In what capacity are X-rays used in security settings?

In security settings, X-rays are primarily utilized for forgery detection, especially in the examination of documents and identification cards. This application takes advantage of the ability of X-rays to penetrate materials and reveal features that are not visible to the naked eye. For instance, X-rays can help identify hidden layers, watermarks, and security features embedded within documents which are designed to prevent counterfeiting. This capability is critical in environments such as airports and government buildings, where verifying the authenticity of identification and other important documents is essential for safety and security.

Other options do have valid uses for X-rays, but they are not applicable in the context of security settings. Medical diagnostics are a primary field for X-ray application, but they do not relate directly to security work. Ultrasound scanning is a completely different technology that uses sound waves, and while it is generally used in medical imaging, it is not relevant in security contexts. The use of X-rays to kill bacteria typically relates to sterilization or medical settings, not the security applications mentioned in the question.