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SAKASHITA UNDERGROUND COSMIC

RAY TELESCOPE

Cosmic-Ray Research Section,

Solar-Terrestrial Environment Laboratory,

Nagoya University

Nagoya 464-8601 JAPAN

Sakashita Tunnel

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I. INTRODUCTION

II. SAKASHITA UNDERGROUND OBSERVATORY

Sakashita Underground Multi-directional

Telescope was planned as one of projects of Japan

program for International Magnetospheric Study (IMS,

1975–1977) to study solar modulation of cosmic rays

in high energy region. For the Telescope Sakashita

Underground Cosmic Ray Observatory, Nagoya

University was constructed in 1976 in a disused railway

tunnel of Japan National Railways (now Central Japan

Railway Co.) at Sakashita-cho, Gifu prefecture, about

100 km north-east of Nagoya. The Underground

Telescope was built, extending detectors with three steps

in 1977 to 1979 to 4 x 15 m2 upper and 3 x 14 m2 lower

finally, and full observations of 10 directional cosmic

ray intensities started in Dec. 1979.

The observatory was constructed in the middle of

a tunnel at Sakashita (35°35’N and 137°32’E in

geographic coorditnates, 334 m above sea level), which

runs 14° westward from the north as shown in Fig. 1.

Topography above the observatory is fairly flat as can

be seen from the vertical cross sections of the soil in

Fig. 2. The vertical depth is estimated at 80 mwe from

the topography and also from a comparison between

cosmic ray fluxes inside and outside the tunnel (Ueno

et al., 1976; Fujii and Jacklyn, 1979).

The detector room is 3.6 x 33 m2 in floor area and

has a cross-section of horseshoe shape as shown in Fig.

3. Inside the room, there constructed two layers, upper

and lower, with iron-frame and plate to lay out detectors.

Thin plastic plate was built along with the coved ceiling

of tunnel to protect detector room from water drop out

of the brick wall. The humidity is kept 70% by seven

300 W dehumidifiers. The seasonal change of the room

temperature is about 1°C from the average 22°C and its

daily variation is about 0.2°C.

Fig.1 Map near Sakashita Observatory

Fig.2 Vertical cross section of the ground

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III. MULTI-DIRECTIONAL UNDERGROUND TELESCOPE

a. Detectors and their arrangements

The multi-directional telescope consists of two

layers of detectors as shown in Fig. 3. Each of detectors

is composed of plastic scintillators of 1 x 1 m2 in area

and 10 cm in thickness, which is set at the bottom of a

pyramid shaped box of 1.6 mm iron plate and is viewed

by a 5" photomultiplier (Hamamatsu Photonics R877).

These detectors are set respectively on the upper and

lower layers separated by 1.75 m in height without

absorber. Configuration of the detector layout was

designed in considering the maximum availability of

space in the tunnel. Centre of each detector on the lower

layer is placed just right under one of the corners of

each detector on the upper layer. Each detector is called

by the number with initial U or L for the upper or lower

layer, as U1, 02, …, U60 and L1. L2, … , L42.

b. Detectors and the adjustments

Pulses from each photomultiplier are amplified by

about 500 times, and then discriminated from noises at

a fixed voltage of 0.5 V by an amplifier/comparator

installed in each detector box. These signal pulses are

sent to a coincidence circuit board by a coaxial cable

(25 m), and reshaped to uniform pulses (5 volts and

200 nsec) to trigger the coincidence circuits. Fig. 4

shows one of plateau curves of detectors, i.e., the

dependence of counting rate of the detector on high

voltage supplied to the photomultiplier. A plateau is seen

at around 850 to 900 volts with a counting rate of about

350 counts per minute, indicating a clear separation of

cosmic ray signals from background noises. Based on

this character, high voltage of each photomultiplier is

adjusted so that the counting rate of every detector is

nearly equal to 400 counts per minute. These

adjustments are made by inserting a proper resister in

series between the photomultiplier and the high voltage

supply set at 950 volts.

Fig.3 Setting of Telescope in the tunnel. Fig.4 Plateau curve (variation of counting rate with

high volate)

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c. Directional telescopes and their counting rates

Directional telescopes are composed of selected

2-fold coincidences of signals from upper and lower

layer detectors. The selection of the coincidences is

illustrated in the top view of the telescope in Fig. 5,

where the upper and lower layer detectors are shown

by squares of thin solid and thin broken lines

respectively. Coincidences for the directional telescopes

are indicated by the relative position of the upper layer

detectors (thick solid lines) to one representative lower

detector or detectors. Central viewing directions and

average counting rates of these directional telescopes

are summarized in Table I. Two directional telescopes

V1 and V2 in the Table are a sum of (NE, SE, SW and

WN) and that of (N, E, S and W) respectively. Effective

depth and effective median primary rigidity were

calculated by Fujimoto et al. (1977) from the yield

function of cosmic rays (Murakami et al., 1979) in

considering the geometrical configuration of detectors

and particle’s path length in the rock with a mean density

of 2.5 g/cm3.

Table 1. Viewing directions, effective depths, median rigidities and counting rates for multi-

directional telescopes at Sakashita

Fig.5 Configuration of upper and lower layer detec-

tors showing two-fold coincidences for directional

telescopes.

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d. Recording system

The output pulses from the coincidence system are

fed into 24 channel recorder. Each channel of the

recorder consists of 12 bits binary counter with 12-bit

gate and 4 digits decimal counter with 4 digits buffer

memory. Each binary counter is designed as a sacling

factor of 1 to 4096 for decimal counter and is set so

that hourly count of the decimal counter becomes about

5000/hour. The accumulated counts in the decimal

counter are transferred to the buffer memory once every

hour and then the counter is cleared up for the next

hour. The contents of the buffer memories as well as

the time code are punched every hour on a paper tape.

The atmospheric pressure and the room temperature are

measured by a digital barograph and thermometers with

V-F converters, and are recorded also on paper tape by

the recorder.

e. Power supplies

In order to avoid interruption of observations due

to AC power line failure, an AC-DC-AC converter

system with a deck of floating batteries (96V, 35AH) is

used for the AC power supply. This system can supply

500 W power to the telescope for 3 hours without AC

power input. High voltage supply and DC power

supplies are highly stabilized against changes of AC

voltage and room temperature. The conventional

stabilized power supplies are used for the coincidence

system and the recording system, as they are not

sensitive to the change of the supply voltage,.

Fig.6 Plateau curves of various telescopes

f. Stability of the telescope

All the instrumental gain drift due to the variations

of the power supply voltage and room temperature are

reflected in the counting rate through slopes of the

plateau curve. Fig. 6 shows the plateau curves for five

representative coincidence components together with

those for total single components of upper and lower

layers. Slopes of the plateaus for coincidence

components are less than 0.2 % counts per 1 volt at

∼950 volts. The change of high voltage due to that of

AC power line voltage has negligible effect to the

counting rate. The main instrumental drift is due to the

variation of room temperature. The room temperature

causes gain changes of the photomultiplier and

amplifier-discriminator system of ∼-0.1 % Gain/°C.

Change of 0.1% gain is equivalent to change of -0.1 V

of high voltage, and the room temperature effect of the

Telescope is estimated to be -0.02 % counts/°C. As the

temperature variation is kept within 0.2°C on daily basis

and 2°C on yearly basis, the instrumental drift of the

coincidence rate is less than -0.004 % in a day and -

0.04 % in a year. The other instrumental drift is due to

the fatigue of the photomultiplier. The local rainfall has

also effect on counting rate by the change of effective

ground mass. As no correction for these effects was

made, the long term drift of about 1 %/year were

observed in the counting rate.

g. Automatic check systems

To check the malfunction of the Telescope and the

long term gain change due to fatigue of photomultiplier,

the automatic check system by a PC records daily means

of single and coincidence counting rates from each

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detector. This system measure also differential pulse

height distribution of each detector by a built-in 256

channel pulse height analyzer, automatically scanning

all detectors once every day. Some of the data are

checked once every day in Nagoya through a telephone

line to the observatory. This automatic check system

has been operated since the middle of 1980.

IV. REDUCTION OF DATA

The paper tape is collected once a month and the

hourly outputs on paper tape are processed by a

computer system in the Institute, to derive the relative

cosmic ray intensity variation. Hourly counts N’s in the

paper tape are converted into the barometer corrected

relative intensity (Wp) using the Natural Logarithmic

Representation (Wada, 1957) as follows,

WP = 104 x (In N - In N

0 + WL) - β(P-P

0)

in unit of 0.01 %

where N0 is average counting rate divided by the scaling

factor and WL is artificially added to make the values

of Wp around 50.00 (%). P is the atmospheric pressure,

P0 is 980 hPa and β is the barometric pressure effect

coefficient (-0.03 %/hPa for all components).

Hourly values of Wp, daily sum and average as well as

the 1st, 2nd and 3rd harmonics coefficients of the daily

variation of each component are computed and tabulated

on daily basis. In the same table, the residual dispersion

of the cosmic ray intensities for one day after removing

the daily variation (up to 3rd harmonics) is tabulated.

Further, these hourly values of Wp are used to produce

hourly value plot for every solar rotation period using

the X-Y plotter attached to the computer system. Check

of the processed data is made, utilizing the hourly data

table and inspecting visually the plotting diagrams. The

final data are written into a magnetic tape and used as

the input to the data processing for final tabulation and

plotting for the publication at NAGOYA University

Computation Center.

REFERENCES

Fuji, Z., and R. M. Jacklyn

Proc. 16th Coemic Ray Conf. Kyoto 4 336 (1979)

Fujimto, K., K. Murakami, I. Kondo, and K. Nagashitna

Proc. 15th Cosmic kay Conf. Plovdiv 4 321

(1977)

Murakami, K., K. Nagasbima, S. Sagisaka, Y. Mishima,

and A. Inoue

Nuovo Cimento 1-2C 635 (1979)

Sagisaka, S., K. Murakami. A. Inoue, Y. Mishima, and

K. Nagashima

Proc. 16th Int. Cosmic Ray Conf., Kyoto 4 235

(1979)

Ueno, H., K. Fujinroto, Z. Fujii, I. Kondo, and K.

Nagashima

Proc. Cosmic Ray Symp. of High Energy C.R.

Modulation 91, (1976)

Wada M.

J. Sci. Res. Inst. 51, 201 (1957)

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Appendix 1 EPILOGUE

Cosmic-Ray Research Laboratory, Nagoya Univer-

sity found in 1958 as a research institute with cosmic

ray telescopes, was reorganized with Research Insti-

tute of Atmospherics, Nagoya University to Solar-Ter-

restrial Environment Laboratory (STEL) in June, 1990.

Sakashita multi-directional underground telescope con-

tinued observations by cosmic ray research section of

STEL until closing of observations of about 23 years in

March, 2000.

Appendix 2 FOR USE OF DATA

Hourly value data of SAKASHITA Multi-directional

underground telescope are available from Web Site of

Solar-Terrestrial Environment Laboratory (http://

www.stelab.nagoya-u.ac.jp) via network . One record

of the hourly value files (92 bytes) consist of 24 data of

4 digits each, WP in 0.01% and Pressure in 0.1 hPa,

shown below.

Digit content

1 - 4 Last digit of year + Day of Year

5 – 8 Hour and minute (start time of record)

9 - 12 Upper Single + Lower Single

13-16 none (for monitoring Telescope)

17-20 V1

21-24 V2

25-28 NE

29-32 ES

33-36 SW

37-40 WN

41-44 N

44-48 E

49-52 S

53-56 W

57-60 NN

61-64 SS

65-68 none

69-72 none

73-76 NNN

77-80 SSS

81-84 none

85-88 Pressure

89-92 Z (=N-S)