The cosmogenic long-lived radionuclides tell us the history of the past cosmic rays. The production rate of cosmogenic nuclides varies because the change in geomagnetic field intensity and the solar activity strongly influences galactic cosmic rays especially at lower part of energy that reach on the Earth's surface. The most promising cosmogenic nuclides having long half-life are (14)C (T(1/2) = 5730 yr), (10)Be (T(1/2) = 1.5 Myr), (26)Al (T(1/2) = 710 kyr) and (36)Cl (T(1/2) = 301 kyr). In the atmosphere, (14)C and (10)Be are produced from nitrogen and oxygen, and (26)Al and (36)Cl are produced from argon. Among these nuclides, only (14)C and (36)Cl can be produced by low energy neutrons with reactions of (14)N(n, p)(14)C, (35)Cl(n, Y)(36)Cl and 36Ar(n, p)(36)Cl, although (35)Cl and (36)Ar are minor component in the atmosphere. The dependencies of production rates on solar modulation and geomagnetic field intensity were estimated using pure physical model (Masarik and Beer, 1999).

The galactic cosmic rays consist of proton (87%), alpha-particles (12%) and heavier nuclei (1%). The proton differential energy spectrum at the Earth can be written in the form (Castagnoli and Lal, 1980):

(1)

where T (MeV) is the proton kinetic energy, Eo its rest energy of the proton in MeV, Φ is the solar modulation parameter (MeV), which varies from ~ 300 MeV during the minimal solar activity up to ~ 1000 MeV during the maximum, A = 9.9 × 108, m = 780 exp(-2.5 × 10(-4) E0), γ = 2.65.

The curves for different Φ values are shown in Figure 1. The solar activity influences low energy cosmic rays efficient more. However, these curves cannot be shown correctly at the energy below 100 MeV, because it is necessary to consider about the influence of solar cosmic rays and anomalous cosmic rays in this energy range. This part of the energy spectrum does not contribute significantly to the production of the cosmogenic nuclides under present geomagnetic field because of the geomagnetic cut-off in the upper atmosphere, and the threshold of the spallation reaction. On the other hand, during the lower geomagnetic field intensities, there is a possibility that energy in this part contributes to the production rates especially the low energy products of (14)C and (36)Cl because the cut-off effect becomes lower.

One of the most valuable records of cosmogenic nuclides is polar ice sheet. The ice cores taken from both polar regions provide us the long time history of the cosmogenic nuclides fluxes as well as the many climatic information. Previous investigations show that the evidence of the correlation of the 14C in tree ring and the (10)Be in the ice core (e.g. Finkel and Nishiizumi, 1997, Yiou et al, 1997, Beer et al, 2002, Muscheler et al, 2005, Horiuchi et al, 2008), and the 205-yr DeVries cycles of the sun are also confirmed in the (10)Be during MIS 3 (Beer et al, 2002).

In this thesis, we have tried to detect the peaks of three cosmogenic nuclides ((10)Be, (26)Al, (36)Cl) concentration during the geomagnetic field excursion in the Antarctic ice core. Especially, (26)Al and (36)Cl without reports are clarified. This study focuses on the estimation of changing the energy spectrum of cosmic ray during the Laschamp geomagnetic excursion using the difference of the change in three cosmogenic radionuclides concentration peaks and/or the change in the amplitude according to the solar modulation cycles reconstructed from the Dome Fuji ice core, Antarctica.

In this study, the first deep ice core drilled at the Dome Fuji station during 1995-1996 was used. A part of this ice core samples are preserved the Institute of Low Temperature Science, Hokkaido University. The samples from 700 - 850 m depth corresponding to 34 -45 kyr BP of DFGT-2006 age (Parrenin et al, 2007) were cut out from this ice core and the surface was thinly cut down with a ceramic knife. The resolution of samples are about 1 m for AMS which corresponding approximately about 75 years of this depth.

As the results, the distinct (10)Be peaks are found that can be attributed as the Laschamp excursion (Fig. 2). The two (10)Be profiles of the Dome Fuji and the Dome C (Raisbeck et al., 2007) are very similar each other. The linear relationship to the core depth was found from corresponding 10Be sub-peaks (Fig. 3). This relation can be allows any data from both cores directly compared during this period.

The (26)Al peak was also found at same depth of the (10)Be and the shape of the peaks very similar (Fig. 2). This is the first data of the continuous record of the cosmogenic (26)Al during the geomagnetic excursions.

The (26)Al/(10)Be ratios are nearly constant during this period and the almost same the present value (Fig. 4). It is suggested that the shape of the energy spectrum of the high energy part of the cosmic rays are also same in present day.

In the (36)Cl results, large discrepancy was found in this period (Fig. 2). Moreover, in the some of depths, concentrations of (36)Cl were higher than the (10)Be results. The (36)Cl/(10)Be ratios also scattered from 0.04 to 9.6 (Fig. 4). It is not observed in the results of shallow section of this study. It is not possible to explain by the production process of spallogenic (36)Cl from (40)Ar in the atmosphere. In addition, it is also hard to assume the change of the transportation process, because no comparable evidences of other climate records had been observed of this period.

Therefore it may suggest that the possibility of the low energy product such as the neutron capture reaction from the (36)Ar (and also (35)Cl possibly) was higher than the spallation product from (40)Ar.

The quantitative evaluation of the energy spectrum of cosmic rays is not yet performed because of the lack of the nuclear data of (36)Cl from Argon. More measurements both upper and deeper part of this core are needed to understand between the (36)Cl anomaly and geomagnetic excursion.

Fig. 1. Differential cosmic ray proton energy spectra. The Φ is a solar modulation parameter (Castagnoli and Lal, 1980). The larger Φ indicates the more activities of the sun.

Fig. 2. The depth profiles of each nuclide. This depth corresponding to 38 - 45 kyr BP from the DFGT-2006 age (Parrenin et al, 2007)

Fig. 3. Comparing with (10)Be fluxes from Dome Fuji and Dome C. The Dome C data were taken from Raisbeck et al., 2007.

Fig. 4. The ratios of (36)Cl/(10)Be and (26)Al/(10)Be compared with (10)Be. The blue line indicates the present production ratio calculated by Masarik and Beer, 1999. The red line also indicates the present value reported by Horiuchi et al., 2007.

これまで,過去の宇宙線変動のプロキシとして(14)Cや(10)Be等の宇宙線生成核種が様々な古気候アーカイブから復元されてきた.本研究では約4万年前の地磁気イベント(Laschamp excursion)を対象として,南極氷床コア中の複数の宇宙線生成核種((10)Be,(26)Al,(36)Cl)の分析を行った.

本論文は6章からなる。第1章はイントロダクションであり、宇宙線生成核種の生成過程と生成後の輸送過程に関する背景について述べられている。氷床コアは過去の宇宙線生成核種の生成率を保持していると考えられており、特に生成率の高い(10)Beは過去の宇宙線変動の研究に応用されてきた。しかし、より低エネルギーの宇宙線からも生成可能である(36)Clについての研究例は南極氷床コアではほとんど行われていない。地磁気イベントや太陽活動変動などに起因した宇宙線スペクトルの変化に対して(36)Cl生成がどのように変化したかを(10)Be等の他の宇宙線生成核種と比較することで評価することが本論文の主旨である。

第2章では分析に使用する南極ドームふじ氷床コアと宇宙線生成核種の分析に使用する東京大学MALTの加速器質量分析計について前処理の手順や核種ごとに異なる測定方法について述べられている。第3章では東京大学MALTの加速器質量分析計を用いた(36)Cl測定法の開発について述べられている。特に本章では測定の妨害となる(36)Sを抑制するために、気体充填型電磁石内のイオンビームの軌道ついてシミュレーション計算を行った。軌道計算の結果から気体充填型電磁石の磁場やビームスリットの位置、検出器入り口の開口径について従来までの条件と比べてより良い条件を発見した。その結果、従来と比べて測定時間は約1/10となり、妨害となる36Sの抑制率は約一桁向上した。第4章では現在から過去千年間の南極ドームふじ氷床コア中の(36)Clの分析について述べられている。地磁気強度が現在と同程度と考えられる過去1000年間の(36)Clの結果からは(10)Beの結果と同様に太陽活動極小期に対応した濃度増加が確認された。降雪速度が低い南極ドームふじにおいても(36)Clが宇宙線変動の記録を保持していることが明らかとなり、宇宙線変動のプロキシとしての(36)Clの信頼性が確認された。第5章では約4万年前の地磁気イベント(Laschamp excursion)を対象とした、南極ドームふじ氷床コア中の宇宙線生成核((10)Be,(26)Al,(36)Cl)の分析について述べられている。分析結果からは、地磁気減少に伴う(10)Beと(26)Alのピークが確認された。(10)Beの結果は同じく南極ドームCで得られた(10)Beとの類似性が確認された。一方、低エネルギー宇宙線で生成可能な(36)Clの結果は(10)Beや(36)Alの結果と大きく異なり、異常な濃度増加が見られた。(36)Clの結果からは(10)Beと同様に太陽活動に起因した205年周期が確認され、(36)Clの異常が同様に宇宙線起源であることが明らかとなった。これはLaschamp excursion時に、地球大気に到達する低エネルギー宇宙線が大幅に増加した事を示している。この期間の千年スケールの(36)Clの大幅な変動は磁極の移動に伴うカットオフ効果の変化、もしくは長周期の太陽活動変化に伴う太陽宇宙線の増加等の可能性があることを明らかにした。第6章では本論文で得られた成果の要約が述べられている。

本研究は加速器質量分析計(AMS)という最先端の機器を用いて,氷床コアに含まれる成分について,(36)Sの抑制率を減じて(36)Clの高精度分析法を開発するとともに,初めて(26)Alの分析を行うことで新しい分析方法を確立したと言える,さらに,南極ドームふじ氷床コア中のLaschamp excursion対象部について,1桁から2桁に及ぶ(36)Clの濃度以上を発見し(36)Cl濃度が低エネルギー領域の宇宙線変動を解析する上で非常に有用な指標であることを初めて示唆した.これらの研究成果は,将来の古環境解析に新しい手法と分野を切り開いたと言える.

なお,共同研究に関しては,横山祐典,永井尚生,松崎浩之,野口真弓,松林宏,本郷やよい,藤村匡胤氏との共同研究の成果である.しかし,論文提出者が主に分析,解析及び解釈を行なったもので,論文提出者の論文への貢献は本質的な部分で特に高く,寄与は十分であると審査委員全員が判断した.

以上の理由より,審査委員会は本論文を提出した阿瀬貴博氏に博士(理学)の学位を授与できると認めた.