Isotopes of iodine

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Isotopes of iodine (₅₃I)

Main isotopes Decay abun­dance half-life (t1/2) mode pro­duct 123I synth 13.2232 h β+100% 123Te 124I synth 4.1760 d ε 124Te 125I synth 59.392 d ε 125Te 127I 100% stable 129I trace 1.614×107 y β− 129Xe 131I synth 8.0249 d β−100% 131Xe 135I synth 6.58 h β− 135Xe
Standard atomic weight Ar°(I)
	126.90447±0.00003[1] 126.90±0.01 (abridged)[2]
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There are 40 known isotopes of iodine (₅₃I) from ¹⁰⁸I to ¹⁴⁷I; all undergo radioactive decay except ¹²⁷I, which is stable. Iodine is thus a monoisotopic element.

Its longest-lived radioactive isotope, ¹²⁹I, has a half-life of 16.14 million years, which is far too short for it to exist as a primordial nuclide. Cosmogenic sources of ¹²⁹I produce very tiny quantities of it that are too small to affect atomic weight measurements; iodine is thus also a mononuclidic element—one that is found in nature only as a single nuclide. Most ¹²⁹I derived radioactivity on Earth is man-made, an unwanted long-lived byproduct of early nuclear tests and nuclear fission accidents.

All other iodine radioisotopes have half-lives less than 60 days, and four of these are used as tracers and therapeutic agents in medicine. These are ¹²³I, ¹²⁴I, ¹²⁵I, and ¹³¹I. All industrial production of radioactive iodine isotopes involves these four useful radionuclides.

The isotope ¹³⁵I has a half-life less than seven hours, which is too short to be used in biology. Unavoidable in situ production of this isotope is important in nuclear reactor control, as it decays to ¹³⁵Xe, the most powerful known neutron absorber, and the nuclide responsible for the so-called iodine pit phenomenon.

In addition to commercial production, ¹³¹I (half-life 8 days) is one of the common radioactive fission products of nuclear fission, and is thus produced inadvertently in very large amounts inside nuclear reactors. Due to its volatility, short half-life, and high abundance in fission products, ¹³¹I (along with the short-lived iodine isotope ¹³²I, which is produced from the decay of ¹³²Te with a half-life of 3 days) is responsible for the largest part of radioactive contamination during the first week after accidental environmental contamination from the radioactive waste from a nuclear power plant. Thus highly dosed iodine supplements (usually potassium iodide) are given to the populace after nuclear accidents or explosions (and in some cases prior to any such incident as a civil defense mechanism) to reduce the uptake of radioactive iodine compounds by the thyroid before the highly radioactive isotopes have had time to decay.

Nuclide [n 1]	Z	N	Isotopic mass (Da)[4] [n 2][n 3]	Half-life[5] [n 4]	Decay mode[5] [n 5]	Daughter isotope [n 6][n 7]	Spin and parity[5] [n 8][n 4]	Isotopic abundance
	Excitation energy[n 4]
108I	53	55	107.94335(11)#	26.4(8) ms	α (99.50%)	104Sb	1+#
					p (0.50%)	107Te
					β+?	108Te
					β+, p?	107Sb
109I	53	56	108.9380860(72)	92.8(8) μs	p (99.986%)	108Te	(1/2+,3/2+)
					α (0.014%)	105Sb
110I	53	57	109.935085(66)	664(24) ms	β+ (71%)	110Te	(1+)
					α (17%)	106Sb
					β+, p (11%)	109Sb
					β+, α (1.1%)	106Sn
111I	53	58	110.9302692(51)	2.5(2) s	β+ (99.91%)	111Te	5/2+#
					α (0.088%)	107Sb
					β+, p?	111Te
112I	53	59	111.928005(11)	3.34(8) s	β+ (99.01%)	112Te	1+#
					β+, p (0.88%)	111Sb
					β+, α (0.104%)	108Sn
					α (0.0012%)	108Sb
113I	53	60	112.9236501(86)	6.6(2) s	β+	113Te	5/2+#
					α (3.310×10−5#%)	109Sb
					β+, α?	109Sn
114I	53	61	113.922019(22)	2.01(15) s	β+	114Te	1+
					β+, p?	113Sb
					α (7.7×10−9#%)	110Sb
114mI	265.9(5) keV			6.2(5) s	β+?	114Te	(7−)
					IT?	114I
115I	53	62	114.918048(31)	1.3(2) min	β+	115Te	5/2+#
116I	53	63	115.916886(81)	2.91(15) s	β+	116Te	1+
116mI	430.4(5) keV			3.27(16) μs	IT	116I	(7−)
117I	53	64	116.913646(27)	2.22(4) min	β+ (77%)	117Te	(5/2)+
					EC (23%)	117Te
118I	53	65	117.913074(21)	13.7(5) min	β+	118Te	(2−)
118mI	188.8(7) keV			8.5(5) min	β+	118Te	(7−)
					IT?	118I
119I	53	66	118.910061(23)	19.1(4) min	β+ (51%)	119Te	5/2+
					EC (49%)	119Te
120I	53	67	119.910094(16)	81.67(18) min	β+	120Te	2−
120m1I	72.61(9) keV			242(5) ns	IT	120I	3+
120m2I	320(150) keV			53(4) min	β+	120Te	(7−)
121I	53	68	120.9074115(51)	2.12(1) h	β+	121Te	5/2+
121mI	2376.9(4) keV			9.0(14) μs	IT	121I	21/2+#
122I	53	69	121.9075901(56)	3.63(6) min	β+ (78%)	122Te	1+
					EC (22%)	122Te
122m1I	314.9(4) keV			193.3(9) ns	IT	122I	7−
122m2I	379.4(5) keV			79.1(12) μs	IT	122I	7−
122m3I	394.1(5) keV			78.2(4) μs	IT	122I	(8+)
122m4I	444.1(5) keV			146.5(12) ns	IT	122I	8−
123I[n 9]	53	70	122.9055898(40)	13.2232(15) h	EC	123Te	5/2+
124I[n 9]	53	71	123.9062103(25)	4.1760(3) d	β+	124Te	2−
125I[n 9]	53	72	124.9046306(15)	59.392(8) d	EC	125Te	5/2+
126I	53	73	125.9056242(41)	12.93(5) d	β+ (52.7%)	126Te	2−
					β− (47.3%)	126Xe
126mI	111.00(23) keV			128 ns	IT	126I	3+
127I[n 10]	53	74	126.9044726(39)	Stable			5/2+	1.0000
128I	53	75	127.9058094(39)	24.99(2) min	β− (93.1%)	128Xe	1+
					β+ (6.9%)	128Te
128m1I	137.851(3) keV			845(20) ns	IT	128I	4−
128m2I	167.368(4) keV			175(15) ns	IT	128I	(6)−
129I[n 10][n 11]	53	76	128.9049836(34)	1.614(12)×107 y	β−	129Xe	7/2+	Trace[n 12]
130I	53	77	129.9066702(34)	12.36(1) h	β−	130Xe	5+
130m1I	39.9525(13) keV			8.84(6) min	IT (84%)	130I	2+
					β− (16%)	130Xe
130m2I	69.5865(7) keV			133(7) ns	IT	130I	6−
130m3I	82.3960(19) keV			315(15) ns	IT	130I	(8−)
130m4I	85.1099(10) keV			254(4) ns	IT	130I	6−
131I[n 10][n 9]	53	78	130.90612638(65)	8.0249(6) d	β−	131Xe	7/2+
131mI	1918.4(4) keV			24(1) μs	IT	131I	19/2−
132I	53	79	131.9079935(44)	2.295(13) h	β−	132Xe	4+
132mI	110(11) keV			1.387(15) h	IT (86%)	132I	(8−)
					β− (14%)	132Xe
133I	53	80	132.9078284(63)	20.83(8) h	β−	133Xe	7/2+
133m1I	1634.148(10) keV			9(2) s	IT	133I	(19/2−)
133m2I	1729.137(10) keV			~170 ns	IT	133I	(15/2−)
133m3I	2493.7(4) keV			469(15) ns	IT	133I	(23/2+)
134I	53	81	133.9097757(52)	52.5(2) min	β−	134Xe	(4)+
134mI	316.49(22) keV			3.52(4) min	IT (97.7%)	134I	(8)−
					β− (2.3%)	134Xe
135I[n 13]	53	82	134.9100594(22)	6.58(3) h	β−	135Xe	7/2+
136I	53	83	135.914605(15)	83.4(4) s	β−	136Xe	(1−)
136mI	206(15) keV			46.6(11) s	β−	136Xe	(6−)
137I	53	84	136.9180282(90)	24.13(12) s	β− (92.49%)	137Xe	7/2+#
					β−, n (7.51%)	136Xe
138I	53	85	137.9227264(64)	6.26(3) s	β− (94.67%)	138Xe	(1−)
					β−, n (5.33%)	137Xe
138mI	67.9(3) keV			1.26(16) μs	IT	138I	(3−)
139I	53	86	138.9264934(43)	2.280(11) s	β− (90.26%)	139Xe	7/2+#
					β−, n (9.74%)	138Xe
140I	53	87	139.931716(13)	588(10) ms	β− (92.40%)	140Xe	(2−)
					β−, n (7.60%)	139Xe
					β−, 2n?	138Xe
141I	53	88	140.935666(17)	420(7) ms	β− (78.8%)	141Xe	7/2+#
					β−, n (21.2%)	140Xe
142I	53	89	141.9411666(53)	235(11) ms	β−	142Xe	2−#
					β−, n?	141Xe
					β−, 2n?	140Xe
143I	53	90	142.94548(22)#	182(8) ms	β−	143Xe	7/2+#
					β−, n?	141Xe
					β−, 2n?	140Xe
144I	53	91	143.95134(43)#	94(8) ms	β−	144Xe	1−#
					β−, n?	143Xe
					β−, 2n?	142Xe
145I	53	92	144.95585(54)#	89.7(93) ms	β−	145Xe	7/2+#
					β−, n?	144Xe
					β−, 2n?	143Xe
146I	53	93	145.96185(32)#	94(26) ms	β−	146Xe
					β−, n?	145Xe
					β−, 2n?	144Xe
147I	53	94	146.96651(32)#	60# ms [>550 ns]	β−?	147Xe	3/2-#
					β−, n?	146Xe
					β−, 2n?	145Xe
This table header & footer: view

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Radioisotopes of iodine are called radioactive iodine or radioiodine. Dozens exist, but about a half dozen are the most notable in applied sciences such as the life sciences and nuclear power, as detailed below. Mentions of radioiodine in health care contexts refer more often to iodine-131 than to other isotopes.

Of the many isotopes of iodine, only two are typically used in a medical setting: iodine-123 and iodine-131. Since ¹³¹I has both a beta and gamma decay mode, it can be used for radiotherapy or for imaging. ¹²³I, which has no beta activity, is more suited for routine nuclear medicine imaging of the thyroid and other medical processes and less damaging internally to the patient. There are some situations in which iodine-124 and iodine-125 are also used in medicine.^[6]

Due to preferential uptake of iodine by the thyroid, radioiodine is extensively used in imaging of and, in the case of ¹³¹I, destroying dysfunctional thyroid tissues. Other types of tissue selectively take up certain iodine-131-containing tissue-targeting and killing radiopharmaceutical agents (such as MIBG). Iodine-125 is the only other iodine radioisotope used in radiation therapy, but only as an implanted capsule in brachytherapy, where the isotope never has a chance to be released for chemical interaction with the body's tissues.

The gamma-emitting isotopes iodine-123 (half-life 13 hours), and (less commonly) the longer-lived and less energetic iodine-125 (half-life 59 days) are used as nuclear imaging tracers to evaluate the anatomic and physiologic function of the thyroid. Abnormal results may be caused by disorders such as Graves' disease or Hashimoto's thyroiditis. Both isotopes decay by electron capture (EC) to the corresponding tellurium nuclides, but in neither case are these the metastable nuclides ^123mTe and ^125mTe (which are of higher energy, and are not produced from radioiodine). Instead, the excited tellurium nuclides decay immediately (half-life too short to detect). Following EC, the excited ¹²³Te from ¹²³I emits a high-speed 127 keV internal conversion electron (not a beta ray) about 13% of the time, but this does little cellular damage due to the nuclide's short half-life and the relatively small fraction of such events. In the remainder of cases, a 159 keV gamma ray is emitted, which is well-suited for gamma imaging.

Excited ¹²⁵Te resulting from electron capture of ¹²⁵I also emits a much lower-energy internal conversion electron (35.5 keV), which does relatively little damage due to its low energy, even though its emission is more common. The relatively low-energy gamma from ¹²⁵I/¹²⁵Te decay is poorly suited for imaging, but can still be seen, and this longer-lived isotope is necessary in tests that require several days of imaging, for example, fibrinogen scan imaging to detect blood clots.

Both ¹²³I and ¹²⁵I emit copious low energy Auger electrons after their decay, but these do not cause serious damage (double-stranded DNA breaks) in cells, unless the nuclide is incorporated into a medication that accumulates in the nucleus, or into DNA (this is never the case is clinical medicine, but it has been seen in experimental animal models).^[7]

Iodine-125 is also commonly used by radiation oncologists in low dose rate brachytherapy in the treatment of cancer at sites other than the thyroid, especially in prostate cancer. When ¹²⁵I is used therapeutically, it is encapsulated in titanium seeds and implanted in the area of the tumor, where it remains. The low energy of the gamma spectrum in this case limits radiation damage to tissues far from the implanted capsule. Iodine-125, due to its suitable longer half-life and less penetrating gamma spectrum, is also often preferred for laboratory tests that rely on iodine as a tracer that is counted by a gamma counter, such as in radioimmunoassaying.

¹²⁵I is used as the radiolabel in investigating which ligands go to which plant pattern recognition receptors (PRRs).^[8]

Iodine-124 is a proton-rich isotope of iodine with a half-life of 4.18 days. Its modes of decay are: 74.4% electron capture, 25.6% positron emission. ¹²⁴I decays to ¹²⁴Te. Iodine-124 can be made by numerous nuclear reactions via a cyclotron. The most common starting material used is ¹²⁴Te.

Iodine-124 as the iodide salt can be used to directly image the thyroid using positron emission tomography (PET).^[9] Iodine-124 can also be used as a PET radiotracer with a usefully longer half-life compared with fluorine-18.^[10] In this use, the nuclide is chemically bonded to a pharmaceutical to form a positron-emitting radiopharmaceutical, and injected into the body, where again it is imaged by PET scan.

Iodine-129 (¹²⁹I; half-life 15.7 million years) is a product of cosmic ray spallation on various isotopes of xenon in the atmosphere, in cosmic ray muon interaction with tellurium-130, and also uranium and plutonium fission, both in subsurface rocks and nuclear reactors. Artificial nuclear processes, in particular nuclear fuel reprocessing and atmospheric nuclear weapons tests, have now swamped the natural signal for this isotope. Nevertheless, it now serves as a groundwater tracer as indicator of nuclear waste dispersion into the natural environment. In a similar fashion, ¹²⁹I was used in rainwater studies to track fission products following the Chernobyl disaster.

In some ways, ¹²⁹I is similar to ³⁶Cl. It is a soluble halogen, exists mainly as a non-sorbing anion, and is produced by cosmogenic, thermonuclear, and in-situ reactions. In hydrologic studies, ¹²⁹I concentrations are usually reported as the ratio of ¹²⁹I to total I (which is virtually all ¹²⁷I). As is the case with ³⁶Cl/Cl, ¹²⁹I/I ratios in nature are quite small, 10⁻¹⁴ to 10⁻¹⁰ (peak thermonuclear ¹²⁹I/I during the 1960s and 1970s reached about 10⁻⁷). ¹²⁹I differs from ³⁶Cl in that its half-life is longer (15.7 vs. 0.301 million years), it is highly biophilic, and occurs in multiple ionic forms (commonly, I⁻ and IO₃⁻), which have different chemical behaviors. This makes it fairly easy for ¹²⁹I to enter the biosphere as it becomes incorporated into vegetation, soil, milk, animal tissue, etc. Excesses of stable ¹²⁹Xe in meteorites have been shown to result from decay of "primordial" iodine-129 produced newly by the supernovas that created the dust and gas from which the solar system formed. This isotope has long decayed and is thus referred to as "extinct". Historically, ¹²⁹I was the first extinct radionuclide to be identified as present in the early Solar System. Its decay is the basis of the I-Xe iodine-xenon radiometric dating scheme, which covers the first 85 million years of Solar System evolution.

Iodine-131 (¹³¹
I
) is a beta-emitting isotope with a half-life of eight days, and comparatively energetic (190 keV average and 606 keV maximum energy) beta radiation, which penetrates 0.6 to 2.0 mm from the site of uptake. This beta radiation can be used for the destruction of thyroid nodules or hyperfunctioning thyroid tissue and for elimination of remaining thyroid tissue after surgery for the treatment of Graves' disease. The purpose of this therapy, which was first explored by Dr. Saul Hertz in 1941,^[11] is to destroy thyroid tissue that could not be removed surgically. In this procedure, ¹³¹I is administered either intravenously or orally following a diagnostic scan. This procedure may also be used, with higher doses of radio-iodine, to treat patients with thyroid cancer.

The ¹³¹I is taken up into thyroid tissue and concentrated there. The beta particles emitted by the radioisotope destroys the associated thyroid tissue with little damage to surrounding tissues (more than 2.0 mm from the tissues absorbing the iodine). Due to similar destruction, ¹³¹I is the iodine radioisotope used in other water-soluble iodine-labeled radiopharmaceuticals (such as MIBG) used therapeutically to destroy tissues.

The high energy beta radiation (up to 606 keV) from ¹³¹I causes it to be the most carcinogenic of the iodine isotopes. It is thought to cause the majority of excess thyroid cancers seen after nuclear fission contamination (such as bomb fallout or severe nuclear reactor accidents like the Chernobyl disaster) However, these epidemiological effects are seen primarily in children, and treatment of adults and children with therapeutic ¹³¹I, and epidemiology of adults exposed to low-dose ¹³¹I has not demonstrated carcinogenicity.^[12]

Iodine-135 is an isotope of iodine with a half-life of 6.6 hours. It is an important isotope from the viewpoint of nuclear reactor physics. It is produced in relatively large amounts as a fission product, and decays to xenon-135, which is a nuclear poison with the largest known thermal neutron cross section, which is a cause of multiple complications in the control of nuclear reactors. The process of buildup of xenon-135 from accumulated iodine-135 can temporarily preclude a shut-down reactor from restarting. This is known as xenon poisoning or "falling into an iodine pit".

Iodine fission-produced isotopes not discussed above (iodine-128, iodine-130, iodine-132, and iodine-133) have half-lives of several hours or minutes, rendering them almost useless in other applicable areas. Those mentioned are neutron-rich and undergo beta decay to isotopes of xenon. Iodine-128 (half-life 25 minutes) can decay to either tellurium-128 by electron capture or to xenon-128 by beta decay. It has a specific radioactivity of 2.177×10⁶ TBq/g.

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Colloquially, radioactive materials can be described as "hot," and non-radioactive materials can be described as "cold." There are instances in which cold iodide is administered to people in order to prevent the uptake of hot iodide by the thyroid gland. For example, blockade of thyroid iodine uptake with potassium iodide is used in nuclear medicine scintigraphy and therapy with some radioiodinated compounds that are not targeted to the thyroid, such as iobenguane (MIBG), which is used to image or treat neural tissue tumors, or iodinated fibrinogen, which is used in fibrinogen scans to investigate clotting. These compounds contain iodine, but not in the iodide form. However, since they may be ultimately metabolized or break down to radioactive iodide, it is common to administer non-radioactive potassium iodide to insure that metabolites of these radiopharmaceuticals is not sequestered by thyroid gland and inadvertently administer a radiological dose to that tissue.

Potassium iodide has been distributed to populations exposed to nuclear fission accidents such as the Chernobyl disaster. The iodide solution SSKI, a saturated solution of potassium (K) iodide in water, has been used to block absorption of the radioiodine (it has no effect on other radioisotopes from fission). Tablets containing potassium iodide are now also manufactured and stocked in central disaster sites by some governments for this purpose. In theory, many harmful late-cancer effects of nuclear fallout might be prevented in this way, since an excess of thyroid cancers, presumably due to radioiodine uptake, is the only proven radioisotope contamination effect after a fission accident, or from contamination by fallout from an atomic bomb (prompt radiation from the bomb also causes other cancers, such as leukemias, directly). Taking large amounts of iodide saturates thyroid receptors and prevents uptake of most radioactive iodine-131 that may be present from fission product exposure (although it does not protect from other radioisotopes, nor from any other form of direct radiation). The protective effect of KI lasts approximately 24 hours, so must be dosed daily until a risk of significant exposure to radioiodines from fission products no longer exists.^[13][14] Iodine-131 (the most common radioiodine contaminant in fallout) also decays relatively rapidly with a half-life of eight days, so that 99.95% of the original radioiodine has vanished after three months.

• Isotope masses from:
  • Audi, Georges; Bersillon, Olivier; Blachot, Jean; Wapstra, Aaldert Hendrik (2003), "The NUBASE evaluation of nuclear and decay properties", Nuclear Physics A, 729: 3–128, Bibcode:2003NuPhA.729....3A, doi:10.1016/j.nuclphysa.2003.11.001
• Half-life, spin, and isomer data selected from the following sources.
  • Audi, Georges; Bersillon, Olivier; Blachot, Jean; Wapstra, Aaldert Hendrik (2003), "The NUBASE evaluation of nuclear and decay properties", Nuclear Physics A, 729: 3–128, Bibcode:2003NuPhA.729....3A, doi:10.1016/j.nuclphysa.2003.11.001
  • National Nuclear Data Center. "NuDat 2.x database". Brookhaven National Laboratory.
  • Holden, Norman E. (2004). "11. Table of the Isotopes". In Lide, David R. (ed.). CRC Handbook of Chemistry and Physics (85th ed.). Boca Raton, Florida: CRC Press. ISBN 978-0-8493-0485-9.

• Iodine isotopes data from The Berkeley Laboratory Isotopes Project's
• Iodine-128, Iodine-130, Iodine-132 data from 'Wolframalpha'