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 in the following pages. 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

 

 


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