Thallium-201m

Properties and data of the isotope ^201mTl.

Contents

Thallium-201m is an excited nuclear state of the nuclide ²⁰¹Tl, characterized by a defined excitation energy (919.47 keV). Its spin and parity quantum numbers differ from those of the ground state (gs). Simultaneously, this metastable nuclear isomer possesses an independent, experimentally measurable lifetime: decay occurs via an isomeric transition to Tl-201 Ground state with a half-life of approximately 211 milliseconds.

See also: List of individual Thallium isotopes (and general data sources).

General data

Name of the isotope:Thallium-201m; Tl-201mSymbol:^201mTl or 201m81TlMass number A:201 (= number of nucleons)Atomic number Z:81 (= number of protons)Neutrons N:120Nucleon pairing (Z - N):odd - evenNuclear ratio (N/Z ratio):1.4814814814815 (= neutron-proton ratio)Neutron excess (N-Z):39Excitation energy:919.47(11) keVIsotopic mass:200.970820(15) u (atomic weight of Thallium-201m)Nuclide mass:200.92639 u (calculated nuclear mass without electrons)Mass excess:-27.18099 MeVMass defect:1.702797512 u (per nucleus)Nuclear binding energy:1586.14566564 MeV (per nucleus)
7.89127197 MeV (average binding energy per nucleon)Half-life:2.11(11) msDecay constant λ:328.50577277723 s^-1Specific activity α:9.891540184834 × 10⁺²³ Bq g^-1
26733892391444 Ci g^-1Spin and parity:
(nuclear angular momentum)(9/2-)Year of discovery:1962

Radioactive Decay

Half-life T_½ = 2.11(11) ms respectively 2.11 × 10^-3 seconds s.

Decay mode	Daughter	Probability	Decay energy	γ energy (intensity)
Iso	²⁰¹Tl	100 %		0.33110(20) MeV [86.1(4) % 0.5883(1) MeV [94.63(7)]

Nuclear Isomers

Nuclear isomers or excited states with the activation energy in keV related to the ground state.

Nuclear Isomer	Excitation Energy	Half-life	Spin
²⁰¹Tl		3.0420(16) d	1/2+

Isotones and Isobars

The following table shows the atomic nuclei that are isotonic (same neutron number N = 120) and isobaric (same nucleon number A = 201) with Thallium-201m. Naturally occurring isotopes are marked in green; light green = naturally occurring radionuclides.

Z	Isotone N = 120	Isobar A = 201
73	¹⁹³Ta
74	¹⁹⁴W
75	¹⁹⁵Re
76	¹⁹⁶Os	²⁰¹Os
77	¹⁹⁷Ir	²⁰¹Ir
78	¹⁹⁸Pt	²⁰¹Pt
79	¹⁹⁹Au	²⁰¹Au
80	²⁰⁰Hg	²⁰¹Hg
81	²⁰¹Tl	²⁰¹Tl
82	²⁰²Pb	²⁰¹Pb
83	²⁰³Bi	²⁰¹Bi
84	²⁰⁴Po	²⁰¹Po
85	²⁰⁵At	²⁰¹At
86	²⁰⁶Rn	²⁰¹Rn
87	²⁰⁷Fr	²⁰¹Fr
88	²⁰⁸Ra	²⁰¹Ra
89	²⁰⁹Ac
90	²¹⁰Th