如何利用核技术帮助犯罪调查？
解读“核”
核技术可以分析有关犯罪的法证样本——从贩毒、谋杀到艺术品赝品。最为常见的技术方法有：X射线技术、中子活化分析、离子束分析和放射性碳测年，这些技术可以补充法证调查中更为传统的技术方法。核技术通过分析材料的化学和物理特性，并辅以其他法医证据（如：DNA或指纹），可以将犯罪现场采集的证据样本与犯罪人关联。
核技术如何帮助调查？
核技术可以帮助专家分析犯罪证据样本的微观痕迹，例如，颜料或头发中的元素。这种技术可以提供其他方式无法发现的信息，如，伪造艺术品的真实年代或某个人是否中毒。此外，与其他方法相比，核技术产生的破坏性要小得多。例如使用化学物质分析，可能会改变证据的原始状态，比如一小块玻璃碎片。但若利用核技术分析，这些易损易碎的证据仍然可以保存下来，以备进一步分析使用。
法证中的X射线技术
X射线技术是法证中使用最广泛的分析工具之一。X射线灵敏且可靠，可以用于非常小的样本，并且大多数情况下并不会破坏证据。X射线技术往往在几分钟内就能产生结果，并且可以快速进行多种分析。此外，便携式X射线设备方便运输，也可以用于现场调查。
X射线辐射可以帮助检测几乎所有类型材料中存在的元素并测量其浓度。还能帮助分析土壤、药物或矿物质的成分并确定其来源。此外，它还有助于绘画颜料、金属或枪击残留物做比较检查以及医疗检查等。
科学家利用X射线荧光光谱仪或同步加速器等设备，通过X射线束轰击来测试材料样本。由于光束与样本中的原子相互作用，释放辐射，因此便可检测识别辐射中原子的来源元素。
X射线技术可以帮助揭露犯罪。图中所示20世纪70年代末美国案件中，检察官将受害者身上的咬痕与被告牙齿的X光照片进行对比，帮助案件定罪。（图片来源：佛罗里达州摄影集）
法证中的中子活化分析 (NAA)
中子活化分析是一种利用中子通量与所研究材料之间的相互作用来确定样本的“元素指纹”的技术，换句话说，就是提供有关其年代和起源信息的准确同位素组成。
中子活化分析过程中，样本受到中子轰击时，其原子会释放出粒子和（或）辐射，通过专业的检测器进行拾取和分析这些粒子和（或）辐射。这种分析可以帮助确定样本的成分、年代以及来源。通常情况下，中子活化分析使用非常小的样本来分析子弹的材料成分，追踪其来源。此外，这种技术可以分析出头发的元素成分，可能会有助于为案件提供新细节，如：体内是否存在药物或其他有毒物质。
中子活化分析还可用于确定优质食品的原产地、识别假冒产品，如：利用中子照射松露，检测其一小部分样本中化学元素原子核所发出的伽马射线。如果检测到这种射线，那么就可以将样本中的元素与特定地理位置进行匹配。此外，该技术还可用于确定其他优质食品的真实性。
近几十年来，调查人员一直使用中子活化分析，帮助确定犯罪证据样本（如，子弹）中的化学元素或特定元素的同位素的精确定量。（图片来源：A. Vargas/国际原子能机构）
法证中的离子束分析 (IBA)
离子束分析是另一种用于推动刑事调查进展的核技术。对于这类分析，利用离子束加速器加速离子或电子，产生带电粒子束。将法证样本放入离子束中，便可对样本所发出的辐射进行拾取和分析。调查人员通过离子可以了解样本的成分和来源，这有助于分析毒品、爆炸物和枪击残留物、伪造的历史文物或艺术品。通过确定样本的年代和成分，可将其作为更有效的调查证据。
例如，枪击残留物可以帮助调查涉及枪支的暴力犯罪。枪弹残留物指的是枪支弹药筒爆炸时释放出的蒸气和颗粒，离子束分析等核技术可以帮助检测枪击事件中产生的无机成分，从而检测出与枪击事件相关物品（如：嫌疑人手上或衣服上）上的微小颗粒，进而确认枪击者的身份。
同时，调查人员可以通过离子束分析进行元素测绘，帮助分析证据中特定部分并检测出犯罪物质中的微小痕迹，如衣服上特别细小部分中残留的微观粒子。
离子束还可用于分析肇事逃逸事故中的玻璃碎片，这有助于涉及车辆的犯罪调查。研究表明，将离子束分析与机器学习相结合，通过车窗玻璃碎片便可追溯各种车型和制造商，其准确率高达80%及以上。
斯洛文尼亚Jožef Stefan研究所利用无损离子束对艺术品进行分析。阅读本文可以获得更多有关使用核技术研究保护文化遗产和艺术品的信息。（图片来源：JSI/Ziga Smit）
法证中的放射性碳测年
放射性碳测年是一种利用碳同位素，即碳-14，来确定由有机材料制成的古代物品的真实年代及检测艺术品真伪的技术。这项技术可以确定长达五万年的物品年代，并在多项调查中帮助检验绘画、雕像及其他具有重要价值的历史物品的真实性。
例如：2019年，专家们利用放射性碳测年法识别出两幅著名画作为赝品，一幅是印象派画作，另一幅是点彩派画作。通过对画布纤维分析，证实两幅画作均为所谓的艺术家去世后所制，因此放射性碳测年法也有助于分辨名家画作真伪。
该加速器质谱仪安装于萨兰托大学测年和诊断中心 (CEDAD)，用于样本的放射性碳测年。（图片来源：萨伦托大学）
国际原子能机构的作用是什么？
- 原子能机构致力于开发和应用用于分析的新型核技术，开发并提供创新的核分析工具和相关框架，以满足不同国家的需求。
- 原子能机构利用其全球国家实验室网络，通过其技术合作计划和协调研究项目为法证专家和调查人员提供具体工具。
- 原子能机构提供在线学习课程，重点关注核技术在文化遗产和法证领域的应用。
- 原子能机构通过其协调研究项目（CRP）研究如何使用核分析技术来获取有关食品来源和真实性的信息，评定艺术品的真实性并分析犯罪现场的证据。
- 原子能机构还以培训、现场工作组访问、科学咨询和咨询的形式为各国提供法证能力建设的技术援助。
- 原子能机构正在努力搭建科学家、执法部门与法证界间联接，以便预防、打击世界各地利用核科学和技术的犯罪活动。
- 原子能机构制作出版材料，宣传各种核技术应用及其为调查、检测欺诈研究中带来的便利好处。
- 原子能机构正在与毒品和犯罪问题办公室、犯罪司法所、工发组织、粮农组织等其他联合国组织机构及其他国际伙伴合作，促进核分析技术在法证界应用。
How Do Nuclear Techniques Help in Investigations?
Nuclear techniques can help specialists analyse microscopic traces of incriminating evidence samples, such as elements in paint or a single hair. This can help to uncover information which could not be discovered otherwise, for example, the true age of forged art pieces or whether an individual was poisoned. Moreover, nuclear techniques are considerably less destructive compared to other methods, such as the use of chemicals for analysis, which can alter the original condition of fragile evidence, for example a tiny shard of glass. Therefore, after being analysed with nuclear techniques, fragile evidence can still be preserved for further analysis.
X-ray Techniques in Forensics
The use of X-ray radiation is one of the most widely used analytical tools in forensics. X-ray emission techniques are sensitive and reliable — they can be applied to very small samples, and in most cases do not destroy the evidence. X-ray techniques tend to produce results in minutes, and multiple analyses can be carried out quickly. In addition, portable X-ray equipment can be easily transported, which enables this technology to be used in on-site investigations.
X-ray techniques can help to detect the presence of, and measure the concentration of, elements in virtually all types of material. They can help analyse the material composition of soil, drugs or minerals and determine their source. X-rays also help in comparative examinations of paint, metals or gunshot residues, and in medical examinations.
Scientists use devices such as X-ray fluorescence spectrometers or synchrotrons to bombard a sample of the test material with X-ray beams. The beams interact with the atoms of the sample, releasing radiation, which can be detected and used to identify the element from which the atoms originated.
X-rays have helped to uncover crimes throughout history. (Photo: Florida State Photographic Collection).
Neutron Activation Analysis (NAA) in Forensics
Neutron activation analysis is a technique which uses the interaction between a flux of neutrons and the material being studied to determine the “elemental fingerprint” of a sample — in other words the exact isotopic composition that provides information about its age and origin.
During NAA, when samples are bombarded with neutrons, their atoms release particles and/or radiation, which can be picked up and analysed by a specialized detector. This can help to determine what the sample is it made of, how old it is and where it came from. NAA is usually used to analyse the material composition of bullets in order to trace their origin, using very small samples. In addition, analysis using this technique can reveal the elemental composition of hair, which may help to uncover new details about a case — for example, the presence of drugs or other toxic substances in the body.
NAA can also be used to establish the origin of premium foods and identify counterfeit products, for example truffles, by irradiating them with neutrons and detecting the gamma radiation emitted by the atomic nuclei of the chemical elements in a small sample of the truffle. When this radiation is detected, it can help to match the elements in the sample to a particular location. It can also be used to determine the authenticity of other premium food products.
NAA has been used by investigators for decades, helping to determine the precise quantity of a chemical element, or specific isotopes of an element, in a sample of criminal evidence — such as a bullet. This technique was used in forensic investigations of the assassination of US President John F. Kennedy. (Infographics: A. Vargas/IAEA).
Ion Beam Analysis (IBA) in Forensics
Ion beam analysis (IBA) is another nuclear method gaining ground in criminal investigations. For this kind of analysis, ion beam accelerators are used to speed up ions or electrons, producing a beam of charged particles. When samples of forensic evidence are placed into the ion beam, they emit radiation which can be picked up and analysed. Using ions enables investigators to understand the composition and origin of the sample, which can help to analyse drugs, explosive and gunshot residues, fake historical artifacts or art. Determining the age and composition of samples means they can be more effectively used as evidence in investigations.
Gunshot residues, for example, can help to investigate violent crimes involving firearms. A gunshot residue is a cloud of vapours and particles released from the explosion of a cartridge in a firearm. Nuclear techniques, such as ion beam analysis, help to detect the inorganic components originating from the shot to detect tiny particles on items related to a shooting (e.g. on the suspect’s hands or clothes) in order to confirm the identity of the shooter.
IBA enables investigators to conduct elemental mapping, which helps in the analysis of specific parts of the evidence and detects tiny traces of incriminating substances, such as microscopic particles of residues on particularly small parts of clothing.
Ion beams have also been used to analyse glass shards from hit and run accidents, which could prove useful in investigations of crimes involving vehicles. Studies show that through a combination of IBA and machine learning, glass fragments from car windows can be traced back to various car models and manufacturers with up to 80 per cent accuracy.
Non-destructive ion beam analysis of art objects has been used at the Jožef Stefan Institute in Slovenia. Read this article to learn more about the use of nuclear techniques for studying and preserving objects of cultural heritage and art (Photo: JSI/Ziga Smit)
Radiocarbon Dating in Forensics
Radiocarbon dating is a method which uses an isotope of carbon — carbon-14 — to determine the true age of ancient objects, made of organic materials, and detect forgeries of art pieces. This technique can determine the age of objects up to 50 000 years old and has already helped a number of investigations to determine the authenticity of paintings, statues and other valuable historical objects.
For example, in 2019 experts used radiocarbon dating to identify forgeries of two well-known paintings — one Impressionist and one Pointillist. Analysis of the canvas fibres revealed that they were manufactured after the death of the purported artists, which helped to determine that the paintings were fake.
This accelerator mass spectrometer, installed at the University of Salento's Centre for Dating and Diagnostics (CEDAD), is used for radiocarbon dating of samples. (Photo: University of Salento)
Other methods
Other common nuclear techniques can also be used in investigations, for example the use of stable isotope analysis for food fraud detection.
Stable isotope analysis is a technique used to determine the origin and authenticity of different foods, enabling specialists to detect and prosecute individuals involved in food adulteration (e.g. selling low-quality food under premium labels for high prices). Stable isotopes can be measured with specialized equipment, which can detect the tiniest differences in the ratios of a chemical element’s light and heavy forms. Such analysis can provide evidence about a particular type of food in order to make sure it is coming from the right region and uses authentic ingredients described on the label (e.g. detecting sugar syrups added to honey).
What is the Role of the IAEA?
- The IAEA contributes to the development and application of novel nuclear techniques for analytical purposes. The Agency develops and delivers innovative nuclear analytical tools and related frameworks to support the needs of different countries.
- Using its worldwide network of national nuclear laboratories, the IAEA provides concrete tools to forensic experts and investigators through its Technical Cooperation Programme and Coordinated Research Projects.
- The IAEA provides e-learning courses, focusing on the application of nuclear techniques in the field of Cultural Heritage and Forensic Sciences.
- Through its Coordinated Research Projects (CRP) the IAEA studies how nuclear analytical techniques can be used to obtain information on food origin and authenticity, to assess art authenticity and to analyse evidence in crime scenes.
- The IAEA also supports countries by providing technical assistance on forensics capacity building in the form of trainings, residential assignments, scientific advisories and consultations.
- The IAEA is working to bridge the gap between scientists, law enforcement and the forensic community to improve prevention and countering of criminal activities around the world using nuclear science and technology.
- The IAEA produces published materials to spread awareness about the use of different nuclear techniques and benefits they bring for investigations and detection of fraud.
- The IAEA is working with other UN organizations like UNODC, UNICRI, UNIDO, FAO, and other international partners to foster nuclear analytical techniques for forensic applications.
This article was first published on 21 July, 2023