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Abstract
Si studiano le proprietà dei contatori ad effetto Čerenkov non focheggiati, con particolare riguardo alle seguenti caratteristiche, in vista di eventuali applicazioni nel campo dei raggi cosmici: 1) rendimento di collezione ottica e sua uniformità; 2) tempo di risposta; 3) grande area sensibile; 4) possibilità di distinguere particelle percorrenti la stessa traiettoria in verso opposto («proprietà antidirezionale»). Per ragionevole generalizzazione, dallo studio quantitativo di casi particolari si conclude:a) i contatori a pareti speculari consentono facilmente di ottenere buone «proprietà antidirezionali», ma non consentono di ottenere l'uniformità di rendimento, non appena l'area sensibile sia grande rispetto a quella del fotocatodo (§ 3);b) i contatori a pareti miste, opportunamente progettati (fig. 8), possono offrire buone «proprietà antidirezionali» sulla base di un rendimento basso, ma non minore di quello dei contatori speculari per quelle particelle che incidono nelle condizioni più sfavorevoli (§8); il cono diffondente, spesso adoperato, sembra invece ben lontano da tali risultati (§7);c) i contatori a pareti diffondenti offrono il massimo rendimento per una data area utile; l'uniformità di rendimento è anche assicurata, mentre ovviamente manca ogni «proprietà antidirezionale»; forme allungate sono sconsigliabili per tali contatori; il tempo di risposta è notevolmente maggiore per essi che per gli altri tipi, ma può ancora essere sufficientemente piccolo per molte applicazioni (v. §§5 e 6).
A scintillation or Čerenkov counter generally consists of three parts: the medium in which the light is produced by moving charged particles; a photomultiplier tube in which the light is converted to electrons which are then multiplied up to a detectable signal; and often, but not always, a light pipe to lead the light from the scintillant or Čerenkov medium to the photomultiplier. Fig. 57 shows a typical scintillation counter. Figs. 75 and 86 show Čerenkov counters. These assemblies must be enclosed in light tight containers so that the relatively weak light flashes from the media will not be swamped by stray light. Part I contains a discussion of the properties of many of the commercially available photomultipliers and a brief treatment of light pipes. In Parts II and III the characteristics of scintillation materials are discussed, the former dealing with inorganic compounds and the latter with organics. Čerenkov counters and their uses are reviewed in Part IV, and the properties of large scintillation and Čerenkov counters for high energy β- and γ-ray spectroscopy are considered in Part V. The important characteristics of these counters which have led to such rapid widespread use include: resolving times down to the region of 10-9sec; large sensitive area (counters with dimensions of the order of one meter have been constructed); proportionality; simplicity of construction; and versatility.
A liquid scintillation counter for '4C with an E2/B of over 1800 has been built and operated on a routine basis for over a year. It has a background of 1.96 ± 0.03 c.p.m. and an efficiency of 60.37. + 0.57. has been measured using 5 ml of 14C benzene prepared from N.B.S. Contemporary Standard Oxalic Acid (14.24 ± .07 d.p.m./g of C). Signals from the last dynodes of the two EMI 9635QB photomultiplier tubes are fed through amplifiers, discrimin-ators and a coincidence system which are entirely contained on three small printed circuit boards mounted near the tube bases. The anode signals of the two tubes are fed into a single N.I.M. preamplifier, amplifier, linear gate (controlled by the coincidence unit) and to a pulse height analyser. As the system gain is high, an EHT of less than 1000 volts can be used. The sources of background are discussed. Optical and electronic methods of substantially reducing crosstalk, the major component of background, without reducing counting efficiency are described.
A description is given of a large-volume Cherenkov counter with diffuse reflecting inside walls. Particular attention is given to the timing properties and it is shown that the chosen combination of the reflectivity of the walls and the number of photomultiplier tubes gives an output rise time of about 12 ns. The performance of the counter in two experiments is discussed.
Riassunto Gli impulsi prodotti in un fotomoltiplicatore (Dumont 6291) dalle particelle ionizzanti veloci della radiazione oosmica vengono
studiati con un dispositivo di coinoidenza, analizzandone la distribuzione in ampiezza in diverse condizioni sperimentali.
Il contributo del. l’effetto Čerenkov nel vetro della finestra viene messo nettamente in evidenza sfruttando le sue caratteristiche
direzionali. L’ampiezza media di un impulso Cerenkov è di ~ 5 fotoelettroni. Viene valutato un limite superiore per l’eventuale
contributo di un meccanismo di scintillazione e discusso un possibile contributo di estrazione diretta di elettroni dal fotocatodo
e dai dinodi.
The distribution of rise time of Čerenkov pulses has been measured for a Čerenkov detector of 23 × 10 × 10 cm3 with reflecting outer surfaces, and constant output level. A method of study of linings is proposed.
An efficiency of détection nearly 100% was measured. This detector is associated with cloud chambers for scattering measurements of Mu mesons.
Summary The design and the performance of a large area, comparatively thin toluene Čerenkov counter are described. The velocity resolution
of non focusing Čerenkov counters and its basic limitation, due to the Landau fluctuations in the emission of fast knock-on
electrons, are discussed and compared with those of scintillation counters. The absolute yield of the Čerenkov light is evaluated
from the measurements and found to be in good agreement with the theoretical yield.
The light collection in large plastic and liquid scintillation counters is analysed and it is shown that it is more efficient for the photomultiplier to be in optical contact with one side of the phosphor than to be viewed from above. Experimental results obtained with a large plastic scintillation counter support this conclusion, as does a survey of other published results. The uniformity of response is about equally good for the two types of counter.
An experiment performed at Echo Lake (Colorado) in order to measure the mean lifetime of charged heavy mesons produced by the cosmic radiation is described. The experimental procedure consists in the electronic measurement (by means of a « chronotron ») of the time lag between the occurrence of a nuclear interaction and the delayed emission of a fast charged secondary in the decay (at rest) of an unstable particle produced in the interaction. The nuclear event is detected through its penetrating secondaries by means of a large scintillation counter and a Geiger counters hodoscope, whereas the decay secondary is detected by means of a large directional water Čerenkov counter. The experimental arrangement is described in Sect.2; the timing errors and the criteria used to select and classify the events are discussed in Sect.3. The delay distributions obtained are analyzed in Sect. 4: they can be resolved into a short life component (lifetime (9.6 ± 0.810-9) s) and a long life component (lifetime (2.2 ± 0.2) μs); the latter is interpreted as due to the electrons of the composite π→μ→e decay of π+-mesons produced locally by the incident nucleons. The possibility that the short life component is due to spurious effects of some kind is discussed and ruled out in Sect. 5 on the basis of the results of auxiliary measurements: the conclusion is that the lifetime obtained should be associated essentially with the Kμ2 decay mode, with a possible small admixture of Kπ2(θ+). A rough estimate of the production ratio of such particles to positive π-mesons is made, taking into account the corresponding detection efficiencies; the value found is rather higher than those (1:30- i : 70) obtained from previous cosmic ray experiments. The lifetime obtained is compared with the recent results of other Authors and the assumptions of a purely exponential decay curve with an unique lifetime is discussed.
Čerenkov detectors with silica aerogel as radiator, and a detector surface of about 18 × 52 cm2, have been tested in a particle beam at the CERN Proton Synchrotron. For 9 cm thickness of silica aerogel the number of photoelectrons for β = 1 particles was found to be 4.6 and 5.5 respectively, depending on the light collection system used.
The paper opens with a brief review of the original discovery of the radiation and the physical principles of the process that gives rise to it. This is followed by an elementary account of the theory of the effect. A general survey of the practical applications to cosmic-ray and high-energy physics is then presented, with two examples of modern detectors discussed in greater detail.
The article concludes with an account of recent experiments carried out on light pulses from the night-sky associated with cosmic-ray showers, found to be due to Cerenkov radiation in the atmosphere.
The published values of the spectral diffuse reflectance of magnesium oxide indicate that it should reflect more highly in the red than in the blue. Using a modification of the method described by Preston, we have measured the absolute value of this quantity between λ=0.24μ and λ=0.85μ. A very carefully prepared surface of MgO on silver has a reflectance of about 0.98 at λ=0.54μ, where there is a very flat maximum. The reflectance of a fresh surface falls to less than 0.96 at 0.4μ, but exposure to strong ultraviolet light bleaches it, the final result being a reflectance increasing continuously from the near-infrared to about 0.4μ, where it begins to fall sharply, reaching 0.96 at about 0.25μ. A sphere lined with such a bleached surface raises the color-temperature of Planckian radiation a little, and the observed color agrees well with that predicted from the new results. It is believed that these results are reliable to ±0.002 from λ=0.4μ to λ=0.7μ, but different surfaces prepared with equal care may differ in reflectance by at least 0.005.
The bleaching is presumed to be caused by the decomposition of small amounts of magnesium nitride.
DOI:https://doi.org/10.1103/PhysRev.78.488
DOI:https://doi.org/10.1103/PhysRev.85.1054
Studies of the luminescence induced in aqueous solutions of radioactive isotopes are described. The relative intensities of the effects observed with various high energy beta - and beta -gamma -emitting species support the hypothesis that the luminescence induced is due to the Cerenkov effect rather than excitation of the substrate. The relative and absolute intensities observed are in general agreement with the theory of Frank & Tamm. Aqueous solutions of alpha -emitters, and of beta -emitters whose beta -particles have energies below the limit for the Cerenkov effect, show a weak luminescence attributable to excitation of the substrate.
Zusammenfassung In der folgenden Arbeit wird die Abhängigkeit des Koeffizienten der diffusen Lichtreflexion von dem Einfallswinkel des Lichtes untersucht. Dabei läßt sich experimentell und theoretisch ein Zusammenhang zwischen der Absorptionsfähigkeit des diffus reflektierenden Körpers, seinem Reflexionsvermögen und dem Einfalls-Winkel des Lichtes feststellen.
In a fluorescent body (e.g., scintillation counter crystal for detecting nuclear radiation) some of the fluorescent light may be trapped within the body because it is totally internally reflected an indefinitely large number of times. This phenomenon occurs only in bodies whose shapes have high symmetry. Equations are derived for the trapping fractions of rectangular parallelepipeds, plane parallel sheets, and spheres. Illustrative numerical values are given for various values of refractive index. For rectangular parallelepipeds of index 1.225, 1.500, and 2.000, the trapping fractions are 0.0, 0.236, and 0.598 respectively. For a sphere of index 1.50, the trapping fraction is 0.414 or 0.264, depending on whether the index with respect to the exciting radiation is taken as 1.00 or 1.50, these alternatives corresponding essentially to excitation by gamma-radiation or by light, respectively.
Cerenkov radiation particle counters of a number of types are in satisfactory use in this laboratory for the counting of high energy mesons, electrons, and protons. The problems associated with Cerenkov counting and some of the uses are discussed together with the design and performance of several types of counters. The uses include high energy electron counting with the exclusion of neutrons and protons, measurement of the velocity of mesons, and the detection of high energy neutrons in the presence of lower energy neutrons by the Cerenkov threshold counting of recoil protons.
Integral size-frequency distribution curves for cosmic-ray bursts of more than 100 particles were obtained by using an ionization chamber shielded by 1.25, 12, and 35 cm of iron. These distribution curves can all be represented by an inverse power law with exponent = 2.0±0.2. The transition curve plotted from these data shows a pronounced maximum. Five G-M counter coincidence sets were arranged to register on the ionization chamber trace so that coincidences between bursts and air showers could be measured. For a thickness of 1.25 cm of iron, about 85 percent of the bursts were coincident with air showers, while for 12 cm of iron, this fraction was about 20 percent and for 35 cm of iron, only 5 percent of the bursts were observed to be coincident with extensive showers. A discussion is given to account for the origin of the bursts which were not coincident with air showers. For the analysis of the transition curve three different types of bursts are considered: bursts due to extensive atmospheric showers, bursts produced by narrow air showers or single high energy electrons, and bursts produced by mesotrons (knock-on and bremsstrahlung processes). Data for bursts of more than 500 particles under 12 cm of lead are compared with the corresponding data obtained under 35 cm of iron in order to determine the dependence of burst frequency on atomic number; the results are in substantial agreement with production of these bursts by bremsstrahlung of the mesotron. Integral size-frequency distribution curves are also plotted for data obtained during a five-year period at Huancayo (3350-meters elevation) and at Cheltenham (72 meters) for bursts under 12 cm of lead. The Cheltenham data are compared with the theoretical calculations of Christy and Kusaka for burst production by mesotrons of spin 0, ½, and 1 and it is concluded that either the spin 0 or the spin ½ curve fits the experimental data but the evidence definitely excludes spin 1. The altitude dependence for burst under 12 cm of lead shows that the ratio of the burst frequencies at Huancayo and Cheltenham is constant for bursts up to 2400 particles, but increases sharply for still larger bursts. This increase is discussed on the basis that bursts under thick shields at higher altitudes may be caused either by extensive showers or by possible spin 1 mesotrons having so short a mean life that most of them fail to reach sea level.
A new method of measuring high energy photons and electrons is reported. Using an experimental arrangement consisting of G-M counters and scintillation counters with large plastic scintillators, the number vs size distribution of cosmic-ray showers produced by photons and electrons in a lead block of one inch thickness was measured. From the results of shower theory the energy spectra of cosmic-ray photons and electrons at Chicago (600 ft above sea level and 51° N geomag. lat) were deduced. The results of the present investigation show that in the energy range of 0.3-2.0 Bev, the differential energy distributions can be represented by a power law of the form E-s, with s=2.71±0.13 and s=2.81±0.13 for the photon and electron components, respectively. The exponent s decreases considerably at energies below 0.2 Bev. An essential part of this investigation is to provide a suitable means of measuring the energies of energetic γ-rays or electrons, or the energy distribution of energetic γ-rays or electrons. The results show that scintillation counter arrangements consisting of large plastic scintillators are suitable instruments for these purposes. This method could also be applied to energy measurements of photons and electrons from high energy accelerators.
Using a Carnegie Model "C" ionization chamber together with Geiger-Mueller tube coincidence circuits, the nature of the radiation which produces large bursts (greater than 200 particles) of cosmic rays under thick absorbers at 11,500-feet elevation has been investigated at Climax, Colorado. The percentage of this radiation which was accompanied by air showers was determined and found to be dependent upon the burst size. The absorption mean free paths for the burst-producing radiation (excluding bursts produced by μ-mesons and by air showers) were measured in lead and iron and found to be 350±40 g/cm2 in lead and 240±20 g/cm2 in iron. The barometric effect on this radiation yielded an absorption mean free path of 65±14 g/cm2 in the atmosphere, and the zenith angle distribution was found to be that due to an absorption mean free path of 75±14 g/cm2. These results are shown to indicate that the radiation responsible for these large bursts consists largely of protons and π-mesons of energy greater than 60 Bev.
A system of Geiger counters and a Čerenkov light detector is described which responds selectively to the passage of slow cosmic ray particles heavier than about 550 electron masses.
Es wird eine neue Methode zur Bestimmung des diffusen Reflexionsvermgens im sichtbaren Gebiet (rotes Licht) angegeben und fr Magnesiumoxyd durchgefhrt, das, durch Verbrennen des Metalls gewonnen, als Normalwei fr relative Messungen empfohlen wird. Bei senkrechter Inzidenz des Lichtes betrgt die in den Halbraum zurckgeworfene Lichtmenge 95,3 Proz. der aufgefallenen Lichtmenge. Von diesem Gesamtreflexionsvermgen ist das Teilreflexionsvermgenr
zu unterscheiden, welches bei gegebener vertikaler Beleuchtung die Intensitt des unter einem bestimmten Winkel (gemessen gegen die Normale) zurckgeworfenen Lichtes bestimmt. Fr das Teilreflexionsvermgensr
, das bei einem ideal diffusen ideal weien Krper fr alle Winkel den Wert 1 hat, wurder
=1–1,3 sin4/2 gefunden. Einige in grnem Licht durchgefhrte Beobachtungen lassen einen Unterschied des Teilreflexionsvermgens in den beiden Farben nicht mit Sicherheit erkennen.
Observations on large cosmic-ray bursts have been performed at the altitude of 3500 meters with a model C meter surrounded by thick lead shields. G-M counters, connected to appropriate electronic circuits, were placed over these lead shields in order to record extensive air showers simultaneously with bursts. It is found that about 2 percent of the 100 particle bursts and about 14 percent of the 1000 particle bursts under 10.7 cm Pb are produced by extensive air showers. This fraction continues to increase with increasing burst size and becomes almost 100 percent for bursts of more than 4000 particles. The experimental observations are consistent with the assumption that the part of the burst-producing radiation which does not consist of air showers or of μ-mesons is attenuated in lead with the absorption mean-free-path of 346±36 g/cm2. The correction for the number of bursts produced by μ-mesons is based on a revision of the calculations of Christy and Kusaka; the revised calculation indicates that μ-mesons (spin 1/2) produce 68±5 percent of the bursts in a model C meter under 10.7 cm Pb at sea level.
Henning eHeuse (23)Zeits f. PhysPokrowski (24)G. I. Pokrowski:Zeits. f. Phys
- Cfr Wright
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- B R Linden
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