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Activity standards in the form of solid sources

Point sources of individual nuclides

Activity standards in the form of point sources are intended for the calibration of photon spectrometers. The radioactive substance is a very thin, compact-grained layer applied to a circular area about 5 mm in diameter, in the middle of the source between two polyethylene foils, each having a mass per unit area of (21.3 ± 1.8) mg·cm-2. By heating under pressure, the two foils are welded together over the whole area so that they are leak-proofed. To facilitate handling, the foil 26 mm in diameter is mounted in a circular aluminium ring (outer diameter: 30 mm, height: 3 mm) from which it can easily be removed if and when required.

As the mechanical stress to which the sources are subject is low during the intended use, they are to be regarded as sealed radioactive sources in accordance with the German Radiation Protection Ordinance. It must, however, be ensured that the sources are not exposed to strong solvents and temperatures above 100 °C. Prior to their delivery, the sources are submitted to a wipe test to check their leak tightness and absence of contamination; they are released only if the activity of the wiped sample does not exceed 20 Bq. An emanation test is performed for Ra-226 sources.

The activity A is stated in the calibration certificate accompanying the source. The emission rate B for photons of a particular energy, which is often of interest, is determined from A and from the emission probabilities p stated in an information sheet according to B = p·A.

The Table "Point sources of individual nuclides" only covers available sources or sources which can be obtained in regular time intervals. It is possible to prepare also solid sources from most of the nuclides stated in the Table "Activity standards in the form of solutions".

Figure 2: Dimensions of sources, in millimetres

Table 4: Point sources of individual nuclides

Nuclide
Half-life
Activity in kBq Uncertainty
in % (3)
Availability
Na-22 950.5 d 4 bis 400 0.7 ex stock
Cr-51 27.71 d 4 bis 400 1 (1)
Mn-54 312.15 d 4 bis 400 0.7 ex stock
Co-57 271.83 d 4 bis 400 1 ex stock
Co-60 1925.3 d 4 bis 400 0.7 ex stock
Zn-65 243.94 d 4 bis 400 1.4 ex stock
Sr-85 64.85 d 4 bis 400 1 (1)
Y-88 106.63 d 4 bis 400 1 (2)
Cd-109 462.1 d 4 bis 400 1.7 ex stock
Ba-133 10.54 a 4 bis 400 1 ex stock
Cs-134 754.0 d 4 bis 400 0.7 ex stock
Cs-137 30.13 a 1 bis 400 1 ex stock
Ce-139 137.66 d 4 bis 400 1 (2)
Eu-152 13.53 a 1 bis 600 1.4 ex stock
Ra-226 1600 a 150 2 ex stock
Am-241 432.2 a
4 bis 400
1 ex stock
(1) The sources are prepared and delivered once a year. The dates of delivery will be announced in good time. Please let us know on the enclosed printed form about which deliveries you wish to be informed.
(2) Raw solutions of this radionuclide are offered only rarely by the producers. The dates of delivery for activity standards are, therefore, undefined.
(3) The uncertainty stated is the relative expanded uncertainty of measurement in percent obtained by multiplying the standard uncertainty by a coverage factor k = 2.

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Point sources with low-energy photon radiation

The sources described under "Point sources of individual nuclides" are only conditionally suitable as activity standards for photon radiation below 100 keV. In this energy range, increasing absorption and scattering effects occur in the relatively thick polyethylene foil. Whereas absorption can be determined rather well by calculation (approx. 1.7 % at 14 keV and 0.5 % at 30 keV), the scattering effects can hardly be quantified. Experiments with Si(Li) detectors have shown that scattering can give the deceptive impression of an up to 2.5 % higher detection probability in the total absorption line.

This is why sources enclosed in considerably thinner foils are prepared for more accurate measurements. In the case of these sources, the radioactive substance is enclosed between two polyester foils, each with a mass per unit area of (2.20 ± 0.07) mg·cm-2. Both foils are welded together and pressed in an aluminium ring (outer diameter: 30 mm). For energies between 14 keV and 25 keV, absorption of the radiation by the foil is less than 0.3 %; for energies above 25 keV it is less than 0.1 %. Scattering effects are negligible.

These sources can be prepared from most of the solutions stated in the Table "Activity standards in the form of solutions". The following nuclides are recommended for the calibration of low-energy photon detectors.

Nuclide Energy in keV


Cr-51 4.95; 5.43
Mn-54 5.41;5.95
Fe-55 5.90;6.49
Co-57 6.40; 7.06; 14.41
Zn-65 8.04; 8.91
Nb-93m 16.58; 18.66
Cd-109 20.10; 25.01; 88.03
Ba-133 30.85; 35.14; 53.16; 79.62; 81.00
Am-241 11.87; 13.93; 17.61; 21.00; 26.35; 59.54

The activity values cover a range between 40 kBq and 200 kBq.

In the case of sources of the nuclides Cr-51, Mn-54, Co-57 and Zn-65, self-absorption of K-x-rays can be observed in the layer containing the radioactivity contrary to absorption in the foil, it can only be estimated with low accuracy. The number of photons emitted per second vertical to the source at a solid angle of 1 steradian is, therefore, stated for these sources, in addition to the activity. For sources of the nuclide Fe-55, only this photon flux is indicated.

As the mechanical stress to which the sources are subject is low during the intended use, they are to be regarded as sealed radioactive sources in accordance with the German Radiation Protection Ordinance. It must, however, be ensured that the sources are not exposed to strong solvents and temperatures above 100°C. Prior to their delivery, the sources are submitted to a wipe test to check their leak tightness and absence of contamination; they are released only if the activity of the wiped sample does not exceed 20 Bq.

Availability: on request.

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Cylindrical sources

These sources are intended for the calibration of NaI(Tl) well-type crystal detectors. The radioactive substance is deposited at the lower end of an aluminium cylinder (outer diameter: 14 mm; height: 38 mm). Sources of the nuclides Co-60, Cs-137, Ra-226 and Am-241 with activities of approx. 1 kBq can be delivered. The relative expanded measurement uncertainty (k = 2) of the activity is 2 %.

Availability: on request.

Figure 3: Construction and dimensions of a cylindrical source.

Point sources with Radium-226

The sources which are intended for the calibration of measuring devices with semiconductor detectors contain Ra-226 in equilibrium with its short-lived daughter products. The Ra-226 of a source is homogeneously distributed in a thin layer of precious metal roller-plated on a silver foil (thickness: 0.2 mm) and covered by a vacuum-evaporated AuPd layer 2 µm in thickness. The active layer is so thin that alpha-particles are still released.

A disc 2.5 mm in diameter with an activity of approx. 100 kBq of Ra-226 was cut from such a foil. The disc has been glued into an aluminium holder (outer diameter: 30 mm, thickness: 3 mm) and covered with an aluminium foil 50 µm in thickness to ensure additional sealing. Release of alpha-particles is prevented by this foil. The photon radiation is absorbed only slightly (0.16 % at = 0.2 MeV, 0.06 % at = 2 MeV). Radon release is below 4 Bq in 12 hours so that this source can be regarded as a sealed radioactive substance in accordance with the German Radiation Protection Ordinance.

When the source is used, the side showing the engraved label should be turned away from the detector, as the silver layer (0.2 mm in thickness) and the bottom of the aluminium holder considerably absorb gamma radiation in this direction.

The activity A of the source is stated in the calibration certificate. The decay data of the source, which are important for its use (half-life T½ and photon emission probabilities p), are stated on the enclosed data sheet. The emission rate B for photons of a particular energy is obtained according to B = p·A.

Figure 4: Sectional view of the sources with Ra-226

Figure 5: Ra-226 metall sources

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Area sources

The determination of the activity of area sources (e.g. filter samples) at short distances from the detector requires calibration sources in which the activity is distributed as uniformly as possible over the area under study. In order to obtain a highly uniform activity distribution, area sources have been developed which are composed of point sources distributed in the form of regular patterns.

For the preparation of these sources, drops of similar mass are taken with a pipette from a calibrated solution and applied to the source backing (plastic foil) in one of the the patterns shown in Fig. 7, the masses of the individual drops vary by ± 5 % at the most. The activity of the source results from the specific activity of the solution used and the total mass of all drops, which is determined by weighing the pipette before and after application of the drops. The total mass ranges from 0.2 g to 2 g, depending on the size of the source. The uncertainty of the total activity is given by the uncertainty of the specific activity of the solution used, as the uncertainty of the drop masses is smaller by one order of magnitude. In the case of photon emitters, the radioactive residue of the dried-up solution is covered with a polyethylene foil with a density per unit area of (12.7 ± 0.4) mg·cm-2; in the case of pure beta emitters, a polyester foil of (2.20 ± 0.07) mg·cm-2 is applied. An adhesive layer on the rear allows the source to be fixed to a suitable support, if necessary, for example to a preparation dish.

On request, such sources are prepared from the mixed solutions and from all standard activity solutions stated in the Table "Activity standards in the form of solutions" - with the exception of H-3, C-14 and Ra-226.

Source sizes: 1 - 5 as indicated in Fig. 7, larger sources on request.

Availability: on request.

Figure 6: Device for the preparation of area sources

Figure 7: Drop patterns for area sources, not to scale

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Alpha-particle sources

These sources serve to determine the detection probability of alpha-particle detectors in well defined geometries. The sources available are 5 mm, 10 mm and 25 mm in diameter and their activities range between 1 Bq and 5 kBq. The radioactive substance is electro-deposited on polished stainless steel discs 30 mm in diameter.

The radionuclide used is Am-241.

Alpha-emitting sources are not provided with a protective layer so that touching of the active area must be avoided. These sources are to be regarded as unsealed radioactive substances in view of the German Radiation Protection Ordinance.

The relative expanded measurement uncertainty (k = 2) of the activity is 2 %.

Availability: on request.

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