Volume 183, number 3,4 PHYSICS LETTERS B 15 January. 1987
SEARCH FOR SUPERMASSIVE RELICS
S. NAKAMURA, K. KAWAGOE, K. YAMAMOTO, S. ORITO Department of Physics, University of Tokyo, Bunkyo-ku, llongo 7-3-1, Tokyo, Japan
H. ICHINOSE, T. DOKE, T. HAYASHI ', H. TAWARA Science and Engineering Research Laboratory, Waseda University, Kikuicho 17, Shinjuku-ku, Tokyo, Japan
K. OGURA College of lndustrial Technology, Nihon University, Narashino-shL lzumi-cho 1-2-1, Chiba, Japan
Received 13 August 1986
We have conducted an underground search for supermassive relic particles by an 80 m 2 array consisting of three layers of track detector CR-39. The non-observation of penetrating tracks places the velocity-dependent flux upper limits down to 2 10 ~4 cm -2 s - t s t - ~ for various electrically or magnetically charged particles under assumptions on their energy losses in the low- velocity region. Assuming that the nuclear recoils are effective to the track formation of CR-39, this experiment sets the best flux limits for such relic particles in the velocity region 4 10- ~ 10- 3 to escape the galaxy, composing an extra-galactic flux.
Various direct searches have been performed for supermassive magnetic monopoles. We refer to recent reviews  for the details of these searches. Here we
Volume 183, number 3,4 PHYSICS LETTERS B 15 January 1987
simply summarize the results, in fig. 3, by showing the most stringent flux limits obtained with different techniques. The induction technique is clearly the best method for the search for monopoles without any principle ambiguity. On the other hand, ionization detectors seem to be the only practical solution at the moment to covering a large area. Since the response of slow monopoles for various ionization detectors is not yet clearly understood, a search in a variety of ways using different techniques seems desirable.
There have been a few monopole searches with track-etch detectors. Doke et al. [ 9 ] exposed 100 m 2 of cellulose nitrate for 3.3 years to obtain a flux limit of 5 10 - ~ 5 cm - 2 s- t sr- ~ at/ /> 0.03. Kinoshita and Price , and subsequently Barwick et al.  made 10 m 2 yr and 16 m 2 yr exposures of CR-39 at mountain altitudes to obtain flux limits of about 10 -13 cm-2s - I sr - I for //>0.02 and fl>0.007, respectively. Price and Salomon [ 12 ] scanned ancient mica to search for tracks due to mono- pole-aluminum bound states, obtaining a flux limit orders of magnitudes below those of the real time experiments. The relevant//region, however, is lim- ited, and some of their assumptions are debatable ~2
As for charged relics, there exist few explict state- ments. Mashimo et al. [ 13-15 ] used scintillation telescopes to search for fractionally as well as inte- grally charged relics to obtain flux limits down to 10 -13 cm-2s - ' sr -~ in the//region 410-4 10 -3, although detailed knowledge of experi- mental conditions is necessary to deduce the flux limits.
We report in this paper on a search for penetrating tracks in a large area array of plastic track detector CR-39 [ 17] placed underground. The search should be sensitive to supermassive electrically or magneti- cally charged relics in certain velocity regions.
The experimental site is situated 700 m under- ground in the Kamioka mine of the Mitsui Mining and Smelting Co. located 300 km west of tokyo. The rock shield over the detector array amounts to mini-
~2 See ref.  for a discussion.
mally 2 105 g/cm 2. The detector has been kept in a tunnel at a constant temperature of (18 _+ 1 ) C. The detector array was composed of 1280 stacks of dimension 280 270 mm 2 each, and had a total fidu- cial area of 76 m 2. Each stack consisted of three lay- ers of 1,55 mm thick CR-39, and was laid down horizontally at the site. Each layer of a stack was brought separately into the mine, in order to elimi- nate coincident background tracks due to heavy cosmic-ray nuclei during the transportation. The three layers of CR-39 were then firmly fixed to each other by small pieces of double-adhesive tapes, and five small holes were drilled through the stack in order to provide the position reference. The stack was then packed in a polymer-coated aluminum bag and was sealed. All these operations were done underground near the site.
The CR-39 sheets were produced by Sola Optical Japan Ltd. using the monomer (allyl diglycol car- bonate) supplied by Pitzberg Plate Glass (Pacific) Industry in Japan. The monomer was cured with 3% of lPP (diisopropyl peroxy dicarbonate) for 21 hours. The uniformity of thickness was 30 ~tm RMS.
The principle : of finding tracks in the plastic track detector is as follows. The passing particle leaves a trail of localized damage (latent trail) mainly in the form of broken chemical bonds, which is more vul- nerable to a chemical attack than the other part of the plastic. By chemically etching the track, a visible etch-cone is developed along the latent trail on each surface of the sheet with a cone angle 0=sin- l (VB/VT), where lib is the bulk etching rate of the plastic itself and lit the etching rate along the track. The V-r or the sensitivity S=VT/VB is an increasing function of the amount of local energy deposited by the particle and depends also on the characteristics of the plastic. If the sensitivity of the track is high enough, a heavy etching can make the two etch-cones from opposite surfaces to connect and to form a penetrated etch-hole, which can be subject to the rapid scanning described below. The condi- tion necessary to produce the etch-hole depends on the S of the track and its zenith angle 0 and is given by ~4
cosO>S-~[D/ (D-d) ]=s inOc[D/ (D-d) ] , (1)
t3 For general references see ref. [ 18]. :4 For more detailed discussions see ref. [ 19].
Volume 183, number 3,4 PHYSICS LETTERS B 15 January 1987
where D and d are the thickness of the plastic before and after the etching, respectively. To eliminate acci- dental background holes, "coincident" holes on mul- tiple layers can be required for a track candidate.
After 286 days of exposure, the two consecutive layers of the stacks were brought out of the mine sep- arately and were etched. The etching was performed in 72 hours in a large tank filled with 6.8N NaOH at 90 C. An efficient heat-shielding of the tank and a uniform flow of the etching solution resulted in a temperature uniformity of _+ 0.05 C, which ensured a uniform etching for all the sheets. By the etching the original thickness of 1.55 mm was reduced to a final average thickness of 290 ~tm.
Etch-holes in the sheets were then searched for by an automatic rapid scanning system . To this end, the thin plastic sheets were first dyed black and then exposed to an intense light source. The light passing through the hole was detected by a CCD camera. The lens of the camera was set slightly out of focus, so that a limited resolution of 401 X 491 pixels per picture would not affect the detection efficiency of small holes. The video picture was digitized and stored in a memory. After simple zero-suppression, the data were read out by a personal computer, fol- lowed by an on-line pattern recognition to detect the holes and to determine their positions and bright- nesses. By this system, etch-holes with a diameter larger than 15 Ixm were detected with 100% effi- ciency. The position of the hole was determined by refering to the five reference holes. The overall posi- tion resolution was determined to be 0.7 mm RMS by calibrations.
The etching condition was such that most of the sheets contained less than three background holes. The "coincident" holes were searched for from two consecutive layers of the same stack in the following way. The distance between a pair of holes in the con- secutive layers was calculated by the computer using the data on hole positions. Fig. 1 shows the distribu- tion of the calculated distances. This distribution is compatible to be solely due to random coincidences, and shows no apparent signal at small distances as expected for true tracks. To be sure, we inspected by stereomicroscope all 14 pairs of holes with distances less than 15 mm. All pairs can easily be rejected as candidates for the track of unchanging ionization,
i 0 i - - - -~ . . . . . ,
d . . . . -n t 7 ~ i ,
i I tl - 'J -qlj _ I . L I . t . .
0 5 10 :5 20 2" 3,0 3'5 4'0 ~,'5 50 c;stcnce "rr-n)
Fig. 1. Observed distribution of distances between a pair of holes which are detected in two consecutive layers of a stack.
either because of a too different hole size or because of a hole shape incompatible with the inclination inferred from the hole position. Thus we have no signal.
This negative result has to be convened to the flux limits of various relic panicles as functions of the velocity. To this end, the sensitivity of CR-39 has to be evaluated for relic panicles at various velocities. The response of CR-39 to charged particles is well measured in the velocity region #>> 10 -2, and it is known that the sensitivity has a good relationship with the restricted energy loss (REL), which is the localized part of the energy loss depositedj very near to the track [ 21 ]. Therefore the calibrations by rela- tivistic ion beams are sufficient in this velocity region to obtain the relation between the sensitivity and REL. For the lower velocity region direct measure- ments are difficult. We assume that also in this region the sensitivity depends on REL and we use the same relation as obtained at fl ~ 1. This probably provides a conservative lower limit of the sensitivity since there are indications  that the sensitivity is higher in the low velocity region than would be expected on the