Defense Update - News Analysis by David Eshel

Wednesday, April 07, 2004

Sheet Explosives - Revisited

Sheet Explosives: The Hidden Terrorist Threat?

Updated on December 20, 2005, Security officials in the State of New Mexico, issued an alert over an apparent theft from a private storage depot, which included amounts of C-4 “sheet explosives”. Experts warned that the quantity was sufficient to demolish an entire building.

According to reports, the unknown thieves used blowtorches to cut the thick steel walls of the bunker. Authorities have no idea who was behind the theft, nor where this dangerous material was now hidden. Also called “deta sheets”, these explosives are especially dangerous as they can be hidden in books or letters but cannot be spotted by an ordinary metal detector.

This week’s incident was not the first time that such high powered explosive material was targeted. Two years ago, in September 2003, New York Police issued an alert warning over missing explosives powerful enough to blow up a commercial airliner. The report mentioned that a quantity of ‘Primasheet’ disappeared under “mysterious” circumstances from a depot at Newark International Airport.

The continuing threat of global terrorism may have entered into a new era, introducing a sofar little publicised substance called Sheet Explosives, which was allegedly used, the first time in Israel, by the two British suicide bombers in the Tel Aviv beachfront pub attack April 30,2003.

Original Post:

What Are Sheet Explosives?

Also named Plastic Bonded Explosives (PBX) this substance normally consists of a mixture of PETN ( Pentaerythritol Tetranitrate) and plastic polymer, such as polystyrene or polyester. These explosives have a high mechanical strength and excellent explosive properties, while extremely stable to shock. They rate among the most powerful and versatile explosives but are, surprisingly, also available from commercial sources, especially in eastern Europe.

The product consists of fine explosive particles embedded in a rubbery elastometric binder and is normally available in rolls with inner layers of waxpaper preventing adhesion of the rolled explosives. Distinctive standard colours indicate the explosive component: C4, or PE4 ( British) is white and Semtex-H orange.

The German Army used a similar product named Nipolit as an early sheet explosive, during WW2, but the first technological break-through happened in the early sixties, when the American firm DuPont perfected the Detasheet as the first reliable sheet explosive, which is widely used in various forms for boosters, steel cutting and explosive welding operations. The American product comes in two basic configurations: the commercial use "Detasheet A" consisting of 85% plasticised PETN with an oil based binder and the military version named "Detasheet B" or "Flex-X", M-118 ( measuring 1/4 inch thick, 3 inches wide and 12 inches long) which has less PETN (65%) and is typified by its olive green colour.
The PBX material is characterised by its smooth rubber -like texture and can be obtained commercially in flat sheet sizes ranging from 1.5mm to 6mm and above.

The sheet explosives pose one of the greatest threats to commerial aviation today, as it can be easily concealed in passenger hand luggage. Organic substances, of which PBX is formed, have low -Z signature and when formed into thin sheets are difficult to detect by conventional transmission methods.

Known Terrorist Uses of PBX

According to Israeli security sources, there have been previous incidents, in which
PBX substances have been used by terrorists, although not directly by Palestinian organisations.
The Israeli daily Maariv mentioned an incident in 1996, in which an unidentified person sent letters, from a post office in Alexandria, Egypt, addressed to leading newspapers in London and Washington, which fortunately were intercepted by security, containing what was suspected as sheet explosives. The sender's intention was to attract attention to a leading Egyptian al Aqeda member incarcerated in a Kansas prison after the 1993 Twin Tower attack in New York.

The two Britons involved in the Tel Aviv bombing attack last week smuggled their explosives in a Muslim holy Koran book. Israeli defence minister Shaul Mofaz announced, that " our investigation has determined that the two terrorists hid small amounts of high-powered explosives in a copy of the Koran to get it accross our checkposts". Security sources claimed that the explosives, devoid of metal outwitted the scanners on the Gaza crossings. Later reports indicated that the material was suspected to be a kind of sheet explosives and that the detonators, possibly, det cord, or Cordex were obtained locally and ferried in by a separate person to the rendez-vous in the Tel Aviv Hostel.

One of the most attractive means used by terrorists in the early seventies was the so-called "Letter Bomb". The first samples contained mechanical and electronic components activating powerful plastic explosives. The PLO "Black September" movement became notorious in mailing such deadly messages to Israeli diplomats between 1972-73 until police forensic experts found ways to intercept and safely defuse these.

The letters contained spring-loaded mechanical devices, activating a miniaturised non-electric detonator. Mounted on cardboard were flat-pressed explosives, but the entire concept was easily detected due to its bulky shape, which attracted suspicion.

A more effective device was "invented" during the eighties, when Musical greeting cards became commercial items. These contain miniature flat batteries, miniaturised circuit boards with electronic components activated by metal leaf springs and a paper or plastic pull tab. Terrorist sabotage experts soon modified these by adding a few components including a miniaturised detonator and PBX sheet explosives. This device exploded on activation of the Musical greeting card.

Two attempts to smuggle explosive devices on to El Al flights occured in 1986. The first ( mentioned in JTIC above) happened in London 17 April 1986, when Jordanian- Palestinian Nizar Hindawi allegedly inserted an explosive sheet device into a double liner of a suitcase, destined to Ben Gurion Airport. Israeli security agents acting on a tip off intercepted the suitcase and neutralised its contents before it was loaded into the cargo bay. Hindawi acted on orders of General Muhammad al-Khouli, the Syrian air force intelligence chief. The same officer was also responsible for another attempt, this time in Madrid airport, only three months later. On 26 June 1986 Nasser Khalil Ali, member of the Syrian-backed Abu Mussa terror organisation, tried to place a similar suitcase on the El Al check-in counter. This time, unfortunately, the device exploded prematurely, wounding 13 bystanders.

New PBX Detection Measures

Due to the nature of PBX conventional detection systems are not effective to prevent smuggling of this substance. X-ray scanners are normally sensitive to heavy-metal (high atomic number) elements and give only limited indication of low-atomic signatures. Vapour detectors are inneffective in meeting either of these missions. Recently, new detection methods have been developed to overcome PBX smuggling at airports and border-crossings.

The new system, named Inelastic-Gamma Ray & Spectroscopy ( IGRIS), is combining the various measurements, analyses them to achieve positive identification of threat material, including PBX.
Specially trained sniffer dogs are also used in this field, trained with special canine testing apparatus.

A leading Israeli forensic expert, responding to the Tel Aviv beachfront attack, claims that the effect of PBX as an explosive device for suicide bombers is limited.

Explaining that the material has considerable blasting power, but disperses widely, thus its effective damage radius is limited. The shock effect is somewhat reduced, compared to other plastic explosive charges used, which normally include metal scrap enhancing casualties, PBX has a low fragmentation effect. The main damage from PBX used in suicide attacks would be from severe burns, as were indeed reported resulting from this incident.

While somewhat ineffective to Suicide bombers, PBX sheet explosives are highly dangerous devices otherwise and pose a serious challenge to security and counter-terror agencies. One of its main charcteristics is the average detonation velocity which reaches 7,800 meters p/sec! Through this phenomen, a high air pressure environment is created on the explosion, which can be extremely lethal to nearby persons. Counter terror experts warn, that future use of PBX and various sheet explosives depend only on the imagination of terrorists.