CERN：物理学を革新する円形加速器の力
世界で最も強力な粒子加速器
CERNのLHCトンネルが位置する地域。ジュネーブおよびレマン湖周辺の位置関係を示す。
© 2001 CERN
全長27 kmにおよぶリング状の超伝導磁石群と複数の加速構造からなる巨大施設が、スイス・フランス国境付近、ジュネーブ近郊の地下100 mに設置されています。世界最大かつ最強の粒子加速器として知られる Large Hadron Collider (LHC) は、CERNの加速器群に加わった最新の装置です。
2010年に加速器が本格稼働を開始して以来、LHCの内部では、光速に限りなく近い速度で反対方向に飛行する2 本の高エネルギー粒子ビームが衝突させられ、そこで生じる大量の粒子シャワーが、リング周囲に配置された専用実験装置によって解析されています。こうした強力な実験により、物理学の分野では数々の重要な発見がもたらされました。その代表が2012年に確認されたヒッグス粒子 (Higgs boson) であり、この画期的成果は、私たちが暮らす世界や宇宙誕生の理解に新たな一歩をもたらしました。
浜松ホトニクスは、フォトニクス分野のイノベーションを牽引する企業として、本プロジェクトおよびその後に予定されているアップグレードにおいてCERNと協力しており、このアップグレードは 2029 年以降のさらなる発見に向けて LHC の性能向上を目指しています。High Luminosity Large Hadron Collider（HL-LHC）と呼ばれるこのアップグレードは、LHC が設計上達成可能な値を基準として、積分ルミノシティを5〜7倍に高めることを目的としています。そのため、実験に用いられるあらゆる技術は、現在以上に厳しい条件に耐え得る性能が求められ、既存技術の限界を押し広げる進化が必要不可欠となっています。
ますます高まる要求性能
CMS 実験（© 2008 CERN、写真提供：Maximilien Brice氏）
浜松ホトニクス製8インチ ピクセルアレイ検出器
HL-LHCと関連する実験のアップグレードは現在進行中であり、より高頻度の陽子—陽子衝突を実現し、ヒッグス粒子の性質をさらに精密に測定するとともに、依然として多くの謎に包まれているダークマターの探索を進めることを目的としています。
衝突頻度を高めれば取得できるデータ量は増加しますが、その一方で放射線量も上昇し、フォトダイオード (PD)アレイへの影響が大きくなります。PDアレイはCMS実験のカロリメータに用いられ、粒子のエネルギーを測定する重要な役割を担っています。
課題となるのは、PDアレイが放射線を浴び続けることで徐々に感度を失っていく点です。単純な対策としては、PDアレイにより高い電圧を印加し、放射線環境下でも高感度を維持する方法がありますが、この対策には限界があります。
さらにCERNは、システム全体のコスト削減とデッドスペース低減のため、より大面積のPDアレイを必要としていました。
最後の大きな課題は、これまで例のない大面積PDアレイを短期間で約2万7000個製造するという点でした。このように、浜松ホトニクスはPDアレイの高感度を維持する最適解を見出しつつ、前例のない製造要件を克服しなければなりませんでした。
世界最大級のフォトダイオード
HL-LHC (高輝度LHC)の土木工事の様子。
© 2018 CERN／Ordan Julien Marius氏
HL-LHC8 (HiLumi)プロジェクト：ポイント1における土木工事完了の様子。
© 2021 CERN／Hertzog Samuel Joseph氏
HL-LHC実験に求められる高度な要件に応えるため、浜松ホトニクスは高エネルギー物理の用途で使用されるフォトダイオード (PD)の中で、世界最大級かつ最高レベルの耐放射線性能を備えたフォトダイオードを設計・開発しました。電離エネルギーの堆積量を測定して粒子や放射線を検出する用途に理想的であり、HL-LHCプロジェクトが要求する高い耐放射線性と大面積を両立しています。
当初、浜松ホトニクスは特に耐放射線性の向上に注力しました。まず、6インチ径の単一ウェハから製造でき、放射線に対して非常に高い耐性を持ち、最大800 Vの高電圧で動作可能な大面積PDアレイの試作に成功しました。
さらに、より大面積のPDアレイを実現するため、浜松ホトニクスは8インチ径ウェハに対応する新しい製造装置を導入しました。これに伴い、ウェハ上に形成される薄膜の膜厚均一性や不純物濃度のばらつきを改善するために、製造プロセス条件全般を一から見直しました。
こうした一連の取り組みにより、浜松ホトニクスは従来品の約2倍の面積を持ちながら、同等の高耐放射線性と性能を維持したPDアレイの製造に成功しました。
世界で最も強力な粒子加速器の拡張に対応するため、これ以外にも多くの技術開発が進められており、現在もその取り組みは続いています。運用開始が近づくにつれ、実験の要求水準はさらに高まり、それに応じて支える技術も進化を続けています。これらすべての一歩一歩が、私たちが宇宙の真の姿へと近づくための大切な前進なのです。
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