Question: What did the United States Army use during Operation Peppermint to prevent the Germans from  disrupting the 1944 Normandy Landings?

Context paragraphs:
Operation Deadstick: Operation Deadstick was the codename for an operation by airborne forces of the British Army that took place on 6 June 1944 as part of the Normandy landings of the Second World War.  The objective was to capture intact two road bridges in Normandy across the River Orne and the Caen Canal, providing the only exit eastwards for British forces from their landing on Sword.  Intelligence reports said both bridges were heavily defended by the Germans and wired for demolition.  Once captured, the bridges had to be held against any counter-attack until the assault force was relieved by commandos and infantry advancing from the British landing zone.

Operation Cobra: Operation Cobra was the codename for an offensive launched by the First United States Army (Lieutenant General Omar Bradley) seven weeks after the D-Day landings, during the Normandy Campaign of World War II.  The intention was to take advantage of the distraction of the Germans by the British and Canadian attacks around Caen, in Operation Goodwood and break through the German defenses that were penning in his troops, while the Germans were unbalanced.  Once a corridor had been created, the First Army would then be able to advance into Brittany, rolling up the German flanks once free of the constraints of the bocage country.  After a slow start the offensive gathered momentum and German resistance collapsed as scattered remnants of broken units fought to escape to the Seine.  Lacking the resources to cope with the situation, the German response was ineffectual and the entire Normandy front soon collapsed.  Operation Cobra, together with concurrent offensives by the British Second Army and the Canadian First Army, was decisive in securing an Allied victory in the Normandy Campaign.

Operation Peppermint: Operation Peppermint was the codename given during World War II to preparations by the Manhattan Project and the European Theater of Operations United States Army (ETOUSA) to counter the danger that the Germans might disrupt the June 1944 Normandy landings with radioactive poisons.

British deception formations in World War II: During World War II the British Army made extensive use of fictional army formations, as part of strategic or tactical military deceptions.  Their use was pioneered by Dudley Clarke during the North African campaign.  Clarke eventually formulated an elaborate order of battle deception (Operation Cascade) to mislead the Axis high command as to the strength of Allied forces in the region.  Based on these successes the London Controlling Section made extensive use of notional formations during Operation Bodyguard - a deception operation ahead of the June 1944 Normandy Landings.

Operation Fortitude: Operation Fortitude was the code name for a World War II military deception employed by the Allied nations as part of an overall deception strategy (code named "Bodyguard") during the build-up to the 1944 Normandy landings.  Fortitude was divided into two sub-plans, North and South, with the aim of misleading the German high command as to the location of the imminent invasion.

Mission Chicago: Mission Chicago was a pre-dawn glider-borne combat assault in the American airborne landings in Normandy, made by elements of the 101st Airborne Division on the early morning of June 6, 1944 during the Normandy landings of World War II.  It was part of Operation Neptune, the assault portion of the Allied invasion of Normandy, codenamed Operation Overlord.  Originally slated to be the main assault for the 101st Airborne Division, the glider operation instead became the first reinforcement mission after the main parachute combat assault, Mission Albany.  Because the area of responsibility for the division was in close proximity to Utah Beach, the use of glider reinforcement was limited in scale, with most division support units transported by sea.

Normandy landings: The Normandy landings (codenamed Operation Neptune) were the landing operations on Tuesday, 6 June 1944 (termed D-Day) of the Allied invasion of Normandy in Operation Overlord during World War II.  The largest seaborne invasion in history, the operation began the liberation of German-occupied northwestern Europe from Nazi control, and contributed to the Allied victory on the Western Front.

Omaha Beach: Omaha, commonly known as Omaha Beach, was the code name for one of the five sectors of the Allied invasion of German-occupied France in the Normandy landings on 6 June 1944, during World War II.  'Omaha' refers to a section of the coast of Normandy, France, facing the English Channel 5 mi long, from east of Sainte-Honorine-des-Pertes to west of Vierville-sur-Mer on the right bank of the Douve River estuary and an estimated 150 foot tall cliffs.  Landings here were necessary to link the British landings to the east at Gold with the American landing to the west at Utah, thus providing a continuous lodgement on the Normandy coast of the Bay of the Seine.  Taking Omaha was to be the responsibility of United States Army troops, with sea transport, mine sweeping, and a naval bombardment force provided predominantly by the United States Navy and Coast Guard, with contributions from the British, Canadian, and Free French navies.

Operation Bodyguard: Operation "Bodyguard" was the code name for a World War II deception plan employed by the Allied states before the 1944 invasion of north-west Europe.  The plan was intended to mislead the German high command as to the time and place of the invasion.  The plan contained several operations, which culminated in the tactical surprise of the Germans during the Normandy landings on June 6, 1944 (also known as D-Day) and delayed German reinforcements to the region for some time afterwards.

Utah Beach: Utah, commonly known as Utah Beach, was the code name for one of the five sectors of the Allied invasion of German-occupied France in the Normandy landings on June 6, 1944 (D-Day), during World War II.  The westernmost of the five code-named landing beaches in Normandy, Utah is on the Cotentin Peninsula, west of the mouths of the Douve and Vire rivers.  Amphibious landings at Utah were undertaken by United States Army troops, with sea transport, mine sweeping, and a naval bombardment force provided by the United States Navy and Coast Guard as well as elements from the British, Canadian, Dutch and other Allied navies.

Supporting paragraphs: Operation Peppermint Normandy landings
Answer: radioactive poisons