Search for alpha and double alpha decays of natural Nd isotopes accompanied by gamma quanta.

From 7 naturally occurring Nd isotopes, 5 are unstable in relation to α decay. If an excited level of the daughter nucleus is populated, or the daughter nucleus is unstable, γ quanta can be emitted. We used an ultra-low background spectrometry system with 4 high purity germanium (HPGe) detectors (about 225 cm³ volume each) to search for such decays using a highly purified Nd-containing sample with mass of 2.381 kg. Measurements were performed at the INFN Gran Sasso underground laboratory (with an overburden of about 3600 m w.e.) during 51,237 h. Half-life limits for α decays of ¹⁴³Nd and ¹⁴⁵Nd were determined to be T_1/2(¹⁴³Nd) > 1.1 × 10²⁰ year and T_1/2(¹⁴⁵Nd) > 2.7 × 10¹⁹ year at 90% C.L. This is an increase of three and two orders of magnitude, respectively, compared with the most restrictive values currently given in literature. A limit for α decay of ¹⁴⁴Nd to the excited level of ¹⁴⁰Ce with E_exc = 1596.2 keV was determined for the first time as T_1/2(¹⁴⁴Nd → ¹⁴⁰Ce^*) > 9.3 × 10²⁰ year. Restriction for the α decay of ¹⁴⁶Nd to the excited level of ¹⁴²Ce with E_exc = 641.3 keV was increased by 3 orders of magnitude to T_1/2(¹⁴⁶Nd → ¹⁴²Ce^*) > 1.4 × 10²¹ year. For α and 2α decays of ¹⁴⁸Nd, first T_1/2 limits were set as 4.2 × 10¹⁸ year and 2.1 × 10²⁰ year, respectively. [ABSTRACT FROM AUTHOR]