介子 : 核廢料的深度探測 ∙ 上
Muons: Probing the depth of nuclear waste
它們，多年來用於觀察石頭，進行考古，但現在科學家們正嘗試著把它當作觀察放射性廢物容器的一扇窗。
Having used them for peering through rocks, scientists are now trying the explore the possibilities of it being the window for the containers of radioactive waste.
圖中右方綠點為介子
The green dot on the right represents muon
比質子和中子還要小的介子，是一種高能亞原子粒子，可以穿過層層緻密物質，考古學家們一直以來在用它進行各種研究，比如埃及石灰石和花崗巖的金字塔內部。
Muons, even smaller than protons and neutrons, are energetic subatomic particles that can pass through thick layer of dense material, which scientists have been using, for example, to look inside the limestone and granite pyramids in Egypt.
平時，介子是在粒子對撞機中產生的，物理學家用它們來識別或研究其他亞原子粒子。然而，它們也會在地球的大氣中自然存在，並且越來越多的科研人員在用這些輕子當作高穿透的探測器。跨越了古代，穿過了時空，介子們現在也是現代能源生產技術的一部分了。
Generally, muons are generated routinely in particle colliders, where physicists use them identify or study other subatomic particles. However, they also occur naturally in the atmosphere, and more and more researches are now using them these common leptons as highly penetrating probes*. Beyond the ancient world, now they also part of the modern energy production technology.
核能，雖然是一種不可再生的能源，但便宜、高效、而且不會產生溫室氣體。儘管目前它僅佔世界能源的10%，但據國際能源組織預測，它是增長最快的能源之一。那，未來越來越多的核電站將會產生成噸的核廢料，我們又該怎麼處理呢？
這些核廢料都是高放射性的，並且含有世界上絕大多數的鈽元素。鈽可是一種可以用來製造核武器的金屬！即使是極少量的鈽元素，在圖謀不軌的人手裡，那也是相當危險的。
因此，為了讓誰也拿不到這些放射性材料，它們都會在沒用後被封存在安全桶中，而介子射線照相(muon radiography)便可以在不打開或移動危險的容器的同時，用來檢查容器內廢物的狀態。
Nuclear power, a non-renewable but cheap, efficient and environmentally friendly energy resource. Though only 10% of the world’s power source now, it was forecasted as one of the fastest growing energy source by the IEO.
So, with all the nuclear power plants, there are tons of nuclear waste created, all being highly radioactive and contains most of the world’s supplie of plutonium - a metal that can be used to make nuclear bomb even with the tiniest amount.
Therefore, with all the those waste deliberately made inaccessible, muon radiography (muography) can be used to check the status of the wastes inside the containers with out opening or moving the hazardous casks*.
圖為核廢料
Nuclear waste in casks
介子雖然比電子重200倍，但只會釋放微小的軔致輻射，很容易穿過大氣層。每分鐘大約有一萬個顆介子到達地球表面的每平方米。而正是它們的高能量，使它們能夠輕鬆穿過巖石和其他密度大的物質。
不過，因為它們的快速闖關，它們還是會因為自己的電離或給電子能量而漸漸失去自己的能量，所以在穿過數百米的巖石後，他們的一生也就結束了。
Muons, being ~200 times heavier than electrons, emit little bremsstrahlung radiation and pass easily through the atmosphere. Around 10 thousand of them arrive one each square meter of Earth’s surface every minute*. It is their high energies allowing them to wiz through rocks and other dense matter.
However, they do lose the energy over time by ionising or giving it to electrons, so eventually disappear after traveling through several hundreds meters of rock.
有了這些特質，科學家們可以利用他們的通量變化來知道他們所經過的物質的密度，從而識別出這些物質。
With this property, scientists can use their variation of flux to know the density of the matter they were travelling though, hence identifying the material.
圖為在埃及用介子探測器研究金字塔的考古人員
Scientists using muon detectors in Egypt
這個方法在1950年左右首次用於測量冰的厚度，但在2003年，杜倫大學洛斯阿拉莫斯分校（Las Alamos)的Christopher Morris和他的同事們提出，使用散射性質相對於吸收性質可能是更好的選擇，特別是對於被包裹得嚴嚴實實的核材料。
This method was first put to work in mid-1950s to measure thickness of ice. But in 2003, Christopher Morris and his co-workers at Durham’s Los Alamos suggested that using the scattering of the muons instead of absorption might be a better choice, especially for concealed nuclear materials.
圖為由介子通量衰減分析模擬的3D地表下構造
Diagram of 3D reconstructed bedrock points under the surface, determined from muon flux attenuation analysis
方案是，原子核中電荷的密集會使介子發生偏轉，材料的原子序數(Z)越大，偏轉就越大。例如，與其他低Z能級物質相比，介子撞擊鈾時，會散射得更多。
因此，通過畫出介子在碰撞前後的軌跡，可以找到材料的結構。只要有足夠的時間和入射角度以及位置，就可以得到關於那個物質的詳細信息。
The idea was, muons are deflected by dense concentrations of charge in nuclei, the larger the material’s atomic number (Z), the more deflection. So, for example, muons will scatter more if they hit uranium, than compared to other lower Z materials.* Therefore, the structure of the material can be found by plotting the trajectories of the muons before and after the collision. With enough time and covering a wide range of incident angle and positions, an accurate image of the content can be seen.
漂移管便是用來實現這一想法的第一種設備。每根管子裡都有一種氣體和一根帶正電的導線。當一個介子通過這個漂移管時，由於它的電荷，它會把電子從氣體的原子中剝離，使其在帶正電荷的導線上留下一個信號。然後當這些漂移管經過複雜的排列組合後，通過介子們的軌跡就可以看到它們在碰撞前後的軌跡，並可以用那些數據來分析具體的物質。
Drift tubes were the equipment used to achieve this idea. Each tube contains gas and a positively charged wire running through its length. When a muon pass though the tube, due to its charge, will knock electrons off the gas atoms, which register a signal along the positively charged wire. By arranging a number of the tubes in a certain way, the before and after trajectories of muons can be seen and used to analyse the object.
圖為由CERN組裝的12 x 8 的 15毫米直徑漂移管
Assembled 12 x 8 package of 15~mm diameter drift tubes by CERN
這個技術第一次測試是由Las Alamos團隊通過對鎢柱的檢測進行的。隨著試驗的成功，小組也繼續開始研發針對檢測隱藏在貨物和貨櫃中的核材料的適當儀器。研發成功後，這款設備很快地被投入市場，如今已經銷售了很多年了。
This technology was first tested with the detection of tungsten cylinders by Los Alamos. With it being successful, the team went on developing proper devices for detecting nuclear materials hidden in cargos and shipping container. The device since then has been in the market for many years.
除了燃料之外，能夠檢測出核廢料同樣也很重要。因此，Morris和他的團隊開發了兩個24層的介子跟蹤器，每一個都是24個1.2米長的漂移管。2016年，他們用對24個幹儲存桶進行了測試，這些桶都是上世紀80年代從西屋公司(Westinghouse)的反應堆中取出來的。實驗目標是檢測每個桶中是否含有輻射物質。經過三個月後，六組中，有四組實驗成功。原因是，實驗中的強風讓兩個檢測儀並未完全對準，導致了這並不完美的結果。
Apart from the fuel, being able to detect nuclear waste is also equally important. So, Morris and his team developed two 24 layered muon trackers, each 24 1.2m-long drift tubes. In 2016, they were tested with a dry storage cask, which was taken out from the Westinghouse reactor in 1980s. The aim was identifying whether each of the 24 fuel-assembly slots were full or not.* After three months of testing, 4/6 of the groupings where identified correctly. It due to the wind, the detectors were out of alignment and caused the errors.
其實，Los Alamos研究小組的最終目標不僅是要知道存放桶是否空了，還要確定裡面放射性物質是否被有被調包，比如把鉛當作代替品。要做到這一點，它需要結合散射和吸收的兩個測量結果。由於偏轉依賴於材料的密度和原子序數，而吸收只依賴於密度，所以從偏轉信息中減去吸收的信息，就可以得到質量數，從而準確地知道隱藏的物體是什麼。
The ultimate aim for the Los Alamos team was not only to know if the cask is empty or not, but also to identify if the radioactive materials have been replaced by dummy objects*, for example lead. To achieve this, it would need to combine measurements for both scattering and absorption. Since deflection depends off both the material’s density and atomic number and absorptions only depend on density, by subtracting the absorption information from the defection information, the mass number can be obtained, hence knowing exactly what the concealed object is.
雖然從理論上講，這種方法是可行的，但還需要實驗來證明。由於之前對幹儲存桶的測試只有4/6成功了，它成為了另一個正在等待資金的項目。
Although, in theory, this method should work, it has to be proven experimentally. With the previous tests in the dry storage cask only 66% successful, it becomes another project waiting for its funds.
那接下來，介子探測器的命運會是怎麼樣的呢？
敬請期待 「介子:核廢料的深度探測 ∙ 下」
圖為正在進行測試的Legnaro國家實驗室的介子探測儀
A muon dector made by Legnaro National Laboratory being tested
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文中除*部分英文信息摘抄於 Edwin Cartlidge 的IOP Physics World 第32期報導，
其餘中英文內容為原創
圖片來自CBC, CERN, Advancing Earth and Space Science 官網
The * parts of English information was from Edwin Cartlidge's article in IOP Physics World issue 32 , the rest of Chinese and English were original.
Pictures from official website of CBC, CERN, Advancing Earth and Space Science.