Performance Evaluation and Utility Assessment of Magneto-Optical Sensor Technology for Detecting and Visualizing Obliterated Serial Numbers in Firearms
RTI International
EXECUTIVE SUMMARY
In the United States, firearms manufactured after 1968 must contain a serial number somewhere on the metal frame. Each number is unique and typically stamped into the metal. During the stamping process, the crystalline structure of the metal is deformed and compacted below the identification number. When criminals destroy the serial number (using a variety of filing, grinding, and peening techniques) to avoid tracing of the firearm to its owner, the serial number may not be visible to the naked eye, but may still be recoverable. Recovery of the obliterated serial number greatly depends on the material used in the manufacturing of the firearm and on the depth of the obliteration.
Over the years, a number of techniques have been developed to detect and restore obliterated serial numbers in firearms. Current techniques fall into one of two categories: destructive and non- destructive. In destructive methods, the most commonly used technique is chemical etching. Restoration by this technique requires extensive sample preparation, the appropriate selection of etchants, and extreme care during etching of the specimen. In non-destructive methods, such as magnetic particle inspection (MPI), the original state of the specimen is preserved. Because of this preservation of material, non-destructive methods are generally preferred by examiners. However, non- destructive methods are extremely messy and suffer from low sensitivity.
The discovery of more effective methods to detect and restore erased markings in firearms, consequently, is of great interest to firearm examiners, in particular the invention of new non- destructive methods. Magneto optical (MO) sensor technology may address this need. The technology was initially developed for the medical engineering field and was recently adapted for use in forensics. The sensor’s operating principle is the Faraday-effect, which allows for the real-time visualization and analysis of the magnetic distribution of sample materials. The technology is manufactured by Matesy GmbH and is commercially available in the United States through Absolute Magnetic Measurements & Solutions, St. Petersburg, FL.
In this project, MO sensor technology was evaluated for the detection and visualization of obliterated serial numbers in five different metals and one composite material. All materials tested are currently used by firearm manufacturers. Technical performance factors included MO sensor sensitivity (detection limit), accuracy, reproducibility, recovery, and selectivity. Non-technical performance factors included sample preparation, system operation, sample processing, worker safety, and cost. The evaluation found that the MO sensor technology is a valid and reliable method for obliterated serial number identification in ferromagnetic materials. The technology requires little to no sample preparation, is easy to use, and allows for the identification of obliterated serial numbers in seconds, with time increasing to minutes at lower detection limits. When compared to the two gold standards in serial number restoration (chemical etching and MPI), MO sensor technology outperforms both in performance, as well as in utility, on the samples tested.
The sensor technology’s limitation is its configuration. For the MO technology to work effectively, the sensor must be in direct contact with the sample. Because of this, the MO sensor technology in its current configuration can only be used on planar surfaces. While this restriction greatly limits the detection and restoration of obliterated serial numbers in firearms, it may not limit the detection and deciphering of alterations in documents using magnetic ink, or the development and visualization of fingerprints using magnetic powder. It is recommended that application of the MO sensor technology in its current configuration be tested and evaluated in questioned document and latent print examinations.
It is also recommended that a different MO sensor system approach be evaluated for the restoration of obliterated serial numbers in firearms. In particular, placement of the MO sensor film directly onto the sample and evaluation of that approach on realistic obliterated firearm serial numbers is recommended. Given the MO sensor technology’s high sensitivity, fast response, and ease of use, the technology, if successful, could replace MPI as a standard in the non-destructive restoration of obliterated serial numbers.
The finding of chemical etching being potentially useful for the restoration of erased markings in Nylon 6 (polyester) was surprising and offers perspective for the future.