NaI(Tl)/ LaBr3(Ce)/ CeBr3

Scintillation Spectrometry [NaI(Tl), LaBr₃(Ce), CeBr₃, SrI₂, … others]

For a long time users avoided nuclear spectrometry with scintillation detectors because resolution (defined as the FWHM of peak at 661.6 keV) is not good and there was no suitable computer software for quantitative analysis of scintillator spectra, in particular for the analysis of small peaks on high background or of overlapping multiplets.

A number of recent developments have initiated a spectacular renaissance of spectrometry with scintillation detectors:

• Technology of NaI(Tl) crystal production has impoved and one frequently finds detectors having resolution well below 7%.
• Modern photomultipliers have significantly improved properties through dynode construction and better conversion of photon energy into free electrons.
• LaBr₃(Ce) detectors have resolution below 3% and their full energy peak efficiency above 100 keV is higher than that of NaI(Tl) detectors.
• CeBr₃ detectors have resolution around 4% and a very high efficiency for higher gamma-ray energies – both advantages at a very reasonable price.
• Because of high average nuclear charge Z in LaBr₃(Ce) and CeBr₃ detectors and the concurrent high electron density in the crystal the full energy peak efficiency is high also for very high photon energies.
• Modern analysis software such as SODIGAM can quantitatively deconvolute complex multiplets measured with new scintillator materials.

Our SODIGAM program is the most advanced software for the analysis of scintillator spectra. SODIGAM is the only program on the market in which spectral properties are derived from physics of interaction of photons with the detector crystal material via Photo- and Compton-effect as well as pair production. This physics-oriented description yields physically correct peak shapes, the course of baseline and background as well as other dependences. The program is best suited to analyse small peaks on high background, complex multiplets or small shoulder peaks.

Thanks to these now available possibilities scintillation spectrometry has gained significant ground. In many applications one uses scintillation detectors rather than cooled high-resolution HPGe detectors and results are found without significant loss of accuracy. Replacement of HPGe by scintillation detectors is dominant in nuclear medicine, biochemistry and environmental supervision as well as in many technical applications such as on-line gauging in steel-mills or purity control during smelting of steel, aluminium or other metals.

There are many application where use of cooled high-resolution HPGe detectors is absolutely necessary; however, many measuring tasks can nowadays be well solved with scintillation detectors.

Significant advantages of scintillators over HPGe are:

• No running cost for liquid nitrogen (LN₂) and operating personnel
• No handling of LN₂
• The spectrometer is small, handy and portable
• The spectrometer operates >5 hours from the notebook battery
• The efficiency of the scintillator crystal is very high
• Investment and maintenance costs are significantly lower than for HPGe

NaI spectrometry
An example:
Portable NaI(Tl) gamma-ray spectrometer with high-precision analysis for in-situ measurements

We assemble and supply such systems, fully configured and tested. An unlimited and cost-free update service is provided for our softwares.

• MCA basic unit: HP or similar high-end Notebook-PC with WINDOWS® 10. The system runs for over 5 hours from the Notebook battery.
• MCA: e.g. bMCA-multichannel analyzer with up to 4096 channels spectra, high voltage up to 1500 Volts, preamplifier, digital amplifier/shaper, 25 MHz flash ADC and MCA, integrated in a 14-pin standard socket, with 3 meters USB cable
• Detector: NaI(Tl)- or BrilLanCe- or CeBr3 detector (2″x 2″ or any other desired size)
• Spectrum analysis: SODIGAM high-precision spectrum analysis and quantification

The batch-feature of SODIGAM is particularly useful for two applications:

1. Manually start a batch file and let the analysis run. As batch files can call other batch files, very complex analyses can be structured and operated in this way.
2. One can define a “procedure” in which all relevant files needed for quantitative analysis (e.g. efficiency, external radiation background, energy calibration, application library, …..) are defined. Also provided is an operational procedure file in which all analysis options are explicitly defined. When the procedure is activated one simply provides the sample mass and decay time (if relevant) and SODIGAM will automatically perform high-precision spectrum analysis. The resulting ASCII protocol file is shown on screen and stored together with the spectrum. The protocol can in addition be

• transfered to the printer for immediate documentation
• stored as .PDF file
• stored on a back-up device for safe data integrity

In this way loss-free documentation of analyses is guaranteed. If the same spectrum is analysed several times result files will be numbered .001 to .999 and no analysis information goes lost.

Preparation and course of measurement
Before the real sample measurement is started one should test proper function of the multichannel analyser and detector by a short measurement of a calibration or reference sample. If no counts appear in the spectrum something is wrong (in most cases power or HV is off) and one must find the reason. We will support you by phone if necessary.

Fill the sample container (Marinelli, beaker, can, ..) to the indicated mark for which an efficiency function was determined. In Marinelli beakers this mark is normally a thin line just under the cover of the Marinelli. Wipe and dry the sample container with a clean rug or cloth in order to avoid contamination of the detector. If contamination is nevertheless possible it is recommended to cover the detector with a thin plastic bag which can be exchanged when necessary. Place the sample container carefully onto the detector and avoid shocks or pressure on the detector – the aluminium has only 0.5 mm thickness! Remember that each puncture or damage leads inevitably to loss of the hygroscopic detector crystal.
Attention! Insertion of a 1-liter Marinelli beaker into a standard lead castle goes best when the beaker is held with crossed hands.

Close the cover of the lead shield, define desired counting time (Livetime if possible) and start the measurement. When measurement is over start spectrum analysis via batch or procedure files. Make sure that the result file is stored and named accordingly so that it can be recovered and assigned properly. The radioactive sample must be collected in a disposal vessel or other appropriate container for radioactive waste. Inactive samples can be disposed of as regular waste.

Marinelli beakers can be rinsed after use, starting with slightly acid water, and dried for repeated use. When significant contamination by for example ¹³¹I is detected one must store the container for 3 months and test integrity before re-use.

Information for applications in radiology and nuclear medicine
When the analyzed activity in below 5 Bq/l of ¹³¹I one can release the content of the tank from which the sample was taken into the public sewage system. According to StrlSchV Anlage III, Tabelle 1 the exempt activity of ¹³¹I is 2 kBq/l; however, this value does not apply for release from supervised areas like radioactive waste decay systems. According to Anlage VII (§§ 29 and 47 StrlSchV) a maximum activity concentration of 5 Bq/l of ¹³¹I is allowed when waste water is released from supervised areas. If the total amount of released water from the decay system is below 10⁵ m³/year then the maximum release activity concentration is 50 Bq/l. The latter limiting value is not acknowledged by all supervising authorities.

Service and Repair
Dr. Westmeier recommend an annual service and recalibration for all systems provided; customers are informed in advance and can order service as proposed. Service works comprise of: check of the technical status of electronics and detector, cleaning and lubrication, check and improvement of calibrations, addition of software updates and back-up of files from the recent year. Small repair actions can be made immediately if the customer agrees. Moreover, the system is inspected and tested according to DGUV V3 and proper status is confirmed with a sticker. A service report, maybe with indication of due or coming repair work, and a Test Protocol according to DGUV V3 are provided to the owner of the spectrometer.
In case of immediate failure we provide Spot-Service and repair within a typical reaction time of 3 working days (plus time to find spare parts, if necessary).
Update versions of our software are provided at no extra cost with every service, provided that it contains relevant improvements.

You can reach us during normal working hours 8 a.m. to 5 p.m. for support, information and consultation; moreover 24/7/12 by FAX and Mail.
Whenever you find peculiarities in your results or you are insecure about an analysis, send us the spectrum and your operations/calibration files as an attachment by mail. We will check results, consult you and solve your problem.

Dr. Westmeier, Möllner Weg 5, D-35085 Ebsdorfergrund, Germany
info@westmeier.com, Tel. +49 (0)6424 923 000, FAX +49 (0) 6424 923 002