A special system of proportional counters to be used as a high pressure gas target without walls

NUCLEAR

INSTRUMENTS

AND METHODS

55 0967) 273-287; (~) N O R T H - H O L L A N D

PUBLISHING

CO.

A S P E C I A L S Y S T E M O F P R O P O R T I O N A L C O U N T E R S T O BE U S E D AS A H I G H P R E S S U R E GAS TARGET W I T H O U T WALLS A. ALBERIGI Q U A R A N T A *, A. BERTIN, G. MATONE +, F. PALMONARI and A. PLACCIt

lstltuto di Fisica dell' Universitd di Bologna and lstituto Nazionale di Fisica Nucleare, Sez. di Bologna, Italy and P. DALPIAZ t and E. ZAVATTINI,

CERN, Geneca, Switzerland Received 9 May 1967 A special assembly of high pressure wire proportional counters

is described. Their operation defines a volume in a gaseous target, the effective limits of which are set mainly by the electric fields of the counters and not by a continuous material wall.

600 MeV synchrocyclotron - while filled with extremely purified hydrogen, is fitted to work with any gas, or mixture of gases, proper to fill gas proportional counters. The device seems suitable to be used in different fields of high-energy physics.

The apparatus, used in a recent experiment - at the CERN

1. Inlroduetion 1.1. We have built a special system of proportional gas counters, the main performance of which is to define an effective volume V in which charged particles can be brought at rest under the following requiremeats: a. In the region V, such particles interact with highly purified gas (hydrogen in our case) at a pressure less than or equal to 10 atm; b. Those particles, which cross the region V without stopping in it, must be detected; c. It must be possible to know whether the decay, or the eventual interactions, of the stopped particles in the volume V give rise to outgoing fast charged products; d. The boundary of the region Vmust not be defined by any material surface (or as little as possible); e. The operation of the special counter has to be fast enough to stand an instantaneous intensity of about 104 particles/see crossing it. We have built and used such a device mainly to measure the nuclear capture rate of /~- mesons by protons according to the reaction #-+p

~ n+v,,

features of this special counter, we give here a short description of the various processes connected with the stopping of a negative muon in hydrogen. 1.2 A p - meson stopping in hydrogen at 8 atm, after being captured into an excited level around a proton 1), reaches the IS singlet state of the newly formed mesoatom in about 10 -a see 2). At this pressure, the subsequent reaction /~p + p --. p/~p,

(2)

takes place for about only 10 % of the formed mesoatoms, due to the fast rate of the competing process

p- ~ e-+~e+v~.

(1)

by stopping negative muons in isotopically pure hydrogen gas at 8 atm, and detecting the outgoing 5.2 MeV neutrons. To clarify and justify some of the main * Now at Universit~t di Modena, Italy. + Supported by a CNR fellowship. t CERN visitor, on leave from Istituto di Fisica dell'Universit~t di Bologna.

(3)

However, before reactions (l), (2) and (3) take place, the pp diffuses throughout the gas, with a diffussion length d = 3.5 m m 2); hence, if no caution is taken, it may reach the wall of the container (which in our device was stainless steel made). In this case, the # - , which is quickly transferred from the mesoatom to an iron atom, is subsequently captured by its nucleus, at a rate 2Ve, much bigger 3) than the rate 2c of reaction (1). Fig. 1, from 2), shows that if negative muons are uniformly stopped in the gaseous hydrogen contained in a cylinder of 30 cm dia., then even after a time t o = 1 psec the number o f / ~ - transferred from the mesoatom to the iron wall is more than 2 % of the number of the pp systems present at time t o. The capture process of the # - mesons by iron nuclei yields therefore a background of delayed neutrons which can make very critical the measurement of the rate of reaction (l).

273

A. A L B E R I G I Q U A R A N T A el al.

274

The device which we are describing in this paper enabled us to overcome this dit~culty, since, as it will be seen later in detail, the inside surface of the gas

ment. In table 1 are summarized the just described processes and their velocities. TABt.I! 1

container is protected by a system of wire proportional counters. Such system rejects m u o n s stopping in the gas at a distance from the walls, which is smaller than a fixed value L (L = 4.5 cm) and which was much bigger than d = 3.5 mm. A volume V (of a b o u t 10 I) is then defined, the effective limits of which are determined, rather than by a wall, by the electric field due to a few thin wires. The gas contained in V can then be considered as a high pressure target without walls. A n o t h e r process which the mesoatom may undergo is the following: pp+Y ~ pY+p,

(4)

where Y is any gaseous impurity within the gaseous hydrogen. Also this process can be a huge source of delayed neutrons, which can mask reaction (I). The speed of reaction (4) has been measured by Basiladze et al. 5) and Alberigi Q u a r a n t a et al.5); their results show that the hydrogen to be used must be kept pure to better than l ppm. The constructive details of the c o u n t e r will be also critically determined by this require-

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1.3. In fig. 2 is shown the experimental set-up used to study reaction (1)*; before going to describe the p r o p o r t i o n a l c o u n t e r assembly, we shall briefly discuss the logic of the experiment, to be able to u n d e r s t a n d the significance of the tests that we have done on the p r o p o r t i o n a l counter, and which will be described later. An event is defined as a possible good one if in a certain time interval (i.e. within a 10 psec large gate) one of the n e u t r o n counters Ni t , and only one, gives a pulse of proper amplitude: the gate, delayed by a b o u t 1 psec, is supplied by a stopping # meson, defined by the coincidence-anticoincidencesign al * The results are on the way of being published. Of the Brooks' type 6).

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Fig. I. Time behaviour of R = ( number of muons transferred to the wall after time tu)/(number of muons decaying after time to). The plot is the result of a Monte Carlo calculation performed for l0 s muons uniformly stopped in a 30 cm dia. vessel, containing gaseous hydrogen at a pressure of 8 atm.

A SPECIAL

SYSTEM

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COUNTERS

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[ 1 2 c t ( A t A 2 A ~ 4 A s i ] , where • is a fast grid proportional counter which detects the incoming /ameson (fig. 2). The possible good event is recognized as a good triggering one, if the following requirements are moreover satisfied: 1. Up to the time t of arrival of the neutron counter pulse, no one of the anticoincidence counters A~ must fire; 2. No pulse must be present in the anticoincidence proportional counters fl and ,/(fig. 2) within a time of about 15 l~sec after the p - stop signal time t 0. As we shall see later, in about 8 ~o of the cases the anticoincidences fl and ), have a time jitter in their response bigger than 15 psec. The good triggering event starts the recording of the various informations which may be of interest [monitor, the (t-to) time interval, the neutron counter amplitudes, etc.] and moreover triggers two double beam oscilloscopes. In these tracks are represented, among other things,

Fig. 2. Schematic layout o f the m u o n capture experimental a p p a r a t u s . T h e target effective v o l u m e // is the one limited by the electric fields o f counters ~t, fl and 7.

the pulses obtained (if there are any) in the anticoincidence proportional counters fl and ~, in a time interval of about 40/~sec after and l0 psec before the the time to. In this way, a further selection for choosing an event due to reaction (l) is done by looking at the tracks films. The 100 MeV/c /~-meson beam entering into the (l, 2) telescope had an intensity of about 8 x l03 particles/sec, with a duty cycle of about 40 ~ *

2. Description of the apparatus 2.1. T H E CONTAINER

The container (fig. 3) which is 130 cm long, is obtained from a 26 cm dia. stainless steel cylinder, Mannesmann processed, and rectified both externally and internally. It was asked to fulfill the following requests t: • T h e experiment has been done at the C E R N synchrocyclotron /z-channel ;). t T h e c o n t a i n e r was supplied by SIAI-LERIC1 ( C o r m a n o , Milano, Italy).

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Fig. 3. Scheme of the proportional counters assembly, a, c, and e are each a high voltage input and a si pressure manometer flange; n, is the high voltage input and the signal output for the counter; 1, thin win tight glass feed-through; 6, high voltage connector; 7, a F counter high voltage electrode; 8, fl counter signal collection wire for ~ counter; 13, nylon insulating wire; 13, silver supporting wire; 14, stainless Ill, second grounded electrode for counter ~. In the figure, gas input and vacuum connection flanges

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tput; b and d are reserved for special electronic operation; 1, is the vacuum gauge flange; m, is the high • beam entrance; 2, high-vacuum O-rings; 3, ~ counter; 4, PTFE insulating support; 5, high voltage pressure ltage electrode; 9, guard electrode for /~ counter; 10, y counter grounded electrodes; 11, high voltage and ronae; 15, stainless steel rod. I, first grounded electrode of counter ~; II, high voltage electrode for counter ~;

lore,d; all distances are expr~sed in ram. The drawing at the top of the figureshows the geometry of the counter/i.

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As it has been emphasized in sect. 1.2., the possibility of studying reaction (1) is critically connected to the degree of purity of the hydrogen, in which only less than I ppm of impurities can be tolerated. Special attention has therefore been paid to purify the gas with great accuracy, and on the other hand to prepare a good vacuum in the tank and in the tubes which the purified gas had to pass through. The used gas is protonium *, an isotopically pure type of hydrogen containing less than 3 ppm of deuterium. This gas reaches the circuit shown in fig. 4 after flowing through a Deoxo purifier (to eliminate oxigen impurities) and a P205 container (to remove mois-

ture); finally, it is sent into a palladium filter t, which purifies it up to a level of less than 10 -2 ppm of impurities. Owing to the low flux allowed by the palladium purifier working conditions, 90 min are requested to fill the container at a pressure of 8 atm. In all the lines which the protonium has to go through the vacuum is made by two rotative pumps, P3 and another one not shown in the figure. The palladium filter output was connected to the container through a very short tube. As to the container, the rotative pump Pt creates a pre-vacuum up to 10--' Torr; afterwards, valve 22 being closed, the two diffusion pumps in series D~ and D2, and the rotative pump P2 begin to operate attaining a static vacuum of 10 - 6 Torr. Two vacuometers (TI and T2) control the vacuum inside the vessel. In the filling system all the connections were done by rubber O-rings treated with silicone. Special care was taken to avoid mechanical couplings between the rotative pumps and the container. The safety of the apparatus is insured mainly by a safety rupture disk (R) breaking at 30 atm, two anti-return valves, which protected the pumps directly connected with the high pressure container, (they opened at a pressure of 400 g/cm2), and by two safety valves, opening at 20 and 27 atm respectively, at the output of the palladium purifier. A continuous vacuum was carried on for 15 days to degas the whole system before beginning the experiment. Before each filling, moreover, the system was washed with protonium at a pressure of 2 atm. To carry out the emptying of the container, the protonium is sent to the recovery circuit, where two compressors pump it into empty bottles. The circuit provides also facilities to fill the container with other gases or mixtures of gases. A measurement was done, which showed that the behaviour of the proportional counter is also a most sensible tool to control the constancy of the degree of purity of the filling gas. 20 ppm of air ware introduced into the counter, before filling it with 8 atm of purified protonium. The result was that the counting rate of counter fl (the most sensitive to the presence of impurities, owing to its large useful volume) decreased of 6 % in comparison to the counting rate observed when no air was present. Before beginning to run the experiment, counter fl was observed to be

* Protoniurn was supplied by Air Liquide, Paris.

t Supplied by Engelhard Industries, Newark, N.J., USA.

I. To contain gas up to a pressure of 25 atm; 2. To be vacuum tight up to 10 -7 Torr; 3. To withstand heating up to about 400~'C in order to be properly degassed; 4. In the central part (facing the neutron counters, fig. 2) to have a wall thickness limited to 2.5 mm. At the edges, the thickness increases gradually up to a proper value to allow soldering of two heavy stainless steel flanges (fig. 3). The front stainless steel cover had a central circular 10 cm dia. hole closed by a 1 mm thick stainless steel window to reduce scattering In the beam entrance; 5. To have an inside surface perfectly smooth to avoid electrostatic discharges. For this purpose (and to make the degassing operation easier) the internal wall has been treated with electrolitic cleaning. Near the end flanges there are 12 (8 cm dia.) holes with small flanges, 4 of which are near the front flange (fig. 3). Their use is assigned as follows: - 1 for a safety rupture disk; - 2 for the vacuum operation (I directly to a rotative pump); - i for the filling operation; - i for a manometer; - 1 for a vacuometer; - 6 for the high voltage supplies and the signal collection from the proportional counters. To avoid mechanical vibrations, the cylindrical container rested on insulating felt supports; such an arrangement was found to be rather important especially to avoid vibrations of the long peripheral counter wires. 2.2. THE FILLING SYSTEM

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® Fig. 5. Front view o f the ~t counter as seen in a section o f the container. 1, v a c u u m gauge flange; m, high pressure m a n o m e t e r flange; n, high voltage input and signal output for counter ~t; o, rupture disk flange; I, first grounded electrode o f counter ~t; II, high voltage electrode for counter 0t; III, second grounded electrode for counter ~t. In the figure are also s h o w n , on a partial sector o f the front corona, the geometrical arrangement o f the mass wires e and o f the high voltage wires v o f the counter 7.

stable within 0.5 ~ for a period of 4 days. During the experiment the pure protonium was recycled every day. 2 . 3 . THE MECHANICAL ARRANGEMENT OF" THE PROPORTIONAL COUNTERS

In fig. 3 is drawn, together with the high pressure container, the assembly of the proportional counters. To make the operation of mounting the wires and assembling the counters as simple as possible, these are

mounted on a frame which, if necessary, moves freely inside the cylindrical container, and can be taken outside for the assembling operation. The frame is essentially constituted by 2 equal 6 mm thick stainless steel coronae, the external diameter of which is equal to the internal diameter of the tank (26 cm). The 2 coronae, the internal diameter of which is 16 cm, are joined together by 4 stainless steel rods, 98 cm long.

A SPECIAL SYSTEM OF PROPORTIONAL COUNTERS To this frame, by means of proper insulators, the following three different systems of wire proportional counters are mounted:

281

a circular row of 16 adjacent, 4.5 cm dia., cylindrical proportional counters, all having an anticoincidence function; this row defined then the effective side surface of the volume V earlier mentioned.

2.3.1. The fast g r i d counter (ct) This counter, which is supposed to have a fast coincidence operation, is placed towards the entrance of the incident meson beam, and is made of 2 circular grids, 9 mm spaced, and with 10 cm (cathode grid) and 13 cm (counting grid) diameters. These grids are both constituted by parallel stainless steel wires, 6 mm spaced and respectively with dia. 100 #m and 50/am (fig. 5). The 2 stainless steel rings, on which the wires are fixed, are clamped to a stainless steel corona (fig. 3): the one with the counting grid by three hollow PTFE insulators (not shown in the figure), and the other one (which is the first seen by the beam) by 4 stainless steel bars (of which only one appears in the schematic drawing of fig. 3). A third 15 cm dia. stainless steel ring with parallel (100 /~m dia.) 8 mm spaced wires, also kept by the corona, is placed after the counting grid at a distance of 14 mm from the latter. The increased value of this distance (in comparison to the one between the cathode and the counting forementioned grids) yields a lower field in the second gap, so that the third grid acts merely as a shielding cathode. The grid planes are set parallel to within 0.1 mm. 2.3.2. The peripheral counter (~) This counter is constituted by 2 concentric circular rows of wires, properly subtended between the 2 big coronae (figs. 3, 5 and 6). The outer row is set with a dia. of 21.5 cm, and it consists of 16 (100 /am dia.) stainless steel wires, alternated to 16 (50 pm dia.) analogous ones, all equally spaced. The inner circular row, the dia. ofwhich is 17 cm, consists of 32 (100 pm dia.) stainless steel wires, also equally spaced. The system of the 50 pm dia. wires, which are held by means of hollow P T F E insulators (details in fig. 3), represent the anode of the counter, whereas the system of all the 100 pm dia. wires, plus the stainless steel wall of the container represent the cathode. Such a geometry (fig. 6) enables one to obtain an electric field distribu-tion, around each of the 50/am dia. anode wires, which is similar to the one obtained around the anode of a usual cylindrical proportional counter. The peripheral counter can therefore be thought of as constituted by

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It is easy to see, in this way, that the effect of transferring the muons from a pp system to the iron atoms (which are now only those belonging to the innermost wires) is reduced by more than a factor 100. The effective length of this peripheral counter is approximately 98 cm and is much longer than the depth of V (which is actually about 50 cm). This type of unconventional proportional counters arrangement has already been adopted in the past, to measure very low radioactive levels of gaseous samples, especially to avoid the cosmic ray radiation background 8). The wires have all been chosen to be stainless steel made for the following reasons: a. The necessity of using, in the proximity of the useful region V, low density materials with good mechanical properties as well as high atomic number Z, in order that any effect due to the # - mesons which are directly stopping in these materials die out in a time as short as possible compared to the p - lifetime (table l).

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b. The possibility of getting rid of superficial impurities by properly heating each wire while assembling the counter. As to the collection of the signals and the distribution of the high voltage, the 16 anodes have been grouped in 4 groups of 4 (connected together by a 12 m m dia. collection wire), thus finally getting 4 nearby anodes. Such a division has been carried out both for symmetry reasons (every group in fact was facing a neutron counter (fig. 2) thus allowing suitable counting controls) and to fraction the rather high total capacity (300 pF) of the system. 2.3.3. The back cylindrical counter (fl) This counter is a conventional type of cylindrical gas proportional counter, constituted by a central 150 pm dia. and 26 cm long silver anode wire, the cathode of which is formed by the inner grounded circular row of wires of the long peripheral counter (fig. 3).

Fig. 7. High voltage distribution system.

The extremes of the anode wire are soldered to two 1.2 mm dia. and 10 cm long silver wires, which act as electric field guards; the ends of this system are anchored: the first to the back corona, through a hollow P T F E insulator; the second (through a short nylon wire) to a very thin silver wire which is subtended between two of the rods that keep the two big coronae together. In this condition, the position of the anode is centred within 2 mm. We found that the small mutual induction among the counters fl and ~,, which exists because of this particular arrangement, is very small. In any case, since these are counters which have both to be put independently in anticoincidence, such an effect is not very important. 2.4. THE HIGH VOLTAGEDISTRIBUTIONAND THE SIGNALS COLLECTIONOF THE COUNTERS The counters are electrically charged by putting the anode to a proper positive high voltage which is filtered by a system of 2 low-pass filters (fig. 7).

A SPECIAL

SYSTEM

OF

PROPORTIONAL

The high voltage is supplied to the anode wires by special high voltage vacuum tight seals * which can stand up to 30 atm, have an insulation up to 25 kV, and through which the counter signals come out. The signals (fig. 7) are sent through a very short 50 ohm coaxial cable, to a charge-sensitive pre-amplifier (Tennelec model 100 C) which was contained in an iron shielding box. Each of the pre-amplifiers output, by means of a doubly shielded coaxial cable (which had to be 10 m long) was connected to the input of an amplifier (Tennelec model 200 C). The negative output of the amplifiers was finally fed into a shaper discriminator, whereas the positive one was sent into a proper resistive mixer, the output of which was connected to a 555 Tektronix oscilloscope, or eventually to a multichannel analyzer (fig. 8). The ground connection of the various parts of the apparatus were made with the utmost care.

TABLE 2

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3. Performance of the apparatus The values of the working voltages for the three different counters ct, fl and 7 are shown in table 2 as a function of the hydrogen pressure.

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good linearity at the output of the ( c o u n t e r + p r e amplifier+amplifier) chain; linearity was in fact not strictly required for the logic handling of the signal involved in the experiment. The proportional counters were asked to stand an instantaneous counting rate of a b o u t 104 particles/sec. We found that the noise of the pre-amplifiers contained some c o m p o n e n t s of very low frequency (around 10 kHz) due to residual mechanical vibrations of the wires. For each of the three counters ct, fl and ,/, a series of measurements have been done, to get information a b o u t the detection etlSciency, and their variations in the time response. The used well collimated beam had a relatively high peak energy (200 MeV/c), and no absorber was placed along its trajectory. We describe these measurements in what follows: 3. I. 1. T h e f a s t g r i d c o u n t e r (~t)

In fig. 8 is shown the counter and electronics arrangement used to measure the efficiency R of counter ct, which we obtained by the ratio between the counts of the coincidence (1, 2, ~, 3) and those of the coincidence (1, 2, 3). The discrimination was set to a very low COUNTER

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Fig. 10. Pulse height spectrum given by counter ct, at a hydrogen pressure of 6 atm. time jitter of the (1, 2, ~t, 3) output coincidence signal, with respect to the m o n i t o r (1, 2, 3) fast coincidence signal, h a s b e e n measured, and is shown in fig. 11. One can see that m o r e than 80 ~ of the beam was counted within an interval of about 500 nsec. 3.1.2. T h e b a c k cylindrical c o u n t e r (l~) For this counter the time variation of the (1, 2, fl, 3') output coincidence signals were measured for different distances of the counter 3' from the axis of the container. The electronics arrangement was quire similar to the one of fig. 8. The scintillation counter 3', placed behind the container, had a reduced area (1 cm 2) and was thus determining a beam of particles of

I

°'i

COUNTS/CHANNEL t

1

ot •

- - ~

.... g

~0

i 1¢

_----~. HIGH

VOLTAGE

~K.V.)

Fig. 9. Behaviour of counter 0t efficiency vs kV power supply. The measurements were done at a hydrogen pressure of 6 aim. threshold. T h e behaviour of the efficiency of ct vs the applied high voltage is shown in fig. 9. (Similar curves have been obtained for the back and peripheral counter.) Fig. 10 shows the amplitude spectrum of the pulses obtained in standard conditions at the output of the ~t counter amplifier. If we accept all pulses bigger than the value corresponding to the abscissa 10 of fig. 10, we get for R the value R = 96 ~ . The

:i 2 / ~- . . . . . . -t~ - , 0 50 tO0 200

t 3 0 ,~

\ i 4,00

~- - . 5110

-~ . . . . . 600 700

~-. - - ~ ~ ~

ID CI'L4NNELS

Fig. I 1. Differential time distribution of counter ~t pulses. The time sorter figure was 2.16 nsec/ch.

A SPECIAL SYSTEM OF PROPORTIONAL COUNTERS

285

COUNTS / CHANNEL

25

2O

j3~ 15--

I

I O,.L

50

fO0

150

200

250

300

350

400

Fig. 12. Differential time distribution of pulses from counter fl for done at a pressure of 6 atm of hydrogen. The small section, nearly parallel to the axis of the container and having from it an average distance D variable with the position of 3'. The chosen values for the distances COUNT$ / ~sec

200 o ' "/ ; ~ ~ ' f ......... 0

15

I .... 30

I 45

,

-----.--~/./leC

Fig. 13. Integral time distribution of pulses coming from y counter. The hydrogen pressure was 8 atm.

450

500

550

600

550

700

CHANNEL S

several positions o f counter 3'. The measurements were time sorter figure was 40 nsec//ch.

of counter 3' from the beam axis have been I, 3, 6 and 9 cm (respectively corresponding to curve fit, f12, f13 and f14 of fig. 12). As to the efficiency, given by the ratio between the counts of coincidence (1, 2, fl, 3') and those of (1, 2, 3'), (when the gate width was set very large), it came out as big as 97.8 9/o. But one has to remember (and the case will be the same for counter 7) that the upper effective value of the efficiency is actually fixed by the maximum time fluctuation which is accepted in the subsequent handling of the signal. We found that, for fast particles parallel to the axis of the container and near the cathode, counter fl gives pulses which have a time fluctuation bigger than 30 #sec in 5 ~ of the cases. 3.1.3. The peripheral counter (7) In this case the measurements were made putting the container with its axis orthogonal to the fast p - beam. The efficiency, given by the ratio between

286

A. A L B E R I G I

QUARANTAet

the coincidence (I, 2, 7, Y') and (!, 2, Y') when the gate width was set very large, was found to be 96 °/o (the plastic oscillation counter 3" was placed behind the container). The time jitter of the (1, 2, "l, 3") output coincidence signals is given in fig. 13. From this figure one can see that in a time interval of about 15 llsec 92 ~o of the total number of pulses were contained. In conclusion, for fast particles, choosing the length of the shaper discriminator for the counters fl and ? to be 15 ysec we can have just more than 90 ~o of efficiency of detection for the mentioned counters. However, it has to be pointed out that the anticoincidence efficiency in the main experiment operation (sect. 1.3) will certainly be better than 90 % for the following reasons: a. During the main experiment we will deal with particles near the end of their range; b. Generally the particles will cross the counters (in particular the peripheral counters y) for a length longer than the one obtained in the test; c. Often a scattered particle crosses both the 7 and fl proportional counters. To give an idea of the actual performance of the proportional counters system, we give in table 4 some data, concerning the behaviour of the counters, obtained during the /a- capture experiment by electronic way only; both anticoincidence signals ,8 and 7 are shaped to have a 15 ysec length.

al.

3. Counters fl and 7 are partially inefficient. We esteem that, anyway, the ratio (I, 2, ct, fl, 9) over (1, 2, :t) would be smaller than I ~o in absence of the effects underlined in 1. and 2. Of course, (as explained in sect. 1.3) by looking at the oscilloscope pictures we will be able to make this ratio even smaller, since the pictures analysis eliminate those residual cases in which the anticoincidence pulses of the counters fl and ), have a time jitter larger than 15 I~sec. 4. Conclusions The illustrated device, which has been working for long periods with hydrogen at high purity, has also been tested in preliminary trials to work while filled with purified gaseous argon, and we think that it is fitted to work also when filled with deuterium, and in general with any gas or mixture of gases proper for a high pressure proportional counter. A test insured besides that, as it has to be expected, given the low electronic attachment of xenon, on the contrary of what was shown at the end of sect. 2.2 for air (which we assume in any case to be the most abundant impurity present in the hydrogen during the I~- capture expzriment), adding 1/1000 of xenon to 8 atm of protonium does not affect the counters characteristics in any appreciable way. This fact has a p~culiar importance for the measurements of the itcapture experiment, because it allows a quick quenching

TABLE Total beam in the (I, 2) telescope

Total beam at the entrance o f the useful region V (1, 2, ~t)

Duty cycle

1500 part./sec

40 %

Total counts (I, 2, ~t, ~, ~) i

10 000 part./sec

i

'

20 part./sec.

I The last column should ideally give a zero value (in fact even /~- stopped in hydrogen, since they later decay giving an electron, should in principle be anticoincided by the proportional counters in anticoincidence). The reported value is not zero because of the following possibilities: I. Muons may stop in the silver electric guard of the back proportional counter; 2. They may also stop in the hydrogen gas, and yield some decay electrons directed towards the front side of the container (region free from anticoincidence counters);

of reaction (1), [due to the fast rate of process (4), Y being now Xe] without altering the proportional counters functions, thus allowing to measure the contribution to the neutron counting rate of all the delayed background coming from anything but the protonium gas, actually in the same experimental conditions fixed for data taking 5). We close this paper by underlining the fact that, provided the work conditions of the (counter+preamplifier+amplifier) chain are chosen taking care of insuring a good linearity, supplementary information may be produced by the described device on the

A SPECIAL SYSTEM OF PROPORTIONAL

energy lost by the ionizing particles in the useful volumes of the counters. This fact yields the possibility of using a similar apparatus to study other kinds of phenomena, as e.g. the scattering of high energy particles by the nuclei of the gas contained in the target, when low momenta are transferred to the nuclei. The fact that we succeeded in coordinating the operation of few-wires counters with plane structure (counter ct) as well as big radius cylindrical ones (counter ~) and small radius quasi-cylindrical ones (counter ~) in the same assembly seems besides encouraging to conclude that no limits are set to any possible effective geometry. We are especially indebted to Prof. B. Ferretti and to Prof. G. C. Bertolini for their useful suggestions helping to clear up the spirit of this technique, and to Prof. G. Torelli for several useful discussions. We also wish to thank lng. Marsili of SIAl LERICI, whose comprehension was essential to the delicate construction of the container, and Per. Ind. L. Pizzirani, of lstituto di Fisica dell'Universitb., Bologna, for his technical assistance in the construction of the

COUNTERS

287

internal frame of the proportional counters. The efforts of Messrs. O. Polgrossi, R. Schillsott, G. Sicher and B. Smith are also greatly appreciated. Our thanks are particularly dine to Prof. P. Bassi and G. Puppi for their encouragement and support.

References 1) A. S. Wightman, Phys. Rev. 77 (1950) 521. ~) A. Alberigi Quaranta, A. Bertin, G. Matone, F. Palmonari, A. Placci, P. Dalpiaz, G. Torelli and E. Zavattini, Nuovo Cimento 47 B (1967) 72. 3) j. C. Sens, Phys. Rev. 113 (1959) 679. ~) G. Conforto, C. Rubbia, E. Zavattini and S. Focardi, Nuovo Cimento 33 (1964) 1001; E. J. Bleser, L. M. Lederman, J. L. Rosen, J. E. Rothberg and E. Zavattini, Phys. Rev. Letters 8 (1962) 128 6) S. S. Basiladze, P. F. Errnolov and K. O. Oganesyan, Z. Exp. Teor. Fiz. 49 (1965) 1042; Sov. Phys. JETP 22 (1966) 725; A. Alberigi Quaranta, A. Bertin, G. Matone, F. Palmonari, A. Placci, P. Dalpiaz, G. Torelli :and E. Zavattini, Nuovo Cimento 47 (1967) 92. 0) F. D. Brooks, Nucl. Instr. and Meth..4:(r9591) 151. 7) A. Citron, C. Delorme, D. Fires, L. Gotlzahl, J. Heintze, E. G. Michaelis, C. Richard and H. Overas, Proc. Conf. Instr. High-Energy physics (1960) p. 286. s) A. Moljk, R.W.P. Drever and S. C. Curran, Proc. Roy. Soc. 239 (1957) A433.