Cast booster

An explosive booster is a sensitive explosive charge that acts as a bridge between a (relatively weak) conventional detonator and a low-sensitivity (but typically high-energy) explosive such as TNT. By itself, the initiating detonator would not deliver sufficient energy to set off the low-sensitivity charge. However, it detonates the primary charge (the booster), which then delivers an explosive shockwave that is sufficient to detonate the secondary, main, high-energy charge.

Unlike C4 plastic explosive, not all explosives can be detonated simply by inserting a detonator and firing it.

An initiator such as a shock tube, cannon fuse, or even a conventional detonator does not deliver sufficient shock to detonate charges comprising TNT, Composition B, ANFO and many other high explosives. Therefore, some form of "booster" is required to amplify the energy released by the detonator so that the main charge will detonate.

At first, picric acid was used as a booster to detonate TNT, though it was superseded due to the inherent danger of picrate formation. Tetryl replaced picric acid because it is more stable, and was once a very popular chemical for booster charges, particularly during World War II. However, since then Tetryl has largely been replaced by other compositions, e.g. a small cylinder or pellet of phlegmatized RDX (e.g. CH-6 or Composition A-5) or PETN (slightly larger than the actual detonator) into which the detonator itself is inserted.

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Wikipedia