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ヒッグス粒子を探る
ヒッグス粒子の存在の確定に貢献したSSDとAPD
宇宙誕生の謎に迫る。
スイス、ジュネーブの郊外。CERN（欧州原子核研究機構）の全長27kmに及ぶLHCプロジェクト。そのセンサ部分には、SSD（シリコン・ストリップ・ディテクタ）、APD、光電子増倍管が採用されています。私たち浜松ホトニクスの光のテクノロジーは、人類の英知が結集する高エネルギー物理学の最前線で活躍しています。
SSD
APD
Keyman Interview
ヒッグス粒子とは
ヒッグス粒子はあらゆる物質に質量を与える粒子で、神の粒子と言われていますが、これまで発見されていませんでした。CERN（欧州原子核研究機構）は、スイス・ジュネーブ郊外にある円周27kmの世界最大の粒子加速器「大型ハドロン衝突型加速器（LHC）」で実験を続ける中で、ヒッグス粒子の存在を確認することを目指していました。2013年、その存在が確定され、半世紀前にヒッグス粒子の存在を予言したフランソワ・アングレール名誉教授、ピーター・ヒッグス名誉教授が2013年のノーベル物理学賞を受賞されました。
円形の赤線の地下約100メートルに周長27kmのLHCプロジェクトのトンネルがある。黄色丸は４つの実験装置の設置場所。［画像提供：CERN］
当社検出器の役割
陽子と陽子を光に近い速度で衝突させると、そのエネルギーから新たな粒子が発生します。浜松ホトニクスの検出器は、その粒子の飛んだ方向（飛跡）とエネルギーを検出して、粒子の性質を調べるために使用されています。ATLAS/CMS実験装置において、粒子の飛跡検出器としてSSDが使用されています（1999年納入開始）。また、粒子のエネルギーを検出するカロリメータ用に微弱光検出器としてATLAS/CMSでは光電子増倍管が、CMSではAPDが使用されています。
ヒッグス粒子崩壊シミュレーション
［画像提供：CERN］
CERNから「The CMS Award」を2003年、2005年に受賞
飛跡検出用センサとしてATLAS/CMS両実験装置に採用されたSSD
ATLAS実験装置では約14,000個、CMS実験装置では約22,000個のSSDが使われています。SSDは、ストリップ形状のダイオードアレイで、粒子の透過した位置を数十μmの分解能で検出します。
CMS実験装置に取り付けられたシリコン･ストライプ･ディテクタ［画像提供：CERN］
シリコンウエハ1枚から作られるSSDは1個
高エネルギー物理実験では、衝突時に発生する粒子のエネルギーの増加に伴って、検出器の大型化が求められています。当社では、6インチシリコンウエハから1チップしか取ることのできない大型のSSDを開発しました。当社の高い検出器製造能力によって、LHCプロジェクトにおいて大面積・高品質のSSDをほぼ予定通り納入することができました。
SSDの形状は検出器の取付け場所に応じて、さまざまな形があります。
LHD加速器に使われたSSDの放射線耐性：1.5Mrad/year = 1.7Sv/h
CERNのLHCプロジェクトには1,232台の超伝導電磁石が設置されていて、その磁力により陽子ビームを曲げています。そのため、LHCプロジェクトでの放射線レベルは自然放射量の約600万倍に達します。SSDはこの過酷な放射線レベルの環境で、5年以上耐えられる高信頼性を実現しています。2020年以降に始まるHL（Hight Luminosity）-LHCプロジェクトなど、次期粒子衝突実験の飛跡検出器の放射線環境は、2008年開始のLHCプロジェクトの5～10倍になる予定で、こうした環境でも使用可能な次世代SSDを開発しています。
加速器の超伝導電磁石[ 画像提供：CERN ]
CMS実験でカロリメータとして使われた浜松ホトニクス製APDの数：130,000個
APDの受光面のサイズは、5mm×5mm。シンチレータと組み合わせて使用されています。高磁場中で放射線が発生する過酷な環境下で、最適な性能が求められるため、高磁場に強い光半導体素子が採用されました。APDの仕様には、「放射線に強く、低ノイズ」で、「高感度、低容量、特性の均一性」であることが求められました。それぞれの仕様が相反する関係にありましたが、構造を見直すことで、必要とされる仕様を実現しました。
CMS実験でカロリメータとして使われたAPDとシンチレータ[ 画像提供：CERN ]
カロリーメータ用APDの量子効率：70％
CMS実験装置では、放出された放射線はシンチレータで可視光に変換し、当社製APDが検出します。その微弱な光をとらえるためAPDは量子効率※70％という高感度を実現しています。
※入射光子1つ当たりの出力電子数のこと。
カロリメータ用APDの概念図
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