Abstract: The half-life of ²⁴⁰Am is of importance to determine the cross section of neutron reaction ²⁴¹Am(n, 2n) ²⁴⁰Am and the currently evaluated datum is 50.8(3) h, which is an experimental result by monitoring the intensity of 987.8 keV γ-ray with a Ge(Li) detector over a period of 6 d, less than 3 times of the half-life of ²⁴⁰Am. In this work, the detection efficiency of the high energy γ-rays for ²⁴⁰Am increases greatly by using a well-type HPGe detector and an absorber made of Pb to attenuate the X-rays and low energy γ-rays. According to the simulation result from Geant4, the detection efficiency of the 888.8 keV and 987.8 keV γ-rays for ²⁴⁰Am are 14.1% and 13.3% when the thickness of Pb absorber is 1 mm. Recently, about 500 μg ²⁴²Pu was irradiated by protons on HI-13 tandem accelerator in China Institute of Atomic Energy to produce ²⁴⁰Am by the reaction of ²⁴²Pu(p, 3n) ²⁴⁰Am. The solution source containing ²⁴⁰Am about 700 Bq was measured with the well-type HPGe detector. The intensities of 888.8 keV and 987.8 keV γ-rays for ²⁴⁰Am were measured continuously more than 18 d and the half-life of ²⁴⁰Am is calculated to be 50.79(5) h by least square method, which is consistent with the evaluated datum.