stein is synonymous with the scien atomic bomb project — an involve 17 March with a very broad pro tist in the popular imagination. Each ment which troubled Einstein for the gramme covering sciences, humani budding genius is foisted with his rest of his life. Another indication of ties, human relations and theology. name. His distinctive features are the esteem in which he was held There is a long list of sponsoring the popular vision of the face of the was the offer in 1952 to become institutions including CERN. Leon scientist. Many of the concepts from President of the State of Israel. Van Hove, CERN Research Director his relativity theory have now Albert Einstein was a man of great General, is Chairman of the Inter seeped into popular culture. stature both as a scientist and as a national Committee and many He had the respect of people from human being. He developed a close other high energy physicists will be all walks of life. He was blessed with relationship with the English phi giving papers or chairing session a great love of music and had consid losopher Bertrand Russell and a very (C.N. Yang, A. Salam, W. Thirring, erable ability as a musician. He had a Russell-like statement, from the V.Weisskopf, E.L Feinberg, A. Zichi- deep sense of social responsibility 1973 book of Einstein's 'Ideas and chi...). and took a firm stand on many of the Opinions', makes a fitting conclusion At the Institute for Advanced important issues which welled up to this short tribute — The ideals Study, Princeton, where Einstein during his life. which have lighted my way... have worked from 1933, an 'Einstein His eminence in science, at a time been kindness, beauty and truth.' Centennial Celebration' will be held when science and technology were Many events are being organized from 4-9 March. The emphasis is on leading to so many changes in every throughout the world to mark the specific aspects of Einstein's scien day life, inevitably led to his being anniversary of Einstein's birth. tific work and many leading figures called into the political arena. This The main event is being held in in high energy physics will be had its most telling moment in 1 939 Bern, Switzerland, where . Einstein involved (C.N. Yang, W. Panofsky, A. when, prompted by Leo Szilard, he was based when he did his work on Pais, E. Amaldi, T. Regge, S.Wein wrote the letter to President relativity. An 'Albert Einstein Cen berg, P. Dirac, R. Feynman, V. Weiss- Roosevelt which initiated the tenary' will be held from 13- kopf, Y. Ne'eman, G. 't Hooft...).
Some developments in detection techniques and applications
It is a rather obvious statement that chine can reveal will be observable where. This article pulls together developments in particle detection by available detectors. some recent developments in detec techniques have been crucial to Since the advent of high energy tion techniques and some indica developments in particle physics. It accelerators, the abilities of particle tions of applications in other fields. is not only the advancing abilities of detection have been revolutionized. accelerators in providing higher en The precision with which it is now Multistep avalanche chambers ergies, intensities and qualities of possible to time the passage of a particle beams that have taken our particle, to locate it in space and to Perhaps the most significant of all understanding of the nature of identify it, and the rate at which this the advances in detection techni matter further but also the abilities of information can be collected, are ques was the invention in 1968 of detectors to untangle the particle orders of magnitude greater than multiwire proportional chambers interactions which are produced. It is they were twenty years ago. and drift chambers by the group led for this reason that every study of a As usual, such technological ad by Georges Charpak at CERN. These new machine carries with it a study vances, which were pushed in the detectors locate the ionization ini of detection systems to ensure that cause of high energy physics, have tiated by the passage of a charged the new interactions that the ma also found many applications else particle through a gas, either by
4 CERN Courier, March 1979 The different steps of the multistep chamber which may prove to be another significant advance in particie detection techniques. The role of the various steps is described in the article.
drawing signals from wires closest to the particle (MWPCs) or by meas uring the time taken for the electrons liberated in the gas by the particle to reach a wire (drift chambers). These detectors, now in universal use in high energy physics Laborato ries, brought high spatial resolution (1 mm for MWPCs, 100 |im for OCs),good time resolution (25 nsfor WIWPCs, 500 ns for DCs), multipar- ticle detection capability, high rate (able to record several million tracks per second), continuous sensitivity to the passage of particles and the ability to draw both spatial co-ordi nates from one detector (which has proved of great use in monitoring neutral radiation). Their applications have extended into other sciences (nuclear physics, solid state physics, astrophysics) and the applied sciences of medicine and biology. There has been a constant effort to improve the abilities of these detectors in many Laboratories and articles and news reports on these study of the basic physics of energy are collected. The tricks are applied developments punctuate CERN deposition in gases and the subse in a 'multistep' chamber in the COURIER pages for the past ten quent behaviour of the ionization following fashion. ears. products. This study has yielded Step 1 is a preamplifier, with elec Now there is particular interest in several surprises and the latest one, trode planes A and B, in which the increasing the rate at which MWPCs again unearthed by the Charpak ionization caused by the passage of can record particles and in improving group, could result in another signif a charged particle is multiplied by their accuracy in locating the particle icant step forward. some factor which can be limited to in space. For example, (i) one poten A major limitation on the possible about 103. Some of the electrons tial application in medicine, where rate appears to be the result of the (perhaps 20%) arriving at the plane high energy proton scattering could length of time needed for the space of anode wires, B, have been be used to do three dimensional charge of the positive ions, produced observed to penetrate into the drift scans of the body, would benefit in the region of the signal wires space between the planes B and C. from the highest possible rate so as where the electron avalanches are In Step 2 the drift time for the elec to keep exposure times down, (ii) concentrated, to die away. trons to travel between B and C can biological studies using X rays gain They decided to investigate ways be set so as to give faster detectors from speed and accuracy so as to of reducing this effect. The first (scintillators and logic elements) study living, moving systems, (iii) the trick is to allow only a selected frac enough time to decide whether to new meson factories flood experi tion (corresponding to interesting record the particle. ments with intense particle beams events) of the ionization, initiated When the decision is positive, and are crying out for detectors to in a chamber by the passage of electrode C is pulsed so as to keep pace with the beam intensi many particles, to develop as a full allow the electrons through into the ties... avalanche. The second trick is to signal collection chamber between A lot of the steady advance in the reduce the 'multiplication' in the planes C and D. Here, Step 3, there abilities of MWPCs has followed the actual chamber where the signals is another stage of multiplication of
CERN Courier, March 1979 5 about 103 so that the total multi to communicate the signal further. The streamer chamber has not, plication from Steps 1 and 3 is In the tests at CERN, positional however, taken over as a universal above 105 which is the customary accuracies of 150 |im along the detection technique. Rather it con figure acceptable to standard anode wires and 250 jim in the tinues to be applied in particular MWPC electronics. However, the direction perpendicular to the wires experiments where its properties are creation of ions in the signal collec (better than the wire spacing given adapted to the particular detection tion chamber is reduced by a factor the possibility of interpolating the needs. One such application has of 1 03. In addition, only signals from charge because of the avalanche aroused a lot of interest over the past wanted particles have been allowed spread) have been achieved. The year. It is the^development of a high through which again reduces the ion rate capability of such multistep resolution streamer chamber by the creation. avalanche chambers is at least two group of Jack Sandweiss at Yale These two manoeuvres, by greatly orders of magnitude better than that They have installed their chamber at reducing the ion space charge of conventional MWPCs. A great Fermilab for a (comparatively) high problem, greatly improve the effi deal of further work needs to be done statistics search for charm parti cient detection of selected events, before practical detectors can be cles. amidst a flood of other radiation, built but the results so far are very The need for high resolution such as is often required in high encouraging. comes from the nature of the energy physics experiments. charmed particles. Their lifetime,
13 (Detector enthusiasts will proba High resolution streamer chamber predicted to be in the region of 10~ bly remark on a similarity with the s, requires good spatial resolution if 'hybrid' chambers developed at Some ten years ago there was their short tracks are to be meas Brookhaven and Karlsruhe in 1970. considerable excitement a.t the de ured. (Up to now, only the emulsion The aim at that time was to reduce velopment of 'streamer chambers' technique has had any impact on this costs by combining a MWPC, via a which offered the possibility of problem.) The Yale team therefore drift space, with a spark chamber gathering full information about an aimed for 10 to 20 jim resolution. where cheaper electronics could be event, as in bubble chambers, while The conventional streamer cham used to draw off the signals.) gaining the ability to trigger on ber is of large volume and operates When pursuing these ideas, the desired events and to have a high with its gas at atmospheric pressure. Charpak group were initially mysti data taking rate. The pioneering Other typical parameters are an fied by the remarkably good unifor work was done by the late G.E. applied voltage of 20 kV per cm, an mity and resolution of the transmis Chikovani in Tbilisi following work a pulse length of 1 5 ns. The high sion process between Step 1, the on wide gap spark chambers by B.I. resolution chamber is of small preamplifier, and the subsequent Dolgoshein in Moscow. By applying volume and is filled with gas under steps. It was eventually realized that high voltage pulses in wide gap high pressure. To a first approxima the main mechanism in the trans chambers charged particle tracks tion, there is a scaling law under mission does not involve a direct are picked out as a series of short which doubling the pressure and the process of charge multiplication but sparks, or streamers, which are not applied field will halve the time involves the photons produced by allowed to develop into full dis necessary to produce a given ioniza inelastic collisions between the elec charges by keeping the applied high tion. Thus the high pressure cham trons accelerated towards the anode voltage pulses very short. ber can collect the same amount of plane and the argon atoms in the The technique was taken up at light from much more compact chamber gas. These photons ob several laboratories such as DESY streamers than the conventional viously 'jump' the cathode plane and SLAC, where there was some type, thus giving the required spatial much more easily and their direc major work with a 2 m chamber. A resolution. tions are not perturbed by the elec helium filled version was operated at Typical operating conditions for tric fields. They are of an energy Argonne and a hydrogen filled the Yale chamber are a pressure of (centred around 10 eV) sufficient to version was tried at CERN. A Munich 24 atmospheres, a field gradient of cause ionization themselves in the team used a streamer chamber in an 300 kV per cm and a pulse length of gas (which is doped with small quan experiment at the ISR and large 0.5 ns (ultraviolet light being used in tities of easily ionized vapour) of the versions, up to 8 m long, have been firing the spark gaps of the Blumlein drift space, liberating more electrons used at Serpukhov. pulse system, a trick used on the
6 CERN Courier, March 1S79 One of the first photographs taken in an avalanche chamber by a Bologna/CERN collaboration searching for quarks in neutrino interactions at the CERN SPS. The hope is that the 'clean' conditions of neutrino inter actions might at last uncover some signs of the elusive free quarks through measure ments of primary ionization.
Serpukhov 8 m chamber). The streamer diameter is then some 0.05 mm rather than 2 mm. The technical difficulties were mainly concerned with the pulse system and the production of trans parent electrodes, through which the streamers can be filmed. The elec trodes have to carry pulse currents of ^ kA. They are made of 25 |im diameter wire with 100 |im spacing. Two lenses view the interaction volume and are now used in conjunction with image intensifiers rather than film to help eliminate a problem with flare. The usual manoeuvre of adding a little sulphur hexafluoride to the gas gives the chamber of memory time of 1 jis during which external detectors can decide whether to record the event. In the Fermilab experiments, the triggering is done on muons pro duced in the semileptonic decay of the charmed particle. Intensities of 106 particles per second can be before the completion of the 1978 calcium atoms in a low background. accepted in the chamber and an physics programme at the SPS. The chamber was to record two interaction to be recorded is ex A Bari / Cracow / Liverpool / electrons with a combined energy acted every ten pulses. From Munich / Nijmegen collaboration is near 4 MeV at an expected rate of amongst them, it is hoped to collect using a streamer chamber as a one event per day. They have set a charmed particle tracks at the rate of vertex detector to record details of lower limit of 1.6 x 1021 years on the about a hundred per month. hadron-hadron collisions at the high lifetime of the double decay pro Two other streamer chamber uses est SPS energies. Data taking is cess. in current experimental programmes scheduled to commence with the In heavy ion work at the Bevalac a are atthe CERN SPS. One is installed start of the 1979 SPS physics streamer chamber has been used by on the neutrino beamline by a Bolo programme, but initial trials show a UC Riverside/Berkeley team to gna/CERN collaboration in a search how well the streamer chamber study pion production from nucleus- for quarks. Tests with cosmic rays copes with the showers of particles nucleus collisions. Using a strong have shown that the chamber, oper produced at these energies. As well magnetic field the pions are easily ating in the avalanche mode, can as recording the streamer chamber identified in the photographs. Other cope with large numbers of second tracks on film, this experiment also heavy ion work is difficult without ary particles giving two track reso uses on-line digitizing techniques, so support from other detectors be lution of about a millimetre. that film and off-line computer cause of the high multiplicities. To photograph the tracks, the analysis can proceed in parallel. Cherenkov counters have been experiment uses the cameras from At Berkeley, streamer chambers part of the detector armoury for the old 2 m bubble chamber, now in have found several uses in the many years. They make use of honourable retirement in a Munich nuclear physics programme. A Co the phenomenon of the production museum. First photographs in the lumbia team took a robust chamber of light (in a medium where a parti experiment proper were taken using 600 m underground to look for cle is travelling faster than the local a 200 GeV antineutrino beam just double beta decay (neutrino-less) of speed of light) which is emitted at an
CERN Courier, March 1979 7 R. Meunier alongside the 4 m pressure vessel of the Spot Focusing Detector, a development of the Cherenkov technique.
(Photo CERN293.10.78)
an experiment at CERN (see Feb ruary 1970 issue). Techniques are being tried, for example, by several groups in the USSR, by a SLAC group led by D. Leith and a Bristol group had some encouraging results at Rutherford just prior to the close down of Nimrod. The CERN group led by R. Meunier, leaning on the expertise in optical techniques tha they have developed over many years, had some fine results with a prototype at the SPS in December. The detector was limited in angu lar acceptance and resolution to keep costs down. It had a 3.8 m nitrogen filled pressure vessel as the Cherenkov light radiator with a maxi mum Cherenkov angle of 35 mrad so as to have sufficient light input despite the short length. The light detecting matrix had 11x11 photo- cathodes 9 mm in diameter. The optical system had an equivalent focal length of 20 m with a resolu tion equivalent to that of a DISC angle to the particle direction which now being successfully used in producing light rings 1.4 m in dia depends upon the velocity of the quantity at the SPS (see September meter! particle. The counter can thus be issue 1 975). It is in the optical system that the used to identify particles in combina Several groups have been pursu skill in using this technique is moi tion with magnetic fields which ing, in different ways, the idea of important. To gather enough light curve trajectories depending upon gaining both velocity and position from single particles within a limited the momenta (mass multiplied by information on single particles by wavelength range while retaining velocity) of the particles. focusing the Cherenkov light via a good resolution is not easy. A combi lens system to a spot or small circle nation of fused silica and water was Spot focusing detectors of light (hence the title 'spot focusing used. It had high transparency and detector'). The light focuses to a spot small refractive index variation over CERN has been prominent in ad for a particular velocity or to a circle the wavelength range (230 to vancing the Cherenkov technique. of radius depending upon the veloc 500 nm) of interest. For example, In the 1 960s, the DISC counter was ity of the particle. The location of the the Cherenkov light transmission at developed (allowing particles with spot (or the centre of the circle) gives 365 nm was 40% despite three velocities very close to one another the direction of the particle. reflections and 28 refractions in the to be distinguished). In the early Such detectors could, in principle, lens system. 1970s, to cope with high energies, handle many particles simulta Tests were carried out with posi a version involving lenses made of neously, have high counting rates tive pions, kaons, protons and deute- fused silica and sodium chloride was (107 per second), have good velocity rons in a 50 GeV/c beam. The pres built at CERN by the R. Meunier resolution (down to one part in 1 06) sure in the tank, which set the refer team and used at Fermilab (see and reasonable spatial resolution. ence velocity for spot focusing, was October issue 1973). A more sim A Bonn group led by R. Giese and constantly monitored by a digital plified version known as CEDAR, G. Schuster saw light corresponding refractometer immersed in the ni developed by the team of C. Bovet, is to single particles for the first time in trogen. Single particle detection
8 CERN Courier, March 1 979 Cherenkov rings obtained using a multistep chamber for detection and amplification of photon signals, followed by an optical spark chamber for visualization. The ring radius determines the particle velocity; the ring centre locates the particle position. This picture was recorded with 1.3 GeV/c protons passing through 5 mm lithium fluoride crystal.
(some twenty photoelectrons re Preliminary results show that a corded for the passage of a single velocity resolution around 1 % can particle, which is good enough in this be obtained for protons in the GeV application), multiparticle simulta energy range using a thin lithium neous detection, good velocity reso fluoride radiator — a rather impres lution (7 x 10"6), good spatial resolu sive result taking into account that tion (0.8 mrad) and rapid response the whole detector was less than (several ns) were achieved. 20 mm thick. A device built on this This type of detector could have a principle, capable of particle identifi seful career as a fast trigger cation over large solid angles, may counter in particular experimental also add to the experimenter's configurations where the size of the arsenal. event source is very small and where limited angular acceptance is not a Detect/on of neutral radiation major consideration. Oddly enough one of the first It is in the detection of neutral radia applications of the multistep cham tion. X-rays, gammas and neutrons, bers described above may be for that most of the 'practical applica Cherenkov ring imaging. By using tions', particularly in medicine and • two or three successive stages of biology, have been found. Increased amplification in these chambers, the abilities in this area can reduce overall gain may be pushed high necessary irradiation times 4 or in enough to detect the slightest crease the accuracy of the informa charge deposition — a single elec tion drawn from the irradiation. tron produced in a gas by a light There have been many articles in quantum could be seen. CERN COURIER on such applica Large gains can be achieved in tions. Pioneering work on the use of conventional MWPCs, by using gas At CERN itself, Alan Jeavons has multiwire proportional chambers in mixtures carefully blended to sup- led work on high density MWPCs (as terleaved with high density material, ress photoelectric effects which described in March 1 977, page 59) to obtain good positron detection for cause spark breakdown, but the light building high resolution positron gamma and X-ray imaging, neutron quantum detection efficiency is then cameras in collaboration with Gen radiography and X-ray crystallogra deliberately made very small. This is eva University. This work has now phy, was carried out by the group of not so in the multistep chamber, extended to solid state studies. Victor Perez-Mendez at Berkeley. where quantum efficiencies above Georges Charpak has had a continu Their detectors use lead honeycomb 50% have been measured for pho ing interest in X-ray imaging (see structures coupled with MWPCs for ton energies exceeding the ioniza October 1976, page 350). MWPCs spatial localization of gammas in the tion potential of an added vapour. have also recently been used with MeV range. Their gamma cameras Working together with Tom Ypsi- X-radiation in the diagnosis of osteo have two large area, large solid angle lantis, who is based at Saclay and is a porosis (bone deterioration) by a detectors of this type operating in pioneer in Cherenkov ring imaging, Rutherford group led by Eddie Bate- coincidence. multistep chambers have been used man. The same disease is being They ensure efficient conversion to yield images of single photons identified using gamma radiation of the gammas by the lead honey emitted in the ultraviolet range by a and sodium iodide detectors by a comb and yet the lead is sufficiently fast particle. Although conventional Hebrew University group. We re thin to allow many of the electrons or techniques could have been used for ported gamma work in the UK and at positrons which are produced out the localization of the photons for a Brookhaven on element concentra into the spaces where the MWPC quick survey, instead a multistep tion in the body (see November can collect signals. Using the back- chamber was coupled to an old- 1978 issue, page 402). The list of projection reconstruction method fashioned optical spark chamber applications of particle detectors in they have good images of bone (revisiting the Brookhaven work). this field is very long. structures, thyroid phantoms, etc.
CERN Courier, March 1979 9 and have demonstrated the three 36 negatively biased wires all The key idea in the detection tech dimensional abilities of the camera. stretched parallel to the axis of the nique was to measure the drift time Spatial resolution was about 6 mm cylinder. The aluminized mylar and of the electron produced by the X- and the high sensitivity gave 1600 an outer cylinder were at earth ray to reach the anode wire. Zero counts per minute perjiCi of radia potential. A sensitive gas control time was set by the time of arrival tion. system, developed at Mainz, en of the antiproton in the gas. The pro To improve efficiency, the Ber sured that the mylar was in a stable bability of the X-ray producing keley team have tried thin walled position throughout a run despite electrons in the chamber increases glass tubes containing a high propor having a range of operating pres rapidly with decreasing X-ray ener tion of lead oxide to build the gamma sures up to 16 atmospheres. gy. Low energy X-rays therefor ray converters. These tubes still give Simultaneous detection of suc tend to create electrons close to thê^ good gamma conversion efficiency, cessive X-rays, emitted as the mylar and the electrons take longer like the lead honeycomb, but the use atomic systems decayed through to drift to the anode. By measuring of glass enables the drift field for the several excited states, and of the characteristic known X-rays, the MWPCs to be more uniform. The use final annihilation, producing charged detector could be calibrated. of microchannel plate converters of particles (mainly pions) marking the A new chamber is now under lead-bismuth oxide with hole dia death of the antiprotonic atom, was construction for a Mainz/Vancouver meters from 20 to 200 jim, which possible because of the multicell collaboration, adding charge division can give electron multiplication of structure. (Forfurther information on and more appropriate field configu 106, is also being investigated. the experiment see the August issue rations so that full three dimensional Another topical example of the 1 978, page 257.) For example, three localization of the X-rays can be detection of neutral radiation is an coincident X-rays with energies achieved. It aims to measure the elegant experiment carried out by of 1.8, 3.9 and 11 keV were seen proton-antiproton atomic system in a Daresbury/Mainz/TRIUMF colla from the atom formed by an detail as it decays to its ground boration at the CERN PS. By meas antiproton and a helium nucleus. state. uring soft X-rays in a detector designed by U. Gastaldi, they identi fied atomic systems which included antiprotons. Low energy antiprotons were directed into a cylinder of hydrogen (which could also be filled with deuterium or helium). The wall of the cylinder was of thin mylar (6 |im) which allowed low energy (1 to 1 5 keV) X-rays, emerging from ato mic systems which were formed as the antiprotons came to rest in the gas, to enter a drift chamber. This chamber was subdivided into 36 independent cells by having 36 anode wires interspaced with
Drift chamber detector for X-radiation used in a study of antiproton atoms at CERN shown removed from its pressure vessel (on the right). An inner mylar tube 6 nm thick appears wrinkled in the absence of dif ferential pressure. A multicell structure enables many X-rays to be measured simultaneously.
(Photo CERN 175.11.78)
10 CERN Courier, March 1 979