Best NDT - The Source for Gamma Radiography

Selenium 75
Iridium 192
Cobalt 60

The decrease of activity of a radioisotope with time is given by the equation:
A(t) = A(0) exp (-0.69315 t/T), Where A(0) is the initial activity, A(t) is the activity after a further time t has elapsed, and T is the half-life of the radioisotope, expressed in the same units as t. The ratio A(t)/A(o) is called the decay factor of the radioisotope for elapsed time t. Decay factors for various radioisotopes and for various elapsed times are tabulated below. To determine the final activity of an isotope, multiply the current activity by the decay factor in the correct table. For example, if on August 23, the activity of Ir 192 is taken at 100 Ci, the calculation to determine the activity on August 30 would be as follows: 100 Ci x 0.9364 = 93.64 Ci

Longer decay periods
To find the decay factor for a longer period than is shown in the table, multiply the decay factors for two or more periods adding up to the desired one. For example, the 10 and 14-year decay factors for cobalt 60 are 0.2677 and 0.1580, respectively. Hence, the 24-year decay factor is: 0.2677 x O. 1580 = 0.0423

Times prior to calibration
To determine the decay factor for a time prior to the initial time, divide 1 by the factor given in the decay table. For example, if 100 Ci of Ir 192 is calibrated on Wednesday, July 13, and the activity as of July 6, is required, the calculation is as follows: 100 Ci x 1/0.9364 = 106.79 Ci

For uniformity, the decay factors are all given to four decimal places, and up to four significant figures. In actual fact, the decay factors are not more accurate than the half-lives, which in some cases are known to only two or three figures. Also some approximations have been introduced; for example, the assumption that a month is exactly the twelfth part of a year, which is not precisely true. For these reasons, the tables should be used with a reasonable amount of caution.

A few commonly-used radioisotopes are so long lived that a decay tabulation would show no significant change over any practical period of time, and have therefore been omitted from the tables. Among the more important of these are carbon 14 (5,730 years), chlorine 36 (301,000 years), radium 226 (1,600 years) and plutonium 239 (29,110 years).

Most of the half-lives used are taken from A Handbook of Radioactivity Measurements Procedures National Council on Radiation Protection and Measurements (NCRP) Report No. 58, Second Edition. It had been published with nuclear data for some biomedically important radionuclides, re-evaluated between August, 1983 and April, 1954. The Table of Radioactive lsotopes by Edgardo Browne and Richard B. Fuestone, published in 1986 by John Wiley & Sons, Inc. was also used in the preparation of these tables.

SELENIUM 75
Half-Life: 119.8 days

days	days
	0	2	4	6	8	10	12	14	16	18
0	1.0000	0.9885	0.9771	0.9659	0.9548	0.9438	0.9329	0.9222	0.9116	0.9011
20	0.8907	0.8805	0.8703	0.8603	0.8504	0.8407	0.8310	0.8214	0.8120	0.8026
40	0.7934	0.7834	0.7752	0.7663	0.7575	0.7488	0.7402	0.7317	0.7232	0.7149
60	0.7067	0.6986	0.6905	0.6826	0.6747	0.6670	0.6593	0.6517	0.6442	0.6368
80	0.6295	0.6222	0.6151	0.6080	0.6010	0.5941	0.5873	0.5805	0.5738	0.5672
100	0.5607	0.5542	0.5479	0.5416	0.5353	0.5292	0.5231	0.5171	0.5111	0.5052
120	0.4994	0.4937	0.4880	0.4824	0.4768	0.4713	0.4659	0.4606	0.4553	0.4500
140	0.4448	0.4397	0.4347	0.4297	0.4247	0.4198	0.4150	0.4102	0.4055	0.4008
160	0.3962	0.3917	0.3872	0.3827	0.3783	0.3740	0.3697	0.3654	0.3612	0.3570
180	0.3529	0.3489	0.3449	0.3409	0.3370	0.3331	0.3293	0.3255	0.3217	0.3180
200	0.3144	0.3108	0.3072	0.3036	0.3002	0.2967	0.2933	0.2899	0.2866	0.2833
220	0.2800	0.2768	0.2736	0.2705	0.2674	0.2643	0.2612	0.2582	0.2553	0.2523
240	0.2494	0.2466	0.2437	0.2409	0.2381	0.2354	0.2327	0.2300	0.2274	0.2248
260	0.2222	0.2196	0.2171	0.2146	0.2121	0.2097	0.2073	0.2049	0.2025	0.2002
280	0.1979	0.1956	0.1934	0.1911	0.1889	0.1868	0.1846	0.1825	0.1804	0.1783
300	0.1763	0.1742	0.1722	0.1703	0.1683	0.1664	0.1644	0.1625	0.1607	0.1588
320	0.1570	0.1552	0.1534	0.1516	0.1499	0.1482	0.1465	0.1448	0.1431	0.1415
340	0.1398	0.1382	0.1366	0.1351	0.1335	0.1320	0.1305	0.1290	0.1275	0.1260
360	0.1246	0.1231	0.1217	0.1203	0.1189	0.1176	0.1162	0.1149	0.1136	0.1122
380	0.1110	0.1097	0.1084	0.1072	0.1059	0.1047	0.1035	0.1023	0.1011	0.1000

IRIDIUM 192
Half-Life: 73.83 days

days	days
	0	1	2	3	4	5	6	7	8	9
0	1.0000	0.9907	0.9814	0.9722	0.9631	0.9541	0.9452	0.9364	0.9276	0.9190
10	0.9104	0.9019	0.8935	0.8851	0.8768	0.8686	0.8605	0.8525	0.8445	0.8366
20	0.8288	0.8211	0.8134	0.8058	0.7983	0.7908	0.7834	0.7761	0.7688	0.7617
30	0.7545	0.7475	0.7405	0.7336	0.7267	0.7199	0.7132	0.7065	0.6999	0.6934
40	0.6869	0.6805	0.6741	0.6678	0.6616	0.6554	0.6493	0.6432	0.6372	0.6313
50	0.6254	0.6195	0.6137	0.6080	0.6023	0.5967	0.5911	0.5856	0.5801	0.5747
60	0.5693	0.5640	0.5587	0.5538	0.5483	0.5432	0.5381	0.5331	0.5281	0.5232
70	0.5183	0.5135	0.5087	0.5039	0.4992	0.4945	0.4899	0.4853	0.4808	0.4763
80	0.4719	0.4674	0.4631	0.4588	0.4545	0.4502	0.4460	0.4418	0.4377	0.4336
90	0.4296	0.4256	0.4216	0.4176	0.4137	0.4099	0.4060	0.4023	0.3985	0.3948
100	0.3911	0.3874	0.3838	0.3802	0.3767	0.3731	0.3697	0.3662	0.3628	0.3594
110	0.3560	0.3527	0.3494	0.3461	0.3429	0.3397	0.3365	0.3334	0.3303	0.3272
120	0.3241	0.3211	0.3181	0.3151	0.3122	0.3093	0.3064	0.3035	0.3007	0.2979
130	0.2951	0.2923	0.2896	0.2869	0.2842	0.2816	0.2789	0.2763	0.2737	0.2712
140	0.2686	0.2661	0.2636	0.2612	0.2587	0.2563	0.2539	0.2516	0.2492	0.2469
150	0.2446	0.2423	0.2400	0.2378	0.2356	0.2334	0.2312	0.2290	0.2269	0.2248
160	0.2227	0.2206	0.2185	0.2165	0.2144	0.2124	0.2105	0.2085	0.2065	0.2046
170	0.2027	0.2008	0.1989	0.1971	0.1952	0.1934	0.1916	0.1898	0.1880	0.1863
180	0.1845	0.1828	0.1811	0.1794	0.1777	0.1761	0.1744	0.1728	0.1712	0.1696
190	0.1680	0.1664	0.1649	0.1633	0.1618	0.1603	0.1588	0.1573	0.1558	0.1544

COBALT 60
Half-Life: 5.26 years

years	months
	0	1	2	3	4	5	6	7	8	9	10	11
0	1.0000	0.9891	0.9783	0.9676	0.9570	0.9466	0.9362	0.9260	0.9159	0.9059	0.8960	0.8862
1	0.8765	0.8670	0.8575	0.8481	0.8389	0.8297	0.8206	0.8117	0.8028	0.7940	0.7854	0.7768
2	0.7683	0.7599	0.7516	0.7434	0.7353	0.7273	0.7193	0.7115	0.7037	0.6960	0.6884	0.6809
3	0.6735	0.6661	0.6588	0.6516	0.6445	0.6375	0.6305	0.6236	0.6168	0.6101	0.6034	0.5968
4	0.5903	0.5839	0.5775	0.5712	0.5649	0.5588	0.5527	0.5466	0.5407	0.5348	0.5289	0.5231
5	0.5174	0.5118	0.5062	0.5007	0.4952	0.4898	0.4844	0.4791	0.4739	0.4687	0.4636	0.4586
6	0.4535	0.4486	0.4437	0.4388	0.4341	0.4293	0.4246	0.4200	0.4154	0.4109	0.4064	0.4019
7	0.3975	0.3932	0.3889	0.3847	0.3805	0.3763	0.3722	0.3681	0.3641	0.3601	0.3562	0.3523
8	0.3485	0.3447	0.3409	0.3372	0.3335	0.3298	0.3262	0.3227	0.3192	0.3157	0.3122	0.3088
9	0.3054	0.3021	0.2988	0.2955	0.2923	0.2891	0.2860	0.2828	0.2798	0.2767	0.2737	0.2707
10	0.2677	0.2648	0.2619	0.2591	0.2562	0.2534	0.2507	0.2479	0.2452	0.2425	0.2399	0.2373
11	0.2347	0.2321	0.2296	0.2271	0.2246	0.2221	0.2197	0.2173	0.2149	0.2126	0.2103	0.2080
12	0.2057	0.2035	0.2012	0.1990	0.1969	0.1947	0.1926	0.1905	0.1884	0.1863	0.1843	0.1823
13	0.1803	0.1783	0.1764	0.1745	0.1726	0.1707	0.1688	0.1670	0.1651	>0.1633	0.1616	0.1589
14	0.1580	0.1563	0.1546	0.1529	0.1513	0.1496	0.1480	0.1463	0.1448	0.1432	0.1416	0.1401
15	0.1385	0.1370	0.1355	0.1340	0.1326	0.1311	0.1297	0.1283	0.1269	0.1255	0.1241	0.1228
16	0.1214	0.1201	0.1188	0.1175	0.1162	0.1149	0.1137	0.1124	0.1112	0.1100	0.1088	0.1076
17	0.1064	0.1053	0.1041	0.1030	0.1019	0.1007	0.0996	0.0986	0.0975	0.0964	0.0954	0.0943
18	0.0933	0.0923	0.0913	0.0903	0.0893	0.0833	0.0873	0.0864	0.0854	0.0845	0.0836	0.0827
19	0.0818	0.0809	0.0800	0.0791	0.0783	0.0774	0.0766	0.0757	0.0749	0.0741	0.0733	0.0725