Thallium-209

Properties and data of the isotope ²⁰⁹Tl.

Contents

General Data

Radioactive Decay

Parent Nuclides

Isotones und Isobares

Literature

Thallium-209 is a radioisotope of the chemical element thallium, which has 128 neutrons in its atomic nucleus in addition to the element-specific 81 protons; the sum of the number of these atomic nucleus building blocks results in a mass number of 209. The very short-lived, only artificially produced, unstable and thus radioactive nuclide has no practical significance; the study of ²⁰⁹Tl is exclusively for academic purposes.

In 1950, the first report of the observation of thallium-209 was published: An actinium-225 solution obtained from a uranium-233 source was chemically processed to isolate the short-lived radioactive decay products. Subsequent measurements of the decay curves in a series of seven observations yielded a half-life of approximately 2.2 minutes, based on which the new nuclide was unambiguously identified as Tl-209 [1].

A later developed, targeted method for producing thallium-209 is the irradiation of lead-210 with 20 MeV tritons (hydrogen-3 or tritium nuclei), in which, according to the nuclear reaction

²¹⁰Pb(t,α)²⁰⁹Tl

a proton is removed from a ²¹⁰Pb atom [2].

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

General data

Name of the isotope:Thallium-209; Tl-209Symbol:²⁰⁹Tl or 20981TlMass number A:209 (= number of nucleons)Atomic number Z:81 (= number of protons)Neutrons N:128Nucleon pairing (Z - N):odd - evenNuclear ratio (N/Z ratio):1.5802469135802 (= neutron-proton ratio)Neutron excess (N-Z):47Isotopic mass:208.985352(7) u (atomic weight of Thallium-209)Nuclide mass:208.940922 u (calculated nuclear mass without electrons)Mass excess:-13.64452 MeVMass defect:1.75758484 u (per nucleus)Nuclear binding energy:1637.17973295 MeV (per nucleus)
7.83339585 MeV (average binding energy per nucleon)Separation energy:S_N = 4.966(6) MeV (first neutron)
S_P = 7.668(31) MeV (first proton)Half-life:2.162(7) minDecay constant λ:0.0053434102725867 s^-1Specific activity α:1.547056217978 × 10⁺¹⁹ Bq g^-1
418123302.15632 Ci g^-1Spin and parity:
(nuclear angular momentum)1/2+Year of discovery:1950

Radioactive Decay

Half-life T_½ = 2.162(7) min (minutes) respectively 1.2972 × 10² seconds s.

Decay mode	Daughter	Probability	Decay energy	γ energy (intensity)
β^-	²⁰⁹Pb	100 %	3.970(6) MeV

Parent Nuclides

Direct parent isotope is: ²¹³Bi.

Isotones and Isobars

The following table shows the atomic nuclei that are isotonic (same neutron number N = 128) and isobaric (same nucleon number A = 209) with Thallium-209. Naturally occurring isotopes are marked in green; light green = naturally occurring radionuclides.

Z	Isotone N = 128	Isobar A = 209
77	²⁰⁵Ir
78	²⁰⁶Pt
79	²⁰⁷Au	²⁰⁹Au
80	²⁰⁸Hg	²⁰⁹Hg
81	²⁰⁹Tl	²⁰⁹Tl
82	²¹⁰Pb	²⁰⁹Pb
83	²¹¹Bi	²⁰⁹Bi
84	²¹²Po	²⁰⁹Po
85	²¹³At	²⁰⁹At
86	²¹⁴Rn	²⁰⁹Rn
87	²¹⁵Fr	²⁰⁹Fr
88	²¹⁶Ra	²⁰⁹Ra
89	²¹⁷Ac	²⁰⁹Ac
90	²¹⁸Th	²⁰⁹Th
91	²¹⁹Pa
92	²²⁰U
93	²²¹Np
95	²²³Am