- Email: [email protected]
Search for cosmic nuclearites at sea level
Volume 161B, number 4,5,6
PHYSICS LETTERS
31 October 1985
SEARCH FOR COSMIC NUCLEARITES AT SEA LEVEL K. N A K A M U R A Department of Physics, University of Tokyo, Tokyo 113, Japan and National Laboratory for High Energy Physics, Oho, Ibaraki 305, Japan 1 H. H O R I E 2, T. T A K A H A S H I a n d T. T A N I M O R I 3 Department of Physics, University of Tokyo, Tokyo 113, Japan Received 13 August 1985
A search has been made at sea level for the recently proposed nuclearites of strange quark matter with the use of high-sensitivity scintillation counters. The velocity range explored is 10-%-10-3c, and the detection threshold is 0.01 /min, where Imin is the pulse height corresponding to the minimum-ionizing muon. A 90% CL flux upper limit of 3.2 × 10 -11 cm -2 s-1 sr-1 has been obtained for nuclearites heavier than 1.5 × 10-13 g.
Cosmic-ray objects as slow as 10 - 3 c - 1 0 - 4 c have been extensively searched for in the context o f superheavy GUT magnetic monopoles. Recently, another possibility o f slow cosmic radiation, "strange m a t t e r " or "nuclearite", has been suggested by some authors [ 1 - 4 ] . Strange matter is quark matter containing roughly equal number o f up, down, and strange quarks. Although it is not conclusive, such quark matter has the possibility o f being absolutely stable for almost any value o f mass number [2], and might be responsible for galactic dark matter [ 1 - 4 ] . De Rtijula and Glashow [3,4] studied phenomenological properties o f lumps o f strange matter, which they call nuclearites * 1. According to them, nuclearites have a net positive charge, and are neutralized b y electron~ Thus, due to the Coulomb barrier, the
nuclearites are protected from direct nuclear interactions with the atoms with which they may collide, and their energy-loss mechanism, passing through the medium with galactic velocities o f 250 km/s, characteristic o f the orbital rotation of the solar system, is mainly elastic and quasi-elastic atomic collisions. The rate o f energy loss for massive nuclearites is given [3,4] by dE / dx = - A pv 2,
(1)
where A is the effective cross-sectional area and u is the velocity o f the nuclearites, and p is the density o f the penetrated medium. The velocity as a function o f distance L is therefore given [3,4] as
0 1 Present address. 2 Present address: Technical Section, Nissan Motor Co., Ltd., Ginza 6-chome, Chuo-ku, Tokyo 104, Japan. 3 Present address: National Laboratory for High Energy Physics, Oho, Ibaraki 305, Japan. .1 With the term "nuclearite", De R6jula and Glashow refer not only to a lump of strange quark matter, but also to a lump of other unusual quark matter which may exist in the galaxy as remnants of the Big Bang or as debris of astrophysical catastrophes. See refs. [3] and [4]. 0370-2693/85/$ 03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
where M is the mass o f the nuclearite. The density o f strange matter is estimated [2] to be 3.6 X 1014 g/ cm 3. For a nuctearite w i t h M < 1.5 × 10 . 9 g , A is determined b y its electronic atmosphere to be of a typical atomic size [3,4], A ~ rr × ( 1 0 - 8 cm)2. On the passage of nuclearites of atomic size with galactic velocity through the medium, a high-temperature shock wave is generated. If the temperature o f 417
Volume 161B, number 4,5,6
PHYSICS LETTERS
the shock wave is high enough, visible light is emitted. Thus, scintillation counters can be sensitive to cosmic nuclearites, even without invoking the emission of scintillation light, simply because they are equipped with photomultipliers. The fraction of dissipated energy appearing as visible light is called luminous efficiency. De Rt~jula and Glashow estimate [3,4] a lower bound on the lumInous efficiency as given by black-body radiation from an expanding cylindrical thermal shock wave. According to the formula given by De Rtijula and Glashow [3,4], the luminous efficiency 7/for a scintillator with a typical cut-off frequency of 410 mm is 7/~ 6.5 X 10 - 6 for nuclearites with velocity v >~ 10-4 c. The rate of energy loss e dissipated as visible light by the nuclearite in a plastic scintillator is given by eq. (1) as
e >~ (n/p)ldE/dxl = n,'lv 2. The photon yield is e/(rr eV)/(g/cm2), where rr eV is a typical visible photon energy. For a typical plastic scintillator, the rate of energy loss of minimum-ionizing muons is emi n ~ 2 MeV/(g/cm 2) and the visible photon yield due to the passage of charged particles is roughly one photon per 100 eV [5]. From eq. (1), nuclearites with o = 250 km/s give d E / p d x ~ 125 GeV/(g/cm2), and this corresponds to a photon yield of more than 13 times that for the minimum-ionizing muons. With ec~. (2), the mass o f nuclearites whose velocity is 1 0 - " c at sea level is given a s M "- 1.5 X 10 -13 g with an initial velocity of 250 km/s and a column density of the atmosphere of ~103 g/cm 2. For such nuclearites, eq. (1) gives d E / p d x ~ 1.75 GeV/(g/cm2), and this corresponds to a photon yield of more than 0.18 times that for the minimum-ionizing muons. Therefore, if the detection threshold of scintillation counters at sea level is lower than 0.18 Imin and if the trigger logic is sensitive to a velocity range of 10 -4 c - 1 0 - 3 c, these counters can detect cosmic nuclearites heavier than 1.5 X 10 -13 g. Previously, at least one scintillation-counter experiment at seal level, which might be sensitive to cosmic nuclearites, was reported by Tarld et al. [6]. Their detector was a single thick slab of scintillators, and was sensitive to an ionization-loss range of (0.6-70)/mi n and a velocity range of 6 X 10-4 c - 2 . 1 X 10 -3 c. They observed two events corresponding to 0.7 lmi n and o = 1.7 X 10 -3 c [6]. If these events could have been due to cosmic nuclearites with a fix418
31 October 1985
ed mass, their flux would be 8 X 10 -13 cm -2 s -1 sr -1 . Tarl6 et al. [6] also observed 820 events with pulse heights greater than 70 Imi n, which saturated the amplifiers. These were probably due to fast cosmic-ray events far out in the Landau tail, or showers, but it would also be possible that some o f them were due to nuclearites. If so, the maximum possible flux of nuclearites would be ~-3 X 10 -10 cm "-2 s -1 sr -1 . Plastic track detectors may also be sensitive to cosmic nuclearites, because their energy loss is of the same order of, or greater than, that required to produce etched holes on plastic sheets. Doke et al. [7] reported a negative result in the search of GUT magnetic monopoles with cellulose nitrate sheets at sea level. If their result is also applicable to cosmic nuclearites, an uj~per limit on the flux would be ~5 X 10 -15 c m - Z s -1 sr "1 , which is much lower than the maximum possible flux based on the observation of Tarld et al. [6]. In any case, these previous experiments should be critically examined whether they were really sensitive to the postulated cosmic nuclearites. The possibility of cosmic nuclearites is so important that searches with a variety of experimental techniques are highly desirable. We have directly searched for slowly moving (o = 10 - 4 c - 1 0 -3 c) cosmic-ray objects with ionizing loss greater than 0.01 Imin, using high-sensitivity scintiUation counters at sea level. This detector was used for searching slow, weakly-ionizing GUT magnetic monopoles [8], but clearly it is also suited for searching cosmic nuclearites with mass heavier than 1.5 X 10 -13 g. The detector consisted of four layers of 3 cm thick plastic scintillators separated by about 30 cm. Each layer was subdivided into five counters. The unit counters for the two upper layers were 20 cm wide and 150 cm long, and those for the two lower layers were 20 cm wide and 100 cm long. Between the counter planes, a 9 mm thick iron plate was inserted. In addition, a 19 mm thick iron plate was put under the lowest counter plane. Each unit counter was viewed by two photomultipliers at both ends (called right and left sides of the counter) via lucite light guides. The photomultipliers used were Hamamatsu R1332 ones, which had highgain and low-noise characteristics due to their GaAsP first dynodes. These characteristics enabled us to count one-photoelectron signals easily. In fact, dark
Volume 161B, number 4,5,6
PHYSICS LETTERS
pulses corresponding to the one-photoelectron signal were clearly observed. Fig. 1 shows a schematic o f the electronics arrangement. Each photomultiplier output was amplified by a X 8 fast amplifier. The gains of the photomultipliers were determined so that the amplifier output corresponding to the one-photoelectron signal was about - 5 0 mV. One of the amplified pulses was discriminated by an updating discriminator (LRS 623) with an output width o f 50 ns. The thresholds of the discriminators were set at - 3 0 mV. The discriminator outputs of the same-side photomultipliers in each layer
31 October 1985
were ORed and vetoed by fast cosmic-ray signals. Subsequently, a coincidence between the right-side OR and left-side OR was formed to produce a layerhit signal. Time-order circuits then recognized either downward (layer 1 -~ 2 ~ 3 ~ 4) or upward (layer 4 3 ~ 2 ~ 1) time-ordered sequences of layer hit signals occurring within a maximum sensitive time o f 60 Us although only the downward trigger is relevant to the detection o f cosmic nuclearites lighter than "0.1 g, which cannot penetrate the earth. The outputs of these time-order circuits were mixed to form a pretrigger.
(..~l~J
•~
~d
,
LAVER,
,--,. :E
LAYER2
5
IT
'AYER,
LN I EARSUM MEMORY MODULE
A N
LAYERI
~
~
~
LAYER 4 LAYER2 LAYER3 LAYER 4
I
" FASTCOSM C I RAYVETO
LAYER3
I
LAYER2
I i
[~
~ 4--3--2-1~ TR G IGER
Fig. 1. Schematic of the electronics. 419
Volume 161B, number 4,5,6
PHYSICS LETTERS
When a pretrigger oc6urred, the time differences between the layer-hit signals were examined, and only if the ratio of the minimum and maximum time differences between adjacent layers was within 1 : 3, the data were recorded. It should be noted that our timing requirement for triggering slow cosmic-ray candidates is not too stringent to reject nuclearites with velocities greater than 10 -4 c, although their velocities are degraded during the passage through the scintillator and the iron shield. A personal computer acquired the data and recorded them on a floppy disk, and monitored the status of data acquisition. The recorded data were timing and pulse-width information of each photomultiplier and summed pulse height of each layer. One of the discriminator outputs of each photo. multiplier was introduced into a CAMAC memory module. At the entrance of each channel of the memory module there was a retriggerable univibrator with a width of about 1 gs. The status of the univibrators was periodically recorded into a 256-bit deep memory with 5 MHz clock, with a write-address counter incremented by the same clock. Therefore, at any given time, the memory module kept 51.2/as history of each photomultiplier before that time. When a pretrigger occurred, the 5 MHz clock was stopped and the memory module retained the data until the decision was made whether the data were to be read out or aborted. It is noted that normal photomultiplier outputs due to low-energy ~,-rays and thermal noises were localized in time and recorded as about five consecutive " l " s in the memory module. However, the photomultiplier output due to a slowly-moving and weakly ionizing object is expected to be a discrete random train of single-photoelectron pulses which would be recorded as a longer train of " l " s in the memory module because of the response of an updating discriminator followed by a retriggerable univibrator, to such photomultiplier output. (Note that if the ionization-energy loss of a slowly moving object in the scintillator is high enough, no broadening of the recorded data occurs.) This fact was used in the analysis to reject background events in the slow-velocity and weak-ionization d o m a i n . To record pulse heights, X 8 amplifier outputs of photomultipliers in the same counter plane were summed up and digitized by a LRS 2249W charge420
31 October 1985
sensitive analog-to-digital converter (ADC). For each counter plane, an independent ADC module was used. A 2/as wide gate was produced by the OR of the layer-hit signals. If no pretrigger occurred within 60/as, a fast clear was issued. The apparatus was contained in a wooden hut located on relatively low-background-level at the site of the National Laboratory for High Energy Physics (KEK). The measurement was performed for five months and about 58 000 events were recorded. After correcting for computer dead time, etc., the net live-time of the apparatus was about 2500 h. During the measurement, the single counts of all the photomultipliers and the rates of the layer hits were perle. dically monitored. The response of the scintillation counters to the minimum-ionizing cosmic-ray muons normally incident on the counters was measured with the use of a charge-sensitive amplifier and a pulse-height analyzer It was found that lmi n corresponded to approximately 100 photoelectrons per photomultiplier, i.e., 200 photoelectrons per counter. The ADC's were then calibrated by using minimum-ionizing cosmic-ray muons again. One photoelectron signal corresponded to about 15 ADC channels, and the ADC's overflowed at "-49.6 Imin" The RMS resolution for low pulse heights was given by the measured pedestal widths to be about 30 channels, i.e., two photoelectrons. In order to select slow cosmic-ray candidates, we imposed the following three requirements: (i) geometrical correlation among the hit counters, (ii) timing correlation, and (iii) pulse-height correlation. For simplifying the analysis, we rejected those events having hits with the same timing in adjacent counters in the same counter plane. This requires that a hit counter had to be traversed by a slow cosmic-ray candidate through its entire thickness. Due to this requirement, the effective solid angle of the apparatus was reduced by about 20%. For requirement (ii), the time differences between the signals from two adjacent counter planes were required to be consistent with the penetration of a cosmic-ray object with a constant velocity less than 10 -3 c. (However, small velocity changes due to energy loss in the scintillator and iron absorber plates were allowed.) To examine requirement (iii), the recorded pulse heights were converted into the numbers of photoelectrons n i (i = 1 ..... 4), and the average number of photoelec-
PHYSICS LETTERS
Volume 161B, number 4,5,6 40 oo d ,° •o
30
I--
lO 0 . 1 5 Imi n i
o
1~0
,
i 20
,
30
h
Fig. 2. Scatter plot of the average number of photoelectrons per layer h versus the time difference T between the signals from layer 1 to layer 4 for geometrically correlated events. trons per layer X was calculated. If the ith ADC overflowed, the number o f photoelectrons corresponding to the overflow level was assigned to n i. Slow cosmicray candidates were required to satisfy
Ini-Xl~< [(3X~) 2 + 2 2 ]
1/2
( i = 1 ..... 4),
where the ADC resolution for low pulse heights was taken into account. Assuming the Poisson distribution for photoelectron statistics, the intrinsic detection efficiency o f our apparatus as a function o f X is given as [1 - e x p ( - X / 2 ) ] 6. In addition, a certain fraction o f candidate events would be discarded because o f the above requirement. The overall detection efficiency was calculated as a function o f X, and was taken into account in calculating the upper limits for the flux. With these requirements, 13 slow cosmic-ray candidates remained. Of these, 8 events were triggered by the downward time-ordered trigger and the rest o f 5 events by the upward time-ordered trigger. Fig. 2 shows a scatter plot o f X against the time difference T between the signals from layer 1 and layer 4 for these 13 events. At this level, no candidate events remained for an energy loss greater than 0.15 Imin, which means that no candidates for cosmic nuclear-
31 October 1985
ites heavier than 1.5 X 10 -13 g were observed. However, considering the rather crude estimates made for the properties o f nuclearites, it would beo worthwhile to examine these low ionization-loss events further. Although details are not mentioned here, these 13 events were completely eliminated b y inspecting the timing structure o f the scintillation-counter response recorded in the m e m o r y module [8]. Based on the null result, we derive the 90% CL upper limit for the flux o f slowly-moving cosmic-ray objects in the velocity range 10 --4 c - 1 0 - 3 c as f < 2.3/ S~2tEo, where SI2 is the detector acceptance, t is the total live time o f the detector, E 0 is the efficiency due to the requirements imposed in the analysis. For our measurement, SI2t = 0.9 × 1011 cm 2 s sr and E 0 is 0.79. Therefore, the present result establishes a 90% CL upper limit o f 3.2 X t 0 -11 cm - 2 s - 1 sr - 1 , or 6.3 X 10 7 km - 2 yr -1 (21r sr) -1 for cosmic nuclearites heavier than 1.5 × 10 -13 g. Although our result is not very far-reaching, especially if the measurement by Doke et al. [7] should be sensitive to cosmic nuclearites, the present experiment is the first one which can directly measure the velocity o f slowlymoving cosmic-ray objects lighter than the GUT magnetic monopole, down to o ~ 10 - 4 c. We wish to thank Professor Tadao Fujii, Professor Hirotaka Sugawara, Professor Kasuke Takahashi, Professor Kazuaki Katoh, and Professor Yoshio Yoshimura for their interest and support.
References [1 ] [2] [3] [4] [5 ]
E. Witten, Phys. Rev. D30 (1984) 272. E. Farhi and R.L. Jaffe, Phys. Rev. D30 (1984) 2379. A. De Rfijula and S.L. Glashow, Nature 312 (1984) 734. A. De Rdjnia, Nucl. Phys. A434 (1985) 605c. L.C.L. Yuan and C.-S. Wu, eds., Methods of Experimental Physics, Vol. 5A (Academic Press, New York, 1961) p. 127. [6] G. Tarl~ et al., Phys. Rev. Lett. 52 (1984) 90. [7] T. Dolce et al., Phys. Lett. 129B (1983) 370. [8] K. Nakamura et al., University of Tokyo preprint UT4-IE85•05 (1985).
421
PHYSICS LETTERS
31 October 1985
SEARCH FOR COSMIC NUCLEARITES AT SEA LEVEL K. N A K A M U R A Department of Physics, University of Tokyo, Tokyo 113, Japan and National Laboratory for High Energy Physics, Oho, Ibaraki 305, Japan 1 H. H O R I E 2, T. T A K A H A S H I a n d T. T A N I M O R I 3 Department of Physics, University of Tokyo, Tokyo 113, Japan Received 13 August 1985
A search has been made at sea level for the recently proposed nuclearites of strange quark matter with the use of high-sensitivity scintillation counters. The velocity range explored is 10-%-10-3c, and the detection threshold is 0.01 /min, where Imin is the pulse height corresponding to the minimum-ionizing muon. A 90% CL flux upper limit of 3.2 × 10 -11 cm -2 s-1 sr-1 has been obtained for nuclearites heavier than 1.5 × 10-13 g.
Cosmic-ray objects as slow as 10 - 3 c - 1 0 - 4 c have been extensively searched for in the context o f superheavy GUT magnetic monopoles. Recently, another possibility o f slow cosmic radiation, "strange m a t t e r " or "nuclearite", has been suggested by some authors [ 1 - 4 ] . Strange matter is quark matter containing roughly equal number o f up, down, and strange quarks. Although it is not conclusive, such quark matter has the possibility o f being absolutely stable for almost any value o f mass number [2], and might be responsible for galactic dark matter [ 1 - 4 ] . De Rtijula and Glashow [3,4] studied phenomenological properties o f lumps o f strange matter, which they call nuclearites * 1. According to them, nuclearites have a net positive charge, and are neutralized b y electron~ Thus, due to the Coulomb barrier, the
nuclearites are protected from direct nuclear interactions with the atoms with which they may collide, and their energy-loss mechanism, passing through the medium with galactic velocities o f 250 km/s, characteristic o f the orbital rotation of the solar system, is mainly elastic and quasi-elastic atomic collisions. The rate o f energy loss for massive nuclearites is given [3,4] by dE / dx = - A pv 2,
(1)
where A is the effective cross-sectional area and u is the velocity o f the nuclearites, and p is the density o f the penetrated medium. The velocity as a function o f distance L is therefore given [3,4] as
0 1 Present address. 2 Present address: Technical Section, Nissan Motor Co., Ltd., Ginza 6-chome, Chuo-ku, Tokyo 104, Japan. 3 Present address: National Laboratory for High Energy Physics, Oho, Ibaraki 305, Japan. .1 With the term "nuclearite", De R6jula and Glashow refer not only to a lump of strange quark matter, but also to a lump of other unusual quark matter which may exist in the galaxy as remnants of the Big Bang or as debris of astrophysical catastrophes. See refs. [3] and [4]. 0370-2693/85/$ 03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
where M is the mass o f the nuclearite. The density o f strange matter is estimated [2] to be 3.6 X 1014 g/ cm 3. For a nuctearite w i t h M < 1.5 × 10 . 9 g , A is determined b y its electronic atmosphere to be of a typical atomic size [3,4], A ~ rr × ( 1 0 - 8 cm)2. On the passage of nuclearites of atomic size with galactic velocity through the medium, a high-temperature shock wave is generated. If the temperature o f 417
Volume 161B, number 4,5,6
PHYSICS LETTERS
the shock wave is high enough, visible light is emitted. Thus, scintillation counters can be sensitive to cosmic nuclearites, even without invoking the emission of scintillation light, simply because they are equipped with photomultipliers. The fraction of dissipated energy appearing as visible light is called luminous efficiency. De Rt~jula and Glashow estimate [3,4] a lower bound on the lumInous efficiency as given by black-body radiation from an expanding cylindrical thermal shock wave. According to the formula given by De Rtijula and Glashow [3,4], the luminous efficiency 7/for a scintillator with a typical cut-off frequency of 410 mm is 7/~ 6.5 X 10 - 6 for nuclearites with velocity v >~ 10-4 c. The rate of energy loss e dissipated as visible light by the nuclearite in a plastic scintillator is given by eq. (1) as
e >~ (n/p)ldE/dxl = n,'lv 2. The photon yield is e/(rr eV)/(g/cm2), where rr eV is a typical visible photon energy. For a typical plastic scintillator, the rate of energy loss of minimum-ionizing muons is emi n ~ 2 MeV/(g/cm 2) and the visible photon yield due to the passage of charged particles is roughly one photon per 100 eV [5]. From eq. (1), nuclearites with o = 250 km/s give d E / p d x ~ 125 GeV/(g/cm2), and this corresponds to a photon yield of more than 13 times that for the minimum-ionizing muons. With ec~. (2), the mass o f nuclearites whose velocity is 1 0 - " c at sea level is given a s M "- 1.5 X 10 -13 g with an initial velocity of 250 km/s and a column density of the atmosphere of ~103 g/cm 2. For such nuclearites, eq. (1) gives d E / p d x ~ 1.75 GeV/(g/cm2), and this corresponds to a photon yield of more than 0.18 times that for the minimum-ionizing muons. Therefore, if the detection threshold of scintillation counters at sea level is lower than 0.18 Imin and if the trigger logic is sensitive to a velocity range of 10 -4 c - 1 0 - 3 c, these counters can detect cosmic nuclearites heavier than 1.5 X 10 -13 g. Previously, at least one scintillation-counter experiment at seal level, which might be sensitive to cosmic nuclearites, was reported by Tarld et al. [6]. Their detector was a single thick slab of scintillators, and was sensitive to an ionization-loss range of (0.6-70)/mi n and a velocity range of 6 X 10-4 c - 2 . 1 X 10 -3 c. They observed two events corresponding to 0.7 lmi n and o = 1.7 X 10 -3 c [6]. If these events could have been due to cosmic nuclearites with a fix418
31 October 1985
ed mass, their flux would be 8 X 10 -13 cm -2 s -1 sr -1 . Tarl6 et al. [6] also observed 820 events with pulse heights greater than 70 Imi n, which saturated the amplifiers. These were probably due to fast cosmic-ray events far out in the Landau tail, or showers, but it would also be possible that some o f them were due to nuclearites. If so, the maximum possible flux of nuclearites would be ~-3 X 10 -10 cm "-2 s -1 sr -1 . Plastic track detectors may also be sensitive to cosmic nuclearites, because their energy loss is of the same order of, or greater than, that required to produce etched holes on plastic sheets. Doke et al. [7] reported a negative result in the search of GUT magnetic monopoles with cellulose nitrate sheets at sea level. If their result is also applicable to cosmic nuclearites, an uj~per limit on the flux would be ~5 X 10 -15 c m - Z s -1 sr "1 , which is much lower than the maximum possible flux based on the observation of Tarld et al. [6]. In any case, these previous experiments should be critically examined whether they were really sensitive to the postulated cosmic nuclearites. The possibility of cosmic nuclearites is so important that searches with a variety of experimental techniques are highly desirable. We have directly searched for slowly moving (o = 10 - 4 c - 1 0 -3 c) cosmic-ray objects with ionizing loss greater than 0.01 Imin, using high-sensitivity scintiUation counters at sea level. This detector was used for searching slow, weakly-ionizing GUT magnetic monopoles [8], but clearly it is also suited for searching cosmic nuclearites with mass heavier than 1.5 X 10 -13 g. The detector consisted of four layers of 3 cm thick plastic scintillators separated by about 30 cm. Each layer was subdivided into five counters. The unit counters for the two upper layers were 20 cm wide and 150 cm long, and those for the two lower layers were 20 cm wide and 100 cm long. Between the counter planes, a 9 mm thick iron plate was inserted. In addition, a 19 mm thick iron plate was put under the lowest counter plane. Each unit counter was viewed by two photomultipliers at both ends (called right and left sides of the counter) via lucite light guides. The photomultipliers used were Hamamatsu R1332 ones, which had highgain and low-noise characteristics due to their GaAsP first dynodes. These characteristics enabled us to count one-photoelectron signals easily. In fact, dark
Volume 161B, number 4,5,6
PHYSICS LETTERS
pulses corresponding to the one-photoelectron signal were clearly observed. Fig. 1 shows a schematic o f the electronics arrangement. Each photomultiplier output was amplified by a X 8 fast amplifier. The gains of the photomultipliers were determined so that the amplifier output corresponding to the one-photoelectron signal was about - 5 0 mV. One of the amplified pulses was discriminated by an updating discriminator (LRS 623) with an output width o f 50 ns. The thresholds of the discriminators were set at - 3 0 mV. The discriminator outputs of the same-side photomultipliers in each layer
31 October 1985
were ORed and vetoed by fast cosmic-ray signals. Subsequently, a coincidence between the right-side OR and left-side OR was formed to produce a layerhit signal. Time-order circuits then recognized either downward (layer 1 -~ 2 ~ 3 ~ 4) or upward (layer 4 3 ~ 2 ~ 1) time-ordered sequences of layer hit signals occurring within a maximum sensitive time o f 60 Us although only the downward trigger is relevant to the detection o f cosmic nuclearites lighter than "0.1 g, which cannot penetrate the earth. The outputs of these time-order circuits were mixed to form a pretrigger.
(..~l~J
•~
~d
,
LAVER,
,--,. :E
LAYER2
5
IT
'AYER,
LN I EARSUM MEMORY MODULE
A N
LAYERI
~
~
~
LAYER 4 LAYER2 LAYER3 LAYER 4
I
" FASTCOSM C I RAYVETO
LAYER3
I
LAYER2
I i
[~
~ 4--3--2-1~ TR G IGER
Fig. 1. Schematic of the electronics. 419
Volume 161B, number 4,5,6
PHYSICS LETTERS
When a pretrigger oc6urred, the time differences between the layer-hit signals were examined, and only if the ratio of the minimum and maximum time differences between adjacent layers was within 1 : 3, the data were recorded. It should be noted that our timing requirement for triggering slow cosmic-ray candidates is not too stringent to reject nuclearites with velocities greater than 10 -4 c, although their velocities are degraded during the passage through the scintillator and the iron shield. A personal computer acquired the data and recorded them on a floppy disk, and monitored the status of data acquisition. The recorded data were timing and pulse-width information of each photomultiplier and summed pulse height of each layer. One of the discriminator outputs of each photo. multiplier was introduced into a CAMAC memory module. At the entrance of each channel of the memory module there was a retriggerable univibrator with a width of about 1 gs. The status of the univibrators was periodically recorded into a 256-bit deep memory with 5 MHz clock, with a write-address counter incremented by the same clock. Therefore, at any given time, the memory module kept 51.2/as history of each photomultiplier before that time. When a pretrigger occurred, the 5 MHz clock was stopped and the memory module retained the data until the decision was made whether the data were to be read out or aborted. It is noted that normal photomultiplier outputs due to low-energy ~,-rays and thermal noises were localized in time and recorded as about five consecutive " l " s in the memory module. However, the photomultiplier output due to a slowly-moving and weakly ionizing object is expected to be a discrete random train of single-photoelectron pulses which would be recorded as a longer train of " l " s in the memory module because of the response of an updating discriminator followed by a retriggerable univibrator, to such photomultiplier output. (Note that if the ionization-energy loss of a slowly moving object in the scintillator is high enough, no broadening of the recorded data occurs.) This fact was used in the analysis to reject background events in the slow-velocity and weak-ionization d o m a i n . To record pulse heights, X 8 amplifier outputs of photomultipliers in the same counter plane were summed up and digitized by a LRS 2249W charge420
31 October 1985
sensitive analog-to-digital converter (ADC). For each counter plane, an independent ADC module was used. A 2/as wide gate was produced by the OR of the layer-hit signals. If no pretrigger occurred within 60/as, a fast clear was issued. The apparatus was contained in a wooden hut located on relatively low-background-level at the site of the National Laboratory for High Energy Physics (KEK). The measurement was performed for five months and about 58 000 events were recorded. After correcting for computer dead time, etc., the net live-time of the apparatus was about 2500 h. During the measurement, the single counts of all the photomultipliers and the rates of the layer hits were perle. dically monitored. The response of the scintillation counters to the minimum-ionizing cosmic-ray muons normally incident on the counters was measured with the use of a charge-sensitive amplifier and a pulse-height analyzer It was found that lmi n corresponded to approximately 100 photoelectrons per photomultiplier, i.e., 200 photoelectrons per counter. The ADC's were then calibrated by using minimum-ionizing cosmic-ray muons again. One photoelectron signal corresponded to about 15 ADC channels, and the ADC's overflowed at "-49.6 Imin" The RMS resolution for low pulse heights was given by the measured pedestal widths to be about 30 channels, i.e., two photoelectrons. In order to select slow cosmic-ray candidates, we imposed the following three requirements: (i) geometrical correlation among the hit counters, (ii) timing correlation, and (iii) pulse-height correlation. For simplifying the analysis, we rejected those events having hits with the same timing in adjacent counters in the same counter plane. This requires that a hit counter had to be traversed by a slow cosmic-ray candidate through its entire thickness. Due to this requirement, the effective solid angle of the apparatus was reduced by about 20%. For requirement (ii), the time differences between the signals from two adjacent counter planes were required to be consistent with the penetration of a cosmic-ray object with a constant velocity less than 10 -3 c. (However, small velocity changes due to energy loss in the scintillator and iron absorber plates were allowed.) To examine requirement (iii), the recorded pulse heights were converted into the numbers of photoelectrons n i (i = 1 ..... 4), and the average number of photoelec-
PHYSICS LETTERS
Volume 161B, number 4,5,6 40 oo d ,° •o
30
I--
lO 0 . 1 5 Imi n i
o
1~0
,
i 20
,
30
h
Fig. 2. Scatter plot of the average number of photoelectrons per layer h versus the time difference T between the signals from layer 1 to layer 4 for geometrically correlated events. trons per layer X was calculated. If the ith ADC overflowed, the number o f photoelectrons corresponding to the overflow level was assigned to n i. Slow cosmicray candidates were required to satisfy
Ini-Xl~< [(3X~) 2 + 2 2 ]
1/2
( i = 1 ..... 4),
where the ADC resolution for low pulse heights was taken into account. Assuming the Poisson distribution for photoelectron statistics, the intrinsic detection efficiency o f our apparatus as a function o f X is given as [1 - e x p ( - X / 2 ) ] 6. In addition, a certain fraction o f candidate events would be discarded because o f the above requirement. The overall detection efficiency was calculated as a function o f X, and was taken into account in calculating the upper limits for the flux. With these requirements, 13 slow cosmic-ray candidates remained. Of these, 8 events were triggered by the downward time-ordered trigger and the rest o f 5 events by the upward time-ordered trigger. Fig. 2 shows a scatter plot o f X against the time difference T between the signals from layer 1 and layer 4 for these 13 events. At this level, no candidate events remained for an energy loss greater than 0.15 Imin, which means that no candidates for cosmic nuclear-
31 October 1985
ites heavier than 1.5 X 10 -13 g were observed. However, considering the rather crude estimates made for the properties o f nuclearites, it would beo worthwhile to examine these low ionization-loss events further. Although details are not mentioned here, these 13 events were completely eliminated b y inspecting the timing structure o f the scintillation-counter response recorded in the m e m o r y module [8]. Based on the null result, we derive the 90% CL upper limit for the flux o f slowly-moving cosmic-ray objects in the velocity range 10 --4 c - 1 0 - 3 c as f < 2.3/ S~2tEo, where SI2 is the detector acceptance, t is the total live time o f the detector, E 0 is the efficiency due to the requirements imposed in the analysis. For our measurement, SI2t = 0.9 × 1011 cm 2 s sr and E 0 is 0.79. Therefore, the present result establishes a 90% CL upper limit o f 3.2 X t 0 -11 cm - 2 s - 1 sr - 1 , or 6.3 X 10 7 km - 2 yr -1 (21r sr) -1 for cosmic nuclearites heavier than 1.5 × 10 -13 g. Although our result is not very far-reaching, especially if the measurement by Doke et al. [7] should be sensitive to cosmic nuclearites, the present experiment is the first one which can directly measure the velocity o f slowlymoving cosmic-ray objects lighter than the GUT magnetic monopole, down to o ~ 10 - 4 c. We wish to thank Professor Tadao Fujii, Professor Hirotaka Sugawara, Professor Kasuke Takahashi, Professor Kazuaki Katoh, and Professor Yoshio Yoshimura for their interest and support.
References [1 ] [2] [3] [4] [5 ]
E. Witten, Phys. Rev. D30 (1984) 272. E. Farhi and R.L. Jaffe, Phys. Rev. D30 (1984) 2379. A. De Rfijula and S.L. Glashow, Nature 312 (1984) 734. A. De Rdjnia, Nucl. Phys. A434 (1985) 605c. L.C.L. Yuan and C.-S. Wu, eds., Methods of Experimental Physics, Vol. 5A (Academic Press, New York, 1961) p. 127. [6] G. Tarl~ et al., Phys. Rev. Lett. 52 (1984) 90. [7] T. Dolce et al., Phys. Lett. 129B (1983) 370. [8] K. Nakamura et al., University of Tokyo preprint UT4-IE85•05 (1985).
421