PROCEEDINGS OF THE 31st ICRC, ŁODZ 2009 1

Cosmic Ray Tracks by the New type of Cloud Chambers

Y. Muraki, F. Kajino and T. Yamamoto ∗∗Department of Physics, Faculty of Science, Konan University, Kobe 658-8501, Japan

Abstract. We have fabricated two different typesof cloud chambers: a diffusion cloud chamber anda Wilson cloud chamber. These chambers were filledwith air and the tracks formed in them by cos-mic rays were investigated. Surprisingly, cosmic raytracks were observed in air containing few aerosolparticles. This finding is highly significant and maybe useful in future studies of the relationship betweensolar activity and global cloudiness.

Keywords: cosmic rays and climate, Maunder min-imum, cloud chambers

I. I NTRODUCTION

Whether cosmic rays affect the global climate is animportant unresolved question in geophysics. This ques-tion was first posed in a paper by Friis-Christensen andSvensmark published in 1997 [1]. In subsequent papers,Svensmark et al. found a correlation between globalcloudiness and the intensity of cosmic rays [2], [3], [4].According to their analysis, global cloudiness increaseswhen the intensity of galactic cosmic rays, i.e., cosmicrays originating from outside the solar system increases.A clear correlation has also been found between thecloudiness at low altitudes (under 3,200 m) and solaractivity. However, no correlation has been discoveredbetween solar activity and the cloudiness at middle andhigh altitudes. Due to the correlation at low altitudesbeing so strong, a vigorous debate has ensued in variousscientific fields.

Cosmic ray physicists have no difficulty understand-ing the correlation between the intensity of cosmic raysand cloudiness, since they use cloud chambers to detectcosmic rays, which produce tracks in the chambersby ionization. However, the intensity of cosmic raysappears to be too low to account for number of waterdroplets in global clouds. Ion pairs induced by cosmicrays probably play only a minor role in acceleratingthe formation of water droplets. Even if cosmic rayswere able to accelerate the formation of a small numberof water droplets by ionization, the clouds producedwould mask the light of the Sun, thereby reducing theglobal temperature. Thus, it might be possible to finda reasonable explanation for the correlation found byFriis-Christensen and Svensmark. In the next section,we discuss a scenario proposed by them.

II. T HE MAUNDER M INIMUM AND CLOUD

FORMATION

During 1645 and 1725, an abnormally low number ofsunspots were observed on the solar surface [5]. Thisphenomenon is known as the Maunder minimum. It

coincided with an approximately 2oC drop in the averageglobal temperature. Due to the weakness of the solarmagnetic field during this period, the intensity of galacticcosmic rays was stronger than at other periods [6], [7],[8]. Thus, the correlation found by Friis-Christensen andSvensmark would be possible to explain the drop inglobal temperatures that occurred during the Maunderminimum [9]. The aim of this present study is to findan explanation of the correlation observed by Friis-Christensen and Svensmark.

As a first step towards this goal, we approach the cor-relation from another point of view. A widely acceptedtheory by meteorologists is that when warm air ascendsto high altitudes, it is cooled and the moisture it containscondenses to form clouds. Both water vapor and aerosolparticles (APs) are necessary to form a cloud. This canbe easily demonstrated by blowing smoke from a matchinto a vessel filled with water vapor, which results in theimmediate formation of a cloud [10]. In summary, boththe water vapor and the APs are necessary for cloudformation [11]. This is the standard theory for cloudformation.

However, the cloud formation process is not sosimple: Aitken particles also play an important role.Aitken particles are typically about 20 nm - 80 nmin diameter, which corresponds to the length of a fewtens of water molecules. They are so small that theycannot be detected by optical counters. The mass ofAitken particles can be determined by measuring thetime that it takes for ionized Aitken particles to driftwhen they are placed in an electric field. Aitken particlesundergo successive collisions with water droplets in airand thereby increase in size until they reach diameters ofabout 1µm, which is the size of a large AP. Observationsat the Mt. Norikura Cosmic Ray Observatory (altitude:2,770 m) indicate that Aitken particles in the atmospheregrow from 10 nm to 30 nm in approximately 8 hours[12]. For simplicity, however, we do not consider therole of Aitken particles in this paper. Rather, we regardAPs as being the principal source of cloud condensationnuclei (CCN).

In addition, the role of the ions created by cosmicrays is not well understood. There may be other routesthrough which ions have function as CCN and formwater droplets. Do ions accelerate the formation processof CCN? Or do ions increase in size by attractingwater particles by themselves, independently of Aitkenparticles and/or APs? This is a very critical point for un-derstanding the Maunder minimum phenomena. In orderto discover the answer to this question, we conducted anexperiment using small cloud chambers. In this paper,

2 MURAKI et al. COSMIC RAYS AND CLOUD CHAMBERS

after describing our experimental method, we present theresults and introduce our future plans.

III. E XPERIMENTAL METHOD

Do ions produced by cosmic rays agglomerate bythemselves? Or do they attach to the surface of Aitkenparticles and/or APs? In order to clarify this point, wefabricated specially designed cloud chambers and usedthem to conduct experiments.

We prepared two types of cloud chambers: a diffusioncloud chamber and a Wilson cloud chamber. Whilethese cloud chambers are similar to conventional cloudchambers, they differed in that the gas inlet and outletwas located on the outside of the chambers. Figures1−3 show schematic views and photographs of thesediffusion and Wilson cloud chambers.

We used purified air and nitrogen manufactured bythe Sumitomo Seika Chemicals Co. which had verylow levels of APs in them (less than one AP per literaccording to the manufacturer). In general, there are highquantities of APs in the air. Table I shows the numberof APs per liter measured in the air in the laboratoryand outside the building. These APs were measured byusing the optical particle counter. In Table I, the numberof aerosols per liter is presented as a function of AP size.

We investigated the formation of tracks by cosmicrays in the purified gases. Ethanol was placed in thebottom of the glass flask and also at the top of the flask.First, we passed gas through both chambers until thetotal amount of gas passed through each chamber ex-ceeded five times the volume of the chamber. Accordingto our calculations, this process increased the purity ofthe gases by a factor of 160 (i.e.,6 × 10−3).

We cooled the diffusion cloud chamber by puttingsolid CO2 at the base of the glass flask and waitingapproximately 30 min until the vessel had cooled downand a saturated layer had formed, which was capableof producing cosmic ray tracks. For the Wilson cloudchamber we just lowered the piston and expanded thevolume; this formed a supercooled region. We haverecognized the cosmic ray tracks by the naked eye, beingilluminated the cloud chamber by the photo diode.

IV. PRELIMINARY RESULTS

Surprisingly, cosmic ray tracks were observed inthe semi-purified air in the same way that they wereobserved in the contaminated gas that contained a highconcentration of APs. We are currently unable to explainthis observation. It appears that ion pairs generated largedroplets without the assistance of APs. Is there a previ-ously unconsidered third route to make CCN? Do ionsform water droplets by themselves? Our experimentalresults suggest that this is possible.

However, there are several questions concerning thequality of our experiment. Even although we used pureair, the purification process was not perfect. Some APsmight remain not only on the chamber wall but also inthe gas itself. The baking of the chamber wall is not easy.

Even if the air contained a small amount of APs, ionpairs may form to the large water droplets. Furthermore,we did not measure the amount of Aitken particles inthe semi-pure air. The air might have been relatively freeof Aitken particles, but it is unlikely that there were noAitken particles present. It is unclear role ethanol vaporplays. Does it behave like Aitken particles in the flaskfor ions?

Therefore we have conducted another experiment us-ing the diffusion cloud chamber. The chamber was filledwith the pure air after the evacuation inside the chamberby the vacuum pump. Neither any track nor fog wasseen. However when we installed a ring fulfilled with aplenty of the Ethanol on the top side of the flask, cleartracks of cosmic rays and fogs were seen even in the“pure” air.

V. COMMENTS AND DISCUSSIONS

Finally we would like to remark on the formationprocess of the cloud. In the old bibliographies [13], theformation process of water droplet in cloud chamber hasbeen given. The ions formed by the passage of cosmicrays in the chamber will attach a water molecule andgrow up by the following process. Water droplet hasa trend to form a cubic shape to minimize the surfacetension (σ). However it is said that in case there is anelectric charge on the water vapor, Coulomb force of theelectric charge is able to weaken the surface tension. Bythe balance of force between the electric force and thesurface tension , the other water vapor can attach on thecharged water vapor and grow up to large size of waterdroplet (≥100nm). The process could be described bythe following equation: RT/Mln(P/P∞) = 2σ/r + dσ/dr- e2/8πr4ϵ.

The authors find several such examples in the plasmaphysics, however we are a little bit skeptical for the ap-plication of above explanation to the formation processof cloud. Because the electric charge is a unit at thebeginning and such statistical electric field could act tothe molecular orbit of electrons effectively? We know ofcourse the water molecule has a electric dipole moment,but from a gross feature of view , they are neutral incharge. The Aitken particle may have an important rolein the formation process of cosmic ray tracks.

Ions generate cosmic ray tracks in the cloud chamber.This is the discovery by Sir Wilson. He investigatedcosmic rays using Wilson cloud chamber. Initially, heobserved plenty of fogs in his cloud chamber. However,after expanding the cloud chamber repeatedly, he sawcosmic ray tracks.

In the initial stages, the water droplets were formedby attaching to APs inside the chamber. However, it isunclear whether the tracks of cosmic rays were formedwith the assistance of Aitken particles or APs that wereleft in the cloud chamber. If this had been true, SirWilson was lucky to see cosmic ray tracks [14]. To con-firm this point, we need to conduct another experimentusing a vessel that has been fulfilled by the purified air.

PROCEEDINGS OF THE 31st ICRC, ŁODZ 2009 3

TABLE I: Numebr of Aerosols in side Air. The number of APs per liter measured in the air inside the laboratoryand outside the building. The data are given as a function of the size of the APs.

Size of Aerosols > 0.3 µm > 0.5 µm > 0.7 µm > 1 µm > 2 µm > 5 µmOutside 309,447 35,578 7,907 2,822 884 87Inside 175,698 14,107 3,634 1,558 675 71

Fig. 1: Wilson type cloud chamber.

We plan to investigate the cloud formation process byusing a vacuum chamber and report the results at theconference in Lodz.

VI. A CKNOWLEDGEMENTS

The authors thank Prof. Takashi Shibata of the Depart-ment of Earth Environment Science, Nagoya Universityfor valuable discussions and lending us his instrument,optical particle counter to measure the size of APs.

REFERENCES

[1] H. Svensmark and Friis-Christensen, J. Atomos. Sol. Terr. Phys.,59 (1997) 1225.

[2] H. Svensmark, Phys. Rev. Lett., 81 (1998) 5027.[3] N.D. Marsh and H. Svensmark, Phys. Rev, Lett., 85 (2000) 5004.[4] H. Svensmark, Astronomy and Geophysics, 48 (2007) 1.18-1.24.[5] J.A. Eddy, Science, 192 (1976) 1189.[6] M.J. Stuiver, Science, 207 (1980) 11, M.J. Stuiver, Radiocarbon,

40 (1998) 1127.[7] J. Beer et al., Nature, 347 (1990) 164, J. Beer, Space Sci. Rev.,

11 (2000) 53.[8] H. Miyahara, K. Masuda, K. Nagaya, K. Kuwana, Y. Muraki and

T. Nakamura, Advances in Space Research, 40 (2007) 1060.[9] H. Kitagawa and E. Matsumoto, GRL, 22 (1995) 2155.

[10] Z. Sorbjan, Hands-on Meteorology Stories, Theories, and SimpleExperiments, published by American Meteorology Society inBoston, http://www.ametsoc.org/AMS.

[11] K.S. Carslaw, R. G. Harrison, and J. Kirkby, Science, 298 (2002)1732.

[12] C. Nishida, K. Osada, M. Kido, K. Matsunaga, and Y. Iwasaka,JGR, 113 (2008) D06202.]

[13] N.N. Gupta and S.K. Ghosk, Rev. of Mod. Phys., 18 (1946) 225.[14] J.G. Wilson, The Principle of Cloud Chamber Technique (1951).

Fig. 2: Diffusion type cloud chamber with inlet andoutlet of the air..

Fig. 3: Flow of clean air inside Wilson cloud chamber

Fig. 4: Fulfillment of clean air inside Wilson cloudcahmber