move-ments less than 1 cm as the age was varied . In suchcases the central point was being determined by thesymmetry of the shower. Showers were also discardedif the plot of the logarithm of the likelihood againstage deviated noticeably from the parabola expected ofgood fits . For showers which passed this criterion thebase age was determined from the maximum of theparabola fitted by least squares.

The decision to reject showers for the two reasonswere quite clear cut and resulted in a set of 436showers. An attempt was made to classify these intofive categories which included two classifications of"array edge showers" . The set of 436 showers was thenreanalyzed using the lognormal distribution (describedabove) for MWPCs which had a nonzero response andthe appropriate Poisson term for MWPCs with zeroresponses. This new maximum likelihood program wasunable to give a satisfactory fit for 30 showers. This left406 showers, fig. 8 shows perspective plots of selectedshowers. Changing the fitting distribution changed thecentral position and the age of the fitted shower . Theshift ranged from zero to 1.1 m when array edgeshowers were excluded . There was a general increaseof age by 0.1 . Figs . 9a and 9b show the ages of showersplotted against (log) size, the latter determined fromthe scintillator data alone. Showers whose cores wereat the edge of the MWPC array were excluded fromthis analysis . Both plots are consistent with the averageage not changing with shower size . This result is to becompared to the experiments at Kiel [5], using neoncounters and at Kobe [6] using a close packed scintilla-tor array. The Kiel results show a slight increase of agewith shower size whereas the Kobe results show aslight decrease . This difference may be, in part, ac-counted for by the different average distances of thedetectors from the shower axis ; for Kiel the range is upto 5 m, for Kobe 3 to 10 m and for our experiment up

L . Horton et al. / MWPCs to a cosmic ray air shower experiment

1616

1 4m 13~° 1 2

10

0s

08

0706

161s

1 4

m 1312

100s

08

07

06

+#+ + +++++.

+ +++

+#++ + + + *zk++ ++

++ +*+

++++

+ ++#++

10 5shower size N

10 410 5shower size N

10'

Fig. 9. Scatter diagram of the age parameter (s) fitted to ashower and its size (N). In (a) the MWPC responses wereassumed to have a Poisson distribution and m (b) a lognormaldistribution was assumed with parameters Fr and Q as given m

the text .

to 4 m. The difference between the response of scintil-lator and gaseous detectors to shower particles mayalso play a part .

7. Discussion and conclusion

Shower particle detectors which are composed of amedium similar to that of the air have the advantage ofmeasuring particle numbers with minimal interference .This advantage however, is partly negated because theshower particles traversing such a detector necessarilyhave a small energy loss which is subject to relativelylarge fluctuations . A further disadvantage is that theenergy loss within the detector varies with energy ofthe traversing particle . For gaseous detectors it risessignificantly from the minimum ionising value . Such arise is significantly less in the case of denser detectors .This variation is important because of the large energyspread of shower particles.

The present work throws some light on whether theadvantages outweigh the disadvantages. When investi-gating possible "core structure" in air showers, i.e .

where there are significant deviations from a smoothdecline of particle density with radius from the axis,there is the possibility that detectors have distorted thedensities measured thus creating core structure arti-facts. We believe that such distortion is much reducedwhen MWPCs are used .

Some of the analysis techniques outlined above, inparticular those described in the section on the re-sponse of MWPCs to air showers, could be fruitfullyapplied to other shower particle detectors such asscintillators . One analyses the response of a localizedgroup of the detectors to distant showers. The his-togram of responses, adjusted for the radial densitygradient in the shower, will show immediately whetherthere are significant fluctuations in addition to those ofPoissonian origin . If such are present then they need tobe used in the subsequent analysis and interpretationof air shower events .

Acknowledgements

The authors have pleasure in acknowledging sup-port from the Science Foundation for Physics withinthe University of Sydney and from University andAustralian Research Grants Schemes. (We thank Prof .Lawrence Peak of the University of Sydney for hiscomments .) We are pleased to acknowledge the carefulwork by Mr . Fred Peterson in constructing the gasmixer.

Appendix: gas mixer

The gas mixture of 94% argon and 6% carbondioxide was used at a rate of 0.9 m3 per day. For thefirst few months of operation the mixture was boughtready mixed at a relatively high cost . The gas mixerdescribed below used welding grade argon with anordinary grade of CO, for about a quarter of the cost .

Fig. 10 is a schematic of the mixer which consistedof a solenoid operated inlet valve IA controlling theingress into a piece of tube of volume VA, of argonregulated to a pressure PA. The outlet valve OA al-lowed the argon out into the tee section where itencountered COZ coming from a similar arrangementof a volume Vc also with valves on either side of it . Vcwas approximately equal to 6% of VA.

The gas leaving the tee section needed to be prop-erly mixed, this was done in the bell jar with theelectric fan F. The mixed gas passed from this into athick walled plastic bag of the type used for wine casks.Its volume of 0.01 m3 smoothed out some of pressurefluctuations produced by the mixing process . The bag'sinflated volume was controlled by the Hall effect dis-placement sensor on top of the bag. When gas was

L Horton et al. / MWTCs in a cosmic ray air shower experiment 333

References

output to MWPCs

Fig. 10 . The MWPC gas mixer. The operation of the mixer isdescribed in the text . Not shown m the diagram are themasses on top of the plastic bag and the needle valve. Bothwere used to control the flow rate of mixed gas into the

MWPCs (of 0.9 m3 per day total) .

used up, the magnetic field on the Hall device reducedand it enabled the following cycle of the valves : all fourvalves closed, both inlet valves open, all valves closed,both outlet valves open, all valves closed, etc. Themixed gas inflated the bag until the sensor disabled thevalve sequence .

The ratio of COZ to Ar was controlled by varyingthe pressures of the gases as they entered the mixerand the rate of flow of the gas to the MWPCs con-trolled by masses on the top of the bag and a needlevalve downstream of the bag. (Neither are shown in thefigure .) Faults producing excess pressure in the gasfeed to the MWPCs had to be avoided at all costs forfear of damaging them . Some protection is given by thefact that the bell jar could lift off its base if both valveshad opened together . More protection was warrantedand we installed a sharp tipped surgeon's scalpel abovethe bag just out of reach of the plastic in normaloperation.

The mixer proved very reliable after the initialadjustments were made . The proportion of CO, wasmonitored by irradiating one of the array's MWPCswith secondary X-rays produced on a silver target by 60keV gamma rays from an americium 241 source . TheMWPCs output produced a peak on a multichannelpulse height analyzer whose channel number wasstrongly dependent on the CO, ratio. The peak's posi-tion also depended on the gas temperature and (tosome degree on gas pressure) . The gas pressure wasslightly above the ambient atmospheric pressure . Weneeded to take the gas temperature into account whenchecking the mixture but we ignored variations in thegas pressure .

[1] Sun Luorui and M.M. Winn, Nucl . Instr. and Meth . 223(1984) 173.

334

[2] H. Sasaki et al ., Proc . 17th Int. Cosmic Ray Conf ., Paris,11 (1981) 338;K. Asakimori et al, Proc . 16th Int. Cosmic Ray Conf .,Kyoto, 8 (1979) 252;A.M. Hillas et al ., ibid ., p. 236.

[3] A.D . Bray et al ., Rev. Sci. Instr. 36 (1965) 587 and Proc .9th Int. Cosmic Ray Conf., London, 2 (1965) 685.

L. Horton et al. / MWPCs in a cosmic ray air shower experiment

[4] K. Greisen, Annu . Rev. Nucl . Sci. 1 0 (1960) 63 .[5] E.R . Bagge, M. Samorski and W. Stamm, Proc . 16th Int

Cosmic Ray Conf ., Kyoto, 13 (1979) 260.[61 K. Asakimori et al ., Proc . 18th Int. Cosmic Ray Conf.,

Bangalore, 11 (1983) 189.