An Analysis of Residual Radiation in
Thoriated Camera Lenses

Jonathan Wäng  and Viktor Henningsson †jonwan@kth.se, † vikhen@kth.se

SA104X Degree Project in Engineering Physics, First Level
Supervisor: Bo Cederwall
Department of Physics
School of Engineering Sciences
Royal Institute of Technology (KTH)
Stockholm, Sweden, 2013

Abstract
The usage of the naturally occurring radioactive isotope thorium-232 in older
camera lenses was implemented to provide better optical properties and a cheaper
production. In this thesis, the resulting radioactivity in camera objectives
containing such lenses is studied via gamma spectroscopy. A number of camera
objectives that potentially contain thorium-232 were measured using a germanium
semiconductor photon detector. The spectra were compared to the backgroundradiation whereby it is possible to determine whether any inherent radiation fromthe thorium-232 decay chain came from the lenses themselves. For one lens, thegamma spectra are then complemented with beta emission spectroscopy. From
this, it is finally possible to establish the activity of the lenses and the radiationdose a photographer would receive from them. The radiation dose can then becompared to national standards of allowed doses. The measurements showed thatthough there clearly was a presence of decaying thorium and daughter nuclides in
the objectives, the activity was small enough not to be hazardous.

2

Contents
1

Introduction.................................................................................................... 4
1.1

Background ............................................................................................... 4

1.2

Scope ......................................................................................................... 4

1.3

Objective ................................................................................................... 5

2

Materials and Methods ................................................................................... 6

3

Results ..........................................................................................................13
3.1

Background Gamma Radiation .................................................................15

3.2

Camera House Gamma Measurements .....................................................17

3.2.1

Nikon F2 .........................................................................................17

3.2.2
3.2.3

Contax .............................................................................................17
Hasselblad 1000F ............................................................................18

3.3

3.3.1

Nikkor 50mm 1:2 ............................................................................18

3.3.2

Zeiss-Opton Sonnar 50mm 1:1.5 .....................................................193.3.3

Carl Zeiss Tessar 80mm 1:2.8 .........................................................20

3.3.4

Nikkor Vivitar 70-150mm 1:3.8 ......................................................21

3.3.5

Sigma super-wide II 24mm 1:2.8 (multi-coated) .............................22

3.3.6

Nikkor 135mm 1:2.8 .......................................................................22

3.3.7

Nikkor-P Auto 105mm 1:2.5 ...........................................................23

3.3.8

Lietz-Wetzlar Macro-Elmarit-R 1:2.8/60 .........................................24

3.4
4

5

Camera Objective Gamma Measurements ................................................18

Beta Measurements...................................................................................24

Discussion .....................................................................................................27
4.1

Activity of the Carl Zeiss Tessar Lens ......................................................27

4.2

Radiation Dose from the Carl Zeiss Tessar ...............................................29

4.3

Error Analysis...........................................................................................31

Summary and Conclusions ............................................................................33

Bibliography .........................................................................................................34
6

Appendix A: Efficiency Calibration ..............................................................35

3

1 Introduction
1.1 Background
Beginning in the end of the 1930’s and up until sometime in the 1980’s, thorium
was sometimes used in the lenses of photographic objectives to improve their
optical properties. Paul De Paolis at Kodak patented some specific formulae for
optical glass with thorium in 1949 [1]. This could be an indicator of whencommercial thoriated lenses were first sold. Thorium-232 was used, both blended
in with the glass and as a coating which gave a higher refractive index as well as alower dispersion. This resulted in a smaller required curvature of the lenses, hence
reducing the cost of production for the lenses [2]. Thorium-232 is a naturally
occurring radioactive isotope which marks the beginning of an extensive decay
chain that ends with the stable lead-208 [3].
Radioactive radiation may be harmful in too large doses. The effects of suchradiation are usually divided into two main categories; deterministic effects andstochastic effects. Deterministic effects will occur if the radiation dose is highenough and increase if the dose is increased and are the result of cell death. The
threshold for these effects lies at around 0.1-1 Sv which is higher than expected
here. The stochastic effects, on the other hand, are more probabilistic. The
probability for stochastic effects increase with a higher dose, but the effects
themselves are no more severe and are instead the result of cell mutation whose
symptoms may take years to show up. There exists no widely accepted thresholdfor stochastic effects but recommended dose limits are based upon extensive
research on both stochastic and deterministic effects [4].
1.2 ScopeThis project will due to financial and material constraints have a restricted area of
research. Since we were only able to obtain objectives through the goodwill ofcamera owner Per-Olof Eriksson this report will restrict itself to smaller cameras.
The scope will also be restricted by the energy range of the used gamma detector. It
has an energy range of 3 keV to 10 MeV and hence no data of the radiation outside
this spectrum will be measured. However, the expected radiation from the thorium232 chain will be covered [3], [5].

4

1.3 Objective
This report aims, primarily, to investigate whether owners of such photographic
objectives have any reason to feel anxious about their health. To do this, we willmeasure gamma radiation from a number of camera lenses, and beta radiation fromone lens. The spectra obtained will then be analyzed and the activity of the lenses
may then tell us something about the possible health hazards.

5

2 Materials and Methods
The practical section of the project began with measuring the gamma radiationemanating from the objectives. The objectives were selected through the use of asimple Geiger counter and the measured objectives and corresponding cameras, inthe case there is one, are listed in table 1 below. Each camera lens and body in thisproject was measured for at least 18 hours.
Table 1: The cameras and camera objectives measured in this project.
CamerasObjectives

Nikon F2

Nikkor 50mm 1:2

ContaxZeiss-Opton Sonnar 50mm 1:1.5

Hasselblad 1000F

Carl Zeiss Tessar 80mm 1:2.8
Nikkor Vivitar 70-150mm 1:3.8
Sigma super-wide II 24mm 1:2.8 (multi-coated)
Nikkor 135mm 1:2.8
Nikkor-P Auto 105mm 1:2.5
Lietz-Wetzlar Macro-Elmarit-R 1:2.8/60

Thorium-232 will go through a certain chain of events when decaying, as shown infigure 1.

6

Figure 1: The decay chain originating at Thorium-232 with type of decay and halftime. Data taken from NuDat: National Nuclear Data Center, see reference [3].
In the figure, every isotope completely decays into the following isotope, except
for bismuth-212. Since it emits both alpha and beta minus radiation it will create
tallium-208 and polonium-212 respectively, which in turn both decay into thestable isotope lead-208. Thorium-232 itself clearly has a substantial half-life andwill therefore be present in the lenses for an extensive period of time. The activityof this decay is expected to be rather low in the measurements.

7

Table 2: The thorium-232 decay chain with some values for the gamma energies
radiated from the daughter nucleus in each decay. Data taken from NuDat:
National Nuclear Data Center, see reference [3].

Q-value Energy of[keV]
gamma [keV]

Mean βenergy [keV]

ParentnucleusHalf life

Decay
mode

Probability of
gamma [%]

Thorium232

1.4*1010
years

α: 100%4082.8

63.81

0.26

Radium-228

5.7 yearsβ-: 100%

45.9

13.52

1.55

7.2

4.4
25.8
15.8

380

-

Actinium228

6.1 minutes β-: 100%2127

462.95911.23969.03

Thorium228

1.9 years

α: 100%5520.1215.980.02-

Radium-224

3.6 days

α: 100%5788.8

242.1

4.1

-

Radon-220

55 seconds α: 100%

6404.67549.730.114-

Polonium216

0.14
seconds

6906.3804.9

0.0019-β-: 100%

569.9

238.7
300.2

43.6
3.3

α: 35.94%

6207.26

10.3

7.0

Bismuth-212 61 minutes β-: 64.06%

2252.1727.26

6.67

3*10-7seconds

8954.12004999.0583.29860.622614.5

85
12.5
99.8

Lead-212

Polonium212

10.6 hours

α: 100%α: 100%Thallium208

3.1 minutes β-: 100%

Lead-208

StableStableStableStable8Stable

100

771
-

560
-

In table 2 above, some of the gamma radiation is shown. The energy and branchingratio of these emissions is seen in column 5 and 6, respectively. This gamma
radiation can easily escape from a radioactive sample even if the radioactivity isembedded in the material. The gamma radiation can be detected and the associatedactivity deduced.
The measurements of gamma radiation were executed using a semiconductor
photon detector of the ORTEC-GMX series (model no. GMX40P4-83) with ahigh-purity germanium crystal. High-purity germanium detectors react with highenergy ionizing radiation such as X-rays or gamma rays by producing charge
carriers when the material interacts with a high energy photon. These charge
carriers are then swept by the electric field inside the detector to the electrodes.
The charge is proportional to the interaction energy of the photon and is converted
to a voltage pulse by a preamplifier. High purity germanium detectors arecharacterized by their excellent energy resolution. This is used here to identify thecharacteristic gamma radiation from different radioactive nuclides. Over time more
and more photons will be detected and thereby the energy of the photons can beplotted against the number of counts, leading to an energy spectrum. The spectrum
can then be analyzed via a computer program [5], [6].

9Figure 2: Inside the gamma measurement chamber. The front end of the detector ismarked on the right in the image. The walls of the measurement chamber include a
5cm lead layer, shielding against the surrounding natural background gamma ray
radiation.
Along with the spectra from objectives and camera bodies, measurements weremade of the background radiation. The background radiation was then subtracted
from the spectra to give a clear view of the radiation from the lenses. The spectracollected were imported into MATLAB where the intensity was plotted against theenergy.
After all the measurements were completed a calibration of the efficiency of the
gamma detector had to be made, since radiation emitted by an objective would
reach the detector with different probability due to the geometry of the measuring
chamber. The detector also has a sensitivity which varies with the gamma rayenergy. See Appendix A for full details of the efficiency calibration.

10

Figure 3: The setup for the beta measurement of the Carl Zess Tessar objective.
The measurement chamber consists of 5 cm lead in order to shield from theexternal background radiation.
Being finished with the measurements of gamma radiation, measurements of thebeta radiation would possibly have given a clearer result. Due to time constraints
only a measurement on the most promising objective was made; the Carl Zeiss
objective belonging to the Hasselblad 1000F camera.
The beta measurements were conducted with a plastic scintillator detector with thegrid taken off. A scintillator detector acts on the basis of the material scintillation
property when excited by incoming ionizing radiation. The radiation energy isabsorbed by the scintillator, which then re-emits the energy in the form of photons
which are typically in the visible range. The photons are registered by aphotodetector, such as a photomultiplier tube which converts the light pulse into an
11amplified electrical signal. The registered radiation may then be displayed as acontinuous energy spectrum of beta radiation.
The plastic scintillator used for the measurements had a diameter of 50 mm. It wasof a few millimeters thickness and therefore mainly sensitive to charged particles.
The setup for the beta measurement is seen in figure 3 above. The lens was placedwith the inner side facing the camera towards turned the detector since this waswhere most radiation was found to be emitted. The lens is also centered to theexact center of the detector at the lowest shelf of the measuring chamber. The
lowest shelf was placed 70 mm from the plastic scintillator.
As with the gamma radiation, measurements of the background and the objective
were done separately. These were imported to MATLAB for further analysis. An
efficiency calibration had to be made for the beta detector as well, but this wasdone taking only the measurement geometry into account. The full details of theefficiency calibrations are given in Appendix A.

12

3 Results
The results below are presented as plots of the gamma spectra for the backgroundradiation and a table of the prominent peaks with the corresponding decay for eachof the camera objectives and houses. Some plots or tables will also be followed bya brief comment, where fitting. For deeper analysis of the results see section 4;
Discussion. Note that the graphs are zoomable when analyzing in the MATLAB
software so that some peaks that are in the table may not be visible in the plots aspresented in this report. The rows marked with “-“ represent values that weredisregarded due to high statistical fluctuations.
In the tables for all measurements, except the background, only the decay from thethorium-232 chain will be included since all the other are of little interest to theproject. The columns in measurements of both the camera houses and camera
objectives are calculated, for each energy according to formulae (1). (2) and (3)below.
(1)

(2)
(3)

13

Figure 4: A zoomed peak in the Carl Zeiss Tessar spectrum.
As an example of the analysis, the zoomed peak from the background spectrum as
seen in figure 4, above, will be used. The energy of the peak is recorded as well as
the area under the peak. The program Tukan 8K will automatically calculate thearea under the peak without the background which mainly is due to Compton
scattering of higher energy photons. The energy is then compared to values fromthe National Nuclear Data Center (NuDat) to characterize which decay the peakcorresponds to [4]. 911.23 keV is seen to match with the beta minus decay ofactinium-228 to thorium-228. The beta decay populates an exited state in thorium228 and a 911.2 keV photon is then emitted. This is only one out of many
characteristic gamma rays from this decay.
The area under the curve is then divided by the live time of the measurement aswell as the efficiency of the detector at this energy and the proportion of the decaythat is represented by this photon energy. The efficiency is derived from the graphdisplayed in figure 13 in Appendix A. This will yield the activity of the parent
nucleus of the decay in the measured object, in this case actinide-228.

14

3.1 Background Gamma Radiation

Figure 5: Gamma spectrum of the background radiation measurement.

15

Table 3: Energy peaks and the corresponding decay for the background radiation.
Nominalenergy[keV]
72.87374.96984.21487.34993.35
185.715238.632240.986
270,245296
300.087338.32351.9321
463.004510
583.187
609.32660.94727.33768.36794.947860.557
911.204934.5
965
968.9711120.61238.122
1377.669
1460.822
1765.44

Decay
210Bi->206Tl
208Tl->208Pb
231Th->231Pa
212Pb->212Bi
228Ac->228Th
235U->231Th
212Pb->212Bi
224Ra->220Rn
228Ac->228Th
210Tl->210Pb
212Pb->212Bi
228Ac->228Th
214Pb->214Bi
228Ac->228Th
222Rn->218Po
208Tl->208Pb
214Bi->214Po
214Bi->214Po
212Bi->212Po
214Bi->214Po
228Ac->228Th
208Tl->208Pb
228Ac->228Th
214Bi->214Po
214Bi->214Po
228Ac->228Th
234Pa->234U
214Bi->214Po
214Bi->214Po
40K -> 40Ar
214Bi->214Po

Populating
particle
decay
mode
alpha
betabetabetabetaalphabetaalphabetabetabetabetabetabetaalphabetabetabetabetabetabetabetabetabetabetabetabetabetabetaepsilon
beta-

Relative
intensity of
gamma[%]
6.30
3.35
6.6
3.97
3.1
57.2
43.6
4.1
3.46
79
3.3
11.27
35.6
4.4
0.076
85
45.49
0.053
6.67
4.894
4.25
12.5
25.8
0.01
0.011
15.8
0.00172
5.834
3.99
10.66
15.3

Measured intensity
[1/s]
0.113
0.423
0.244
0.080
0.071
0.068
0.219
0.067
0.026
0.214
0.017
0.056
0.386
0.024
0.127
0.155
0.449
0.072
0.039
0.045
0.022
0.025
0.147
0.027
0.023
0.081
0.135
0.055
0.035
0.897
0.133

Table 3, above, shows the background radiation measured in the measuring
chamber (see figure 2) of our lab at Alba Nova of the Royal Institute ofTechnology, Sweden with no source of radiation inside. Notable peaks are the one
from potassium-40 and radon-222, radioactive isotopes that are common in
Sweden. Some peaks seen here also correspond to decay originating from thorium232, the decay chain of interest in this thesis, and these are marked with bold
letters. If there is indeed radiation from the lenses and camera houses, the16intensities of these will be higher than those derived from Table 3. The energies in
the Nominal energy column are taken from [3].

3.2 Camera House Gamma Measurements

3.2.1 Nikon F2Table 4: Energy peaks from the thorium-232 decay chain for Nikon F2.
Nominalenergy[keV]
87.349238.632240.986338.32583.187
727.33911.204
968.971

Decay
212Pb->212Bi
212Pb->212Bi
224Ra->220Rn
228Ac->228Th
208Tl->208Pb
212Bi->212Po
228Ac->228Th
228Ac->228Th

Measured
Proportion of
gamma
BackgroundTotal isotope
intensity [Bq] intensity [%]Efficiency
activity [Bq]
0.251
115
0.023
0.062
111
0.017
0.168
109
0.013
0.083
103
0.008

3.20
3.15
1.27
1.81

3.2.2 Contax

Table 5: Energy peaks from the thorium-232 decay chain for Contax.
Nominalenergy[keV]
87.34993.35
238.632240.986270.245300.087338.32463.004
583.187
727.33794.947860.557
911.204
968.971

Decay
212Pb->212Bi
228Ac->228Th
212Pb->212Bi
224Ra->220Rn
228Ac->228Th
212Pb->212Bi
228Ac->228Th
228Ac->228Th
208Tl->208Pb
212Bi->212Po
228Ac->228Th
208Tl->208Pb
228Ac->228Th
228Ac->228Th

Measuredgamma
intensity[1/s]

Proportion of
Background
intensity [%]

0.278
0.067
0.185
0.088

127
119
120
109

17

Efficiency
0.023
0.017
0.013
0.008

Total isotope
activity [Bq]
5.95
5.50
2.85
5.82

3.2.3 Hasselblad 1000F

Table 6: Energy peaks from the thorium-232 decay chain for Hasselblad 1000F.
Nominalenergy[keV]
87.34993.35
238.632240.986270.245300.087338.32463.004
583.187
727.33794.947860.557
911.204
968.971

Decay
212Pb->212Bi
228Ac->228Th
212Pb->212Bi
224Ra->220Rn
228Ac->228Th
212Pb->212Bi
228Ac->228Th
228Ac->228Th
208Tl->208Pb
212Bi->212Po
228Ac->228Th
208Tl->208Pb
228Ac->228Th
228Ac->228Th

Measuredgamma
intensity[1/s]

Proportion of
Background
intensity [%]

0.250
0.064
0.173
0.040
0.157
0.086

114
113
112
102
106
106

Efficiency

Total isotope
activity [Bq]

0.023
0.017
0.013
0.011
0.009
0.008

3.15
3.80
1.75
1.50
4.22
3.79

3.3 Camera Objective Gamma Measurements

3.3.1 Nikkor 50mm 1:2

Table 7: Energy peaks from the thorium-232 decay chain for Nikkor 50mm 1:2.
Nominalenergy[keV]
87.34993.35
238.632240.986270.245300.087338.32463.004
583.187
727.33794.947

Decay
212Pb->212Bi
228Ac->228Th
212Pb->212Bi
224Ra->220Rn
228Ac->228Th
212Pb->212Bi
228Ac->228Th
228Ac->228Th
208Tl->208Pb
212Bi->212Po
228Ac->228Th

Measuredgamma
intensity[1/s]

Proportion of
Background
intensity [%]

0.246
0.058
0.169
-

113
104
109
-

18

Efficiency
0.023
0.017
0.013
-

Total isotope
activity [Bq]
2.75
1.07
1.34
-

3.3.2 Zeiss-Opton Sonnar 50mm 1:1.5

Table 8: Energy peaks from the thorium-232 decay chain for Zeiss-Opton Sonnar
50mm 1:1.5.
Nominalenergy[keV]
87.34993.35
238.632240.986270.245300.087338.32463.004
583.187
727.33794.947860.557
911.204
968.971

Decay
212Pb->212Bi
228Ac->228Th
212Pb->212Bi
224Ra->220Rn
228Ac->228Th
212Pb->212Bi
228Ac->228Th
228Ac->228Th
208Tl->208Pb
212Bi->212Po
228Ac->228Th
208Tl->208Pb
228Ac->228Th
228Ac->228Th

Measuredgamma
intensity[1/s]

Proportion of
Background
intensity [%]

0.232
0.059
0.164
-

106
105
106
-

19

Efficiency
0.023
0.017
0.013
-

Total isotope
activity [Bq]
1.26
1.32
0.91
-

3.3.3 Carl Zeiss Tessar 80mm 1:2.8

Figure 6: Gamma radiation spectrum for the measurement of the Carl Zeiss Tessar
lens.
Table 9: Energy peaks from the thorium-232 decay chain for Carl Zeiss Tessar
80mm 1:2.8.
Nominalenergy[keV]
87.34993.35
238.632240.986270.245300.087338.32463.004
583.187
727.33794.947860.557
911.204
968.971

Decay
212Pb->212Bi
228Ac->228Th
212Pb->212Bi
224Ra->220Rn
228Ac->228Th
212Pb->212Bi
228Ac->228Th
228Ac->228Th
208Tl->208Pb
212Bi->212Po
228Ac->228Th
208Tl->208Pb
228Ac->228Th
228Ac->228Th

Measuredgamma
intensity[1/s]

Proportion of
Background
intensity [%]

0.247
0.080
0.029
0.189
0.046
0.026
0.028
0.168
-

113
142
122
122
117
117
111
114
-

20

Efficiency
0.023
0.017
0.015
0.013
0.011
0.010
0.009
0.009
-

Total isotope
activity [Bq]
2.78
12.22
8.03
3.16
9.37
9.21
2.39
9.46
-

In table 9 one can clearly see an increase in the activity from decays in thethorium-232 chain. The actinium-228 decay has several observable gamma
radiation peaks that give the total activity of actinium-228. Most of these give
similar results.

3.3.4 Nikkor Vivitar 70-150mm 1:3.8

Table 10: Energy peaks from the thorium-232 decay chain for Nikkor Vivitar 70150mm 1:3.8.
Nominalenergy[keV]
87.34993.35
238.632240.986270.245300.087338.32463.004
583.187
727.33794.947860.557
911.79969.68Decay
212Pb->212Bi
228Ac->228Th
212Pb->212Bi
224Ra->220Rn
228Ac->228Th
212Pb->212Bi
228Ac->228Th
228Ac->228Th
208Tl->208Pb
212Bi->212Po
228Ac->228Th
208Tl->208Pb
228Ac->228Th
228Ac->228Th

Measuredgamma
intensity[1/s]

Proportion of
Background
intensity [%]

0.222
0.158
-

102
102
-

21

Efficiency
0.023
0.013
-

Total isotope
activity [Bq]
0.345
0.32
-

3.3.5 Sigma super-wide II 24mm 1:2.8 (multi-coated)

Table 11: Energy peaks from the thorium-232 decay chain for Sigma super-wide II
24mm 1:2.8.
Nominalenergy[keV]
87.34993.35
238.632240.986270.245300.087338.32463.004
583.187
727.33794.947860.557
911.204
968.971

Decay
212Pb->212Bi
228Ac->228Th
212Pb->212Bi
224Ra->220Rn
228Ac->228Th
212Pb->212Bi
228Ac->228Th
228Ac->228Th
208Tl->208Pb
212Bi->212Po
228Ac->228Th
208Tl->208Pb
228Ac->228Th
228Ac->228Th

Measuredgamma
intensity[1/s]

Proportion of
Background
intensity [%]

0.267
0.071
0.189
0.161
0.092

122
127
122
109
114

Efficiency

Total isotope
activity [Bq]

0.023
0.017
0.013
0.009
0.008

4.81
7.73
3.23
6.35
8.78

3.3.6 Nikkor 135mm 1:2.8

Table 12: Energy peaks from the thorium-232 decay chain for Nikkor 135mm
1:2.8.
Nominalenergy[keV]
87.34993.35
238.632240.986270.245300.087338.32463.004
583.187
727.33794.947860.557
911.204
968.971

Decay
212Pb->212Bi
228Ac->228Th
212Pb->212Bi
224Ra->220Rn
228Ac->228Th
212Pb->212Bi
228Ac->228Th
228Ac->228Th
208Tl->208Pb
212Bi->212Po
228Ac->228Th
208Tl->208Pb
228Ac->228Th
228Ac->228Th

Measuredgamma
intensity[1/s]

Proportion of
Background
intensity [%]

0.243
0.062
0.189
0.158
0.089

111
110
122
107
111

22

Efficiency
0.023
0.017
0.013
0.009
0.008

Total isotope
activity [Bq]
2.43
2.98
3.19
4.89
6.72

3.3.7 Nikkor-P Auto 105mm 1:2.5

Table 13: Energy peaks from the thorium-232 decay chain for Nikkor-P Auto
105mm 1:2.5.
Nominalenergy[keV]
87.34993.35
238.632240.986270.245300.087338.32463.004
583.187
727.33794.947860.557
911.204

Decay
212Pb->212Bi
228Ac->228Th
212Pb->212Bi
224Ra->220Rn
228Ac->228Th
212Pb->212Bi
228Ac->228Th
228Ac->228Th
208Tl->208Pb
212Bi->212Po
228Ac->228Th
208Tl->208Pb
228Ac->228Th

Measuredgamma
intensity[1/s]

Proportion of
Background
intensity [%]

0.132
0.049
0.046
0.016
-

117
122
121
112
-

23

Efficiency
0.023
0.017
0.013
0.011
-

Total isotope
activity [Bq]
3.75
6.23
3.01
6.73
-

3.3.8 Lietz-Wetzlar Macro-Elmarit-R 1:2.8/60

Table 14: Energy peaks from the thorium-232 decay chain for Leitz-Wetzlar
Macro-Elmarit-R 1:2.8/60
Nominalenergy[keV]
87.34993.35
238.632240.986270.245300.087338.32463.004
583.187
727.33794.947860.557
911.204
968.971

Decay
212Pb->212Bi
228Ac->228Th
212Pb->212Bi
224Ra->220Rn
228Ac->228Th
212Pb->212Bi
228Ac->228Th
228Ac->228Th
208Tl->208Pb
212Bi->212Po
228Ac->228Th
208Tl->208Pb
228Ac->228Th
228Ac->228Th

Measured
gamma
activity[Bq]

Proportion of
Background
intensity [%]

0.260
0.063
0.179
0.046
0.156
-

119
111
116
117
106
-

Efficiency
0.023
0.017
0.013
0.011
0.009
-

Total isotope
activity [Bq]
4.13
3.27
2.27
9.35
4.07
-

3.4 Beta Measurements
Due to a shortage of time, only one measurement of beta radiation was made in
addition to the background radiation, a measurement of the Carl-Zeiss Tessar
objective. This objective showed the largest enhancement in activity over the
background in gamma ray measurements.

24

Figure 7: Beta spectrum of both the background (green) and objective (blue). The
mean beta energy per decay of most isotopes in the thorium-232 decay chain are
indicated.

25

Figure 8: Zoomed image of the beta spectrum.
In the spectrum of beta radiation separate peaks cannot be isolated; it is acontinuous spectrum. Instead of observing specific decays, the whole activity istherefore calculated by integrating the entire spectrum.
As with the gamma radiation Tukan 8k is used to get a total area below the
spectrum to yield the total number of counts from which the number of counts
from the background is subtracted. This is then divided by the time measured andthe absolute efficiency of the beta detector to yield the results in table 15.

Table 15: The measured activity of the beta radiation for the Carl Zeiss Tessar lens.
Measured intensity of
Background intensity [1/s] Carl Zeiss Tessar [1/s]
Efficiency
3.19
3.71

26

Total beta activity
[Bq]
0.098
5.41

4 Discussion
From the gamma spectra, it is clear that most camera objectives and the camera
houses rarely give higher intensities than 115% of the background radiation. Sincea simple Geiger meter was used to assist the choosing of which objectives tomeasure more precisely, no objectives were chosen that gave off little or noradiation. Thereby, the increase in activity - however slight - observed on allmeasurements was expected.
The objective measurement that gave the most significant activity increase was theCarl Zeiss Tessar with most discernible peaks from the thorium-232 chain and an
average gamma ray enhancement over the background intensity of around 20%.
Since the aim of the thesis is to see whether or not thoriated camera lenses pose ahealth hazard, this case is of major interest. Therefore, the Carl Zeiss Tessar willserve as the main study for the remainder of this report.

4.1 Activity of the Carl Zeiss Tessar Lens
The Hasselblad 1000F camera was introduced on the market in the 1953 [7]. Since
the Carl Zeiss Tessar that was measured was originally mounted on this camera
model, it is safe to assume that the lens was manufactured around the same time, ora few years after. The assumption is made that the Tessar objective was
manufactured 55 years ago and that, since thorium-232 has such a long half life
(1.4*1010 years) it can be considered an inexhaustible reservoir. By assuming thatthorium-232 has an arbitrary starting activity it is possible to calculate thepercentage activities of the rest of the decays in the thorium-232 chain, knowing
their half lives. This calculation was done through a simple simulation using theformulas below.
(4)
(5)
Where
is the activity for the n:th nuclide in the decay chain, and is the decay
constant for nuclide in the decay chain. Note thatin the denominator of(5). The resulting activity ratio is shown in table 16 below together with theactivities measured in this thesis.

27

Table 16: Table of the measurement results as well as calculated activity ratios
Parent nucleus
Th-232
Ra-228
Ac-228
Th-228
Ra-224
Rn-220Po-216
Pb-212
Bi-212
Po-212
Tl-208Half life
1.4*1010 years
5.7 years
6.1 minutes
1.9 years
3.6 days
55 seconds
0.14 seconds
10.6 hours
61 minutes
3*10^(-7) seconds
3.1 minutes

Activity from
Activity Ratio
gamma
Mean activity[%]
measurements [Bq] [Bq]100.00
9.56
99.87
9.55
99.87
9.73
9.55
99.80
9.55
99.80
9.55
99.80
9.55
99.80
9.55
99.80
9.55
99.80
9.37
9.55
63.94
6.12
35.86
2.78
3.43

The activity from gamma has been calculated through a mean of the values fromtable 9. Here, some values have been disregarded due to a large discrepancy fromthe other values. The discrepancy could be explained by the fact that other decays
radiate gamma of around this energy, resulting in a larger intensity than the actual
intensity of the specific decay of interest. The low energy level discrepancies canalso be explained by the fact that the measuring chamber is made of lead. The lead
can then be activated by a radioactive sample in the chamber, resulting in lead Xrays. These are prominent in the low energy range of the spectrum. The mean
activity is calculated using the activity ratio, given from the simulation describedearlier, and the measured values of the activity for actinium-228 and bismuth-212.
The activities seem to correlate quite closely to the calculated ratio. It is also
noticeable how the activity ratios correspond to the half lives of the parent nuclei.
For example, radium-228 acts as a bottle neck, but when it finally decays thefollowing isotopes decay very rapidly. The last two values arise since bismuth-212
decays both with alpha and beta decay. The ratios of these decays (as seen in table2) correspond perfectly to the values in table 16. The total activity of the Carl Zeiss
Tessar is calculated, from this, to be 96 Bq.
It is also very interesting to compare the total beta decay radiation from the gamma
measurements to the beta measurements. By summing up the activities of the beta
decays in table 19, it is calculated that the gamma radiations give a total beta
activity in the Tessar lens of 38.2 Bq whereas the beta measurement only gave 5.4
Bq. The large difference could, partially, be explained by the purely geometrical28

beta detector efficiency calibration resulting in an overestimation of the efficiency.
A second explanation is the fact that most Q-values for decays in the thorium-232
chain lay outside the range of the beta detector. Yet a third reason is given by the
cut-off that the beta detector makes at low energy levels. The result from the
gamma radiation experiment is therefore regarded more precise.

4.2 Radiation Dose from the Carl Zeiss Tessar
With the activity of the lens established, it is desirable to calculate the radiationdose a photographer would absorb while using the Carl Zeiss Tessar objective. This
will possibly illuminate the radiation related hazards involved with the lens. In
order to do this, the first thing to do is to go over the radiation dose limits set by
the Swedish Radiation Protection Authority. This will give values to compare theresults to. The yearly maximum effective dose for a person not working withradioactive materials is 1 mSv [9]. If the radiation is incident on the eye’s lens,
however, the limit is set at 15 mSv equivalent dose [9]. Beta radiation is stopped
much easier than gamma since beta radiation interacts with other particles quite
readily, much more frequently than gamma particles. Therefore, one can assume
that the beta radiation will be incident on the user’s eye whereas the gamma
radiation will instead be evenly absorbed by the user’s body.
The mean beta energy in table 2 shows, for each beta decay in the thorium-232
decay chain, the mean beta energy radiated per decay. This yields, for the beta
activities deduced in the gamma measurements, a total mean beta energy persecond of 14 MeV/s, which is equal to 2.2*10-12 J/s, for the entire thorium-232
decay chain.
Assuming that a photographer works for 240 days per year and that he has thecamera held towards his face for one hour every day, his eye would be exposed tothe beta radiation for 240 hours per year. The radiation does not all go directly into
the eye, but rather is emitted evenly in a sphere around the camera lens. By
assuming that the eyes lens has a diameter of 10 mm and that the camera lens islocated 150 mm away from the eye, it can be calculated that aroundof thebeta radiation reaches the eye. The mass of an eyes lens, L (in mg), can becalculated by the use of formula (6) below [10].
(6)29Here, A is the age of the person. An average professional photographer is assumed
to be of age 40. This gives a mass of the eye’s lens as 193 mg. This would give anannual absorbed dose, , according to formula (7).
(7)
To be able to compare this to the maximum dose set by the Swedish Radiation
Protection Authority (15 mSv/year), the absorbed dose must first be converted tothe equivalent dose,. This is done using the formula below.
(8)
Here,
is the radiation weighting factor which is equal to 1 for electrons.
Therefore, 2.77 μGy absorbed dose translates to 2.77 μSv equivalent dose. From
this, it is seen that the contribution to the annual maximum radiation dose to theeye’s lens from this camera lens is 0.2 ‰.This is indeed a contribution, but a very
insignificant one. This amount of usage of the Carl Zeiss Tessar lens is therefore
deemed non hazardous in terms of beta radiation.
The gamma radiation absorbed by a photographer is equally interesting. By
observing the intensity of gamma rays at different energy levels emitted from acertain decay, a mean gamma energy per decay is found [3].
Table 17: Table of the gamma energy released from the decays in the thorium-232
chain for the Carl Zeiss Tessar.
Parent nucleus
Th-232
Ra-228
Ac-228
Th-228
Ra-224
Rn-220Po-216
Pb-212
Bi-212
Po-212
Tl-208Mean activity[Bq]
9.56
9.55
9.55
9.55
9.55
9.55
9.55
9.55
9.55
6.11
3.43

Mean total gamma
energy released
Emitted gamma power[keV/decay][keV/s]
14.5
139
13.2
126
478
4570
27.3
261
216
201
550
5250
805
7680
154
1470
561
5350
0.00
0.00
1480
5070

30

By adding the emitted gamma power a total energy release of the thorium-232
chain for the Carl Zeiss Tessar is found to be 32 MeV/s which is equal to 5*10-12
J/s. Since gamma radiation is not shielded by the camera itself, a photographer
using the Carl Zeiss Tessar would receive gamma radiation to the whole torso
whilst carrying the camera. The assumption is made that an average photographer
carries his camera for all eight hours of his working days. One third of the gamma
radiation is assumed to be absorbed by the whole body of the photographer who
has a mass of 70 kg. The absorbed dose, , is then calculated by the following
formula.
(9)This is converted to effective dose,, according to formula (10).
(10)

In this case, the whole body is irradiated, meaning that the tissiue weighting factor,, is equal to 1.
is given using the same calculation as in formula (9) with. This gives an effective dose from gamma radiation from the Carl Zeiss
Tessar lens on the whole body of the photographer of 0.17 μSv. This, compared tothe standards set by the Swedish Radiation Protection Authority, is only 0.17‰ ofthe maximum allowed dose.

4.3 Error Analysis
The systematic errors of the measurements done in this project consisted mainly ofthe efficiency calibrations. All of the measurements for the efficiency calibrationof the gamma detector displayed large amount of pile-up. Therefore, thegeometrical efficiency had to be extrapolated from the best point of measurement.
This means that the efficiency of the gamma detector may very well differ from theresult in this report. The efficiency calibration of the beta detector was done purely
geometrical. The lens was assumed to radiate as a point source located at the centerof the objective which, of course, is inaccurate. This means that the distances fromthe edges of the objective were unaccounted for. The intrinsic efficiency of thedetector itself was also assumed to be perfect. These assumptions ultimately resultin an overestimated efficiency of the beta detector.

31

The absolute error of the calculated areas, , (as described in the beginning ofsection 3: Results) propagates through all calculations according to the error
propagation formulae.
This error analysis has only been done for the Carl Zeiss Tessar since this objective
has been the focus of the rest of the discussion. The table below shows the result of
this error analysis.
Table 18: The original area errors and the resulting error for total isotope activity
for the Carl Zeiss Tessar
ParentNominalAbsolute area Absolute area
Absolute total
nuclideenergy [keV] error for theerror for theisotope activity
Carl Zeissbackgrounderror [Bq]
Tessar peaks. peaks.
212Pb
228Ac
228Ac
208Tl
212Bi
228Ac
208Tl
228Ac

238.632
338.32463.004
583.187
727.33794.947860.557
911.204

770
609
412
451
324
272
284
383

823
560
445
488
410
356
338
466

1.38
5.25
11.5
0.76
9.00
13.2
4.76
3.32

This generates, through the continued use of the error propagation formulae, anabsolute error for the total activity of the Carl Zeiss Tessar of 10.5 Bq. The
propagation of the errors is highly noticeable since the resulting errors are
obviously very high, even though only areas with relative errors below 10 % were
used.

32

5 Summary and Conclusions
Gamma radiation measurements were made on eight different camera lenses andthree camera houses. The gamma spectra produced showed that most lenses and,indeed, also camera houses had some radioactive activity. One camera lens, theCarl Zeiss Tessar, stood out amongst the rest with a higher activity. A
complementary beta radiation measurement was conducted on this lens. Efficiency
calibrations were made on both detectors to give the efficiency curve, shown infigure 7, for the gamma detector and an efficiency value of 9.8% for the beta
detector.
The total measured activity of the Carl Zeiss Tessar lens was measured to be 96
10 Bq. The total beta energy released per second from the thorium-232 decay chain
from the Carl Zeiss Tessar objective was about 14 MeV/s. This would give acontribution to the maximum annual radiation dose of the eye’s lens, set by the
Swedish Radiation Protection Authority for a person not working with radioactive
materials, of merely 0.2 ‰ for an average professional photographer.
The total gamma energy released every second from the Carl Zeiss Tessar, on theother hand, was almost 32 MeV/s. The contribution from this for a photographer tothe maximum annual dose to the whole body was calculated to be just 0.17 ‰.
This is comparable to the increased dose from beta radiation. Thereby the beta andgamma radiation from the Carl Zeiss Tessar are equally non hazardous.
These ratios are so close to zero that the conclusion drawn in this thesis is that
there are no radiation related health hazards involved in the usage of any of thecamera lenses measured.

33

Bibliography[1] Paul F. De Paolis Optical Glass, US Patent 2,466,392[2] Thoriated Camera Lens (ca.1970s). (Last updated: 2001-09-24) Available:
http://www.orau.org/ptp/collection/consumer%20products/cameralens.htm[2013-04-23][3] National Nuclear Data Center Available: http://www.nndc.bnl.gov/nudat2/[2013-04-12][4] Radiation Safety Information Avalible:
http://www.nuclear.kth.se/courses/lab/latex/radsafe.pdf [2013-05-16][5] GMX Series Coaxial HPGe Detector Product Configuration Guide
Available: http://www.ortec-online.com/download/gamma-x.pdf [2013-04-23][6] Germanium Detectors Available:
http://www.canberra.com/products/detectors/pdf/Germanium-Det-SSC36151.pdf [2013-04-23][7] Hasselblad 1000F Instruction Book Avalible:
http://hasselbladhistorical.eu/PDF/Brochures/1000F_Man.pdf [2013-05-19][8] What types of radiation are there?, Avalible:
http://hps.org/publicinformation/ate/faqs/radiationtypes.html [2013-05-10][9] Swedish Radiation Safety Act (Strålskyddslagen), SFS 1988:220[10]
Spencer, RP. (1976). Change in Weight of the Human Lens with Age.
Annals of Ophthalmology, Apr;8(4), 440-1.

34

6 Appendix A: Efficiency Calibration
The efficiency calibration was done by placing four known sources of radiation(americium-241, barium-133, cesium-137 and cobalt-60) at five different points inthe chamber. These are marked with red crosses in figure 9. The points werechosen to represent the geometry of the investigated objectives and camera bodies
and the radioactive samples were chosen to cover the energy range of interest for
the thorium-232 decay chain. Then, the measured activity in each point wascompared with the nominal activity of the calibration sources. In this way, themeasured efficiency curve should represent both intrinsic and geometricalefficiency.

Figure 9: The points of measurement in the chamber for the efficiency calibration.
Compare to figure 2.
For the gamma detector, the four control samples had their activity measured on
the 1st of October, 1988, and the calibration took place on 11th of April, 2013,giving a discrepancy of 8 957 days.
The current activity is calculated as(11)
where T1/2 is the half-life in days, and t = 8 957 days. This yields the resultspresented in Table 19 below.
35

Table 19: The activity of the control samples.
SampleHalf-life(days)

Initial activity (kBq)

Current activity (kBq)

Americium-241

153004463.98446

Barium-133

3838.7387.02

77.3

Cesium-137

11019

391.46222.84

Cobalt-60

1925.2401.82

15.99

Figure 10: Spectrum of gamma radiation from control samples in point (0,0).

36

Figure 10, above, shows the measured radiation in point (0, 0), as displayed infigure 9, during the efficiency calibration. This spectrum displays massive amountsof pile-up. Pile-up refers to when too much radiation is incident on the detector for
it to handle resulting in the wider type of peaks seen in the plot. This renders thevalues from this measuring point to be useless.

Figure 11: Spectrum of gamma radiation from control samples in point (5, 5.95).
All points exhibited pile-up, but the point (5, 5.95) had the best spectrum in termsof the least pile up. This spectrum is shown in figure 11, above. Therefore this plot
was used in the calculations of the detectors efficiency. By comparing themeasured activity for different energy levels with the calculated activity in table 19
above, the efficiencies in point (5, 5.95), as shown in figure 9, are calculated for
different energy levels.

37

By assuming the geometrical efficiency to be dependent on the distance to thedetector crystal squared, the geometry was accounted for by calculating theefficiency for different energy levels in the remaining points. Figure 3 displays the
distances from all points to the center of the detector which are used in formula(12) below.
(12)
Here,

is the total efficiency of the detector, with geometry accounted for,is the distance from point (5, 5.95) to the center of the detector andis the mean value of the inverse squared distances from all points to the detector
center. From this a mean value of efficiency for all points could be calculated and a
cubic spline was then fitted to this data.
Table 20: Energy peaks from control samples, point (5, 5.95).
Experimental
Energy [keV]
59.54
80.9979302.8508
356.0129383.8485
661.657
1173.231332.492

Decay
241Am > 273Np
133Ba ->
133Cs
133Ba ->
133Cs
133Ba ->
133Cs
133Ba ->
133Cs
137Cs ->
137Ba
60Co ->
60Ni
60Co ->
60Ni

Measuredgamma
intensity
Intensity
[1/s]
[%]

Total
activity[Bq]

Nominalactivity[kBq]

Efficiency

174.6

35.9

485.84446

0.00110

121.2

32.9

367.7477.3

0.00480

17.08

18.34

92.92

77.3

0.00121

28.5

62.05

45.76

77.3

0.000597

21.25

8.94

237.4177.3

0.00310

42.89

85.1

50.37

222.840.00220

3.424

99.85

3.42

15.99

0.0002142.723

99.9826

2.72

15.99

0.00017

38

Table 21: The mean value of efficiency at different energies for the gamma
detector.
Efficiency
Efficiency
Efficiency Efficiency
Efficiency at
Energyat point (0,at point (0,at point
at point
point (5,Efficiency
Level [keV] 0)
5.95)(5, 0)(2.5, 3.3)
5.95)
Mean Value
59.54
0.057190.057190.001100.033690.001100.0032980.9979
0.05637
0.056370.00480
0.033200.004800.00431302.8508
0.029410.029410.001210.017320.001210.00180356.0129
0.026850.026850.00060
0.015820.00060
0.00132
383.84850.018110.018110.003100.01067
0.003100.00242661.657
0.010680.01068
0.002200.006290.002200.007211173.230.009640.009640.000210.005680.000210.00040
1332.492
0.057190.057190.000170.033690.000170.00033

Figure 12: Cubic spline of the gamma detector efficiency over our energy interval
for point (5, 5.95)

39

Figure 13: Cubic spline of the gamma detector efficiency over our energy intervalwith geometry accounted for.
For the beta detector, the assumption is made that the detector itself is 100%efficient given that a beta particle is incident on the detector. The plastic
scintillator has a diameter of 50 mm, giving an area of
m2. The lens
was placed on the lowest shelf of the chamber which was 70 mm from the detector,
as seen in figure 3. The lens itself was 50 mm high. Assuming that the lens radiates
as a point source located at the center of the lens, and that this point source radiates
evenly in all directions the geometrical efficiency of the detector will then be theratio of the area of the detector and the area of the sphere with center at the point
source and radius as the distance between the point source and the detector.
(13)
The efficiency of the beta detector is thereby 9.8% with this setup. The uncertaintyof this value is of the order of a few percent.

40