determination of the dead-time losses in nai(tl) gamma-ray...
TRANSCRIPT
Determination of the dead-time losses in
NaI(Tl) gamma-ray spectrometer
D. Grozdanov1,2
, I. Ruskov1,2,*
, N. Janeva1, Yu.N. Kopach
2, S.I. Negovelov
2, Yu.D. Mareev
2
1Institute for Nuclear Research and Nuclear Energy (INRNE) of the Bulgarian Academy of Sciences
(BAS), Tzarigradskochausseblvd. 72, 1784 Sofia, Bulgaria
2Frank Laboratory of Neutron Physics (FLNP) of the Joint Institute for Nuclear Research (JINR),
Joliot Currie 6, 141980 Dubna, Moscow region, Russia
* Corresponding author Tel.: + 7-496-216-2785; fax: +7-496-216-5085. E-mail : [email protected].
Abstract
The 12 NaI(Tl)-detector gamma-ray spectrometry system (GSS) “Romashka”[1,2] has
recently been equipped with a new computer controlled HVSys 16 channel high-voltage
power supply with preselected single channel polarity [3] (Fig.1).
In order to do some preliminary checks of the GSS chains’ characteristics, a single
channel gamma-spectrometry tract was commissioned. It was consist of a single Romashka’s
section - NaI(Tl) crystal, optically coupled with a photo-multiplier tube FEU-110, a
POLONTM
Active Filter Amplifier and an EG&G ORTEC TRUMPTM
-2k Multichannel
Buffer (MCB) PC- compatible plug-in card, run by MaestroTM
MCA Emulation and Analysis
Software for Windows®, having real-time and life-time acquisition presets [4].
Two calibration sources method was applied for obtaining the effective dead-time (and
dead-time losses) of a single spectrometry chain. It was used, also, to determine the dead-time
of the chain when a ParsekLLC
desktop 8k multichannel analyzer (MCA) with no live-time
measured/indication possibility [5]. The testing of the whole gamma-ray spectrometry system
with the new HV power supply has been recently accomplished.
It is planned to use the modernized “Romashka” GSS at the JINR FLNP resonance
neutron time-of-flight (TOF) spectrometer IREN, for investigation of the gamma-rays
emission from neutron inelastic scattering, capture and fission reactions, using the multiplicity
method [2].
Keywords: Gamma-rays, spectrometry, multichannel analyser, dead-time.
Multi detector NaI(Tl) gamma-ray spectrometry system
The multi-detector gamma-ray spectrometer "Romashka" consists of 12 NaI(Tl)
crystals of total volume about 16.6l, which are grouped in two sections by 6 crystals in a
common casing (Fig.1). Each crystal is optically coupled with FEU-110. The outer diameter
of the sections is 30cm, the inner diameter is 4cm, and thus a solid angle of ~4 can be “seen”
by the sample when situated in the center between the both sections.
Fig.1. Twelve crystals gamma-ray spectrometry system "Romashka": 1– Six NaI(Tl) crystals in a
single Al cylinder wrapped with Al arc-seals, 2– Photo multiplier tubes FEU-110 and high-
voltage divider, 3– Pb shielding-collimator, 4 and 5– Lower and upper Pb shielding arcs, 6–
Adjustable rigid iron support, 7– Movable iron trolley, 8– 16 channels HVSys power supply,
9– Parsek Desktop 8k MCA.
Counting system dead-time
Every radiation detector takes a time to measure and record an event, during which it
is unable to cope with the next ones [6]. This is the signal detecting system dead-time. During
this time-interval some events are not recorded - they are lost. Two types of dead-time can be
distinguished:
Non- paralyzable dead-time – this type of dead-time can be constant or variable, but it
doesn’t depend on the events which enter the system during it. In such a case, the average
counting rate n can be determined by the following formula [7]:
or (1)
Here, no is the intensity of the incoming events; is non- paralyzable dead-time; n and
no are generalized parameters, which characterize the counting rate and dead-time.
Paralyzable dead-time – this type of dead-time is added after every event, whether
registered or not. For this prolonged dead-time the average counting rate can be determined
by the formula:
or ; (2)
Here is the paralyzable dead-time.
When the experimental setup consists of several devices, which obey different types of dead-
time, the determination of the system’s “effective” dead-time cannot be an easy task.
Experimental setup
The block-scheme of the experimental setup for determination of the effective dead-
time is shown in Fig.2. Two different data acquisition systems (DAQs) have been tested
simultaneously. One of them utilizes an EG&G ORTEC TRUMPTM
-2k Multichannel Buffer
(MCB) PC-compatible plug-in card, driving by MaestroTM
MCA Emulation and Analysis
Software running under Windows® and having factory real-time and life-time acquisition
presets [4]. The second DAQ system is using an USB powered and controlled8k ADC board
and software from ParsekLLC
[5], with no dead-time calculation or correction features. By
comparing the spectra taken simultaneously with the both DAQs one can determine the
"live/dead" time of the second one.
Fig.2.Block-scheme of the experimental setup: Source–137
Cs and/or 137
Cs’; HV–HVSys computer
controlled high-voltage; Oscilloscope– TDS 2014C, Polon Timing-filter Amplifier 1101,
ADC-1– EG&G Ortec 2k MCA, ADC-2–Parsek8k MCA.
Table 1. Main characteristics of the calibration -sources
Date № Source Activity
(Bq)
Error
(%) E
(keV)
E
(keV)
I
rel
I
(%)
I
16.04.2012
R1 137
Cs 88383.08 2.0
661.657 0.003 100 85.1 0.5 R2 137
Cs' 99229.92 1.7
R 137
Cs+137
Cs' 187613.00 2.6
Fig.3. Two calibration sources positioning before the detector face.
The dead-time of each spectrometric system was determined by the two sources
method [2, 4], using two 137
Cs calibration sources R1 and R2 of different activities, according
to the procedure, depicted on Fig. 3, following the steps below:
- acquiring an amplitude spectrum with source R1;
- acquiring an amplitude spectrum with both sources R (R1 and R2 together);
- acquiring an amplitude spectrum with source R2;
The dead-time loses (DTL) (%), obtained with Ortec software [4], are shown in Table 2.
Table 2.
Source Real time (s) Live-time (s) DTL (%)
R1 600 569 5.17
R2 600 569 5.17
R 600 541 9.83
The true count rate from the both -sources no can be written as a sum of the true count
rates from each (n01 and n02) of them [7]:
; (3)
Suggesting that the dead-time of the spectrometric chain used is upmost of non-
paralyzed type, from the formulas (1)-(3), one can write the following relations, from which
the dead-time, live-time and dead-time losses (DTL) values can be determined
straightforward as follows from:
where τ, and a are expressed by the following formulas:
; (4)
; (5)
;
;
; (6)
If then .
If then or then .
; (7)
; (8)
; (9)
Experimental results
The procedure for determination of Parsek 8k MCA DAQ system [5] dead-time, from
the acquired amplitude spectra (Fig. 5 7), is the following:
1) The amplitude spectra counts are summed over all the channels;
2) Photo-peak areas are determine by fitting of the corresponding amplitude
spectra by OriginLab©
Data Analysis and Graphing Software Origin 9.1,using a Gauss Area
function;
3) The dead-time losses are calculated from the obtained photo-peak areas and
formulas 4 9;
4) In order to compare the both DAQ systems, the same procedure was applied to
the Maestro 2k MCA data;
5) The results obtained are shown in Tables 3 through 6.
0 100 200 300 400 5000
10k
20k
30k
40k
50k
60k
70k
Maestro 2048 channels MCA - Amplitude spectre
C
ou
nts
Channel
R
R1
R2
0 1000 2000 3000 4000 5000 60000
1k
2k
3k
4k
5kParsek 8192 channels MCA - Amplitude spectre
Co
un
ts
Channel
R
R1
R2
Fig.4. Amplitude spectra collected with Maestro2k MCA (left) and Parsek8k MCA (right).
100 120 140 160 180 200 220 240 2600
10k
20k
30k
40k
50k
60k
70k
Co
un
ts
Channel
Maestro 2048 channels MCA Photo-peak R1
1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 30000
1k
2k
3k
4k
5kParsek 8192 channels MCA Photo-peak R1
Co
un
ts
Channel
Fig.5. Gauss function fittedto the -rays amplitude spectra from R1-source, collected with Maestro
2kMCA (left) and Parsek8kMCA (right).
100 120 140 160 180 200 220 240 2600
10k
20k
30k
40k
50k
60k
70k
Maestro 2048 channels MCA Photo-peak R2
Co
un
ts
Channel
1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 30000
1k
2k
3k
4k
5kParsek 8192 channels MCA Photo-peak R2
Co
un
ts
Channel
Fig.6. Gauss function fittedto the -rays amplitude spectra from R2-source, collected with Maestro
2kMCA (left) and Parsek 8kMCA (right).
100 120 140 160 180 200 220 240 2600
10k
20k
30k
40k
50k
60k
70kMaestro 2048 channels MCA Photo-peak R
C
ou
nts
Channel
1000 1200 1400 1600 1800 2000 2200 2400 2600 2800 30000
1k
2k
3k
4k
5kParsek 8192 channels MCA - Photo peak R
Co
un
ts
Channel
Fig.7. Gauss function fittedto the -rays amplitude spectra from both sources R (R1 and R2), collected
with Maestro 2kMCA (left) and Parsek 8kMCA (right).
Table 3.
Source Maestro 2k
MCA
N
(imp)
n
(imp/s)
(s)
(s)
Real
(s)
Live
(s)
DTL
(%)
R1
Sum over
the channels
1 - 1938
3202692 5337.8
0.908 9.1 0.3
600 571 4.85
R2 3188109 5313.5 600 571 4.83
R1 + R2 6390801 10651.3 600 542 9.68
R 6095842 10159.7 600 545 9.23
Table 4.
Source Parsek 8k
MCA
N
(imp)
n
(imp/s)
(s)
(s)
Real
(s)
Live
(s)
DTL
(%)
R1 Sum over
the channels
1 - 7430
2200764 3667.9
0.869 16.9 0.2
600 563 6.19
R2 2771986 4620.0 600 553 7.80
R1 + R2 4972750 8287.9 600 516 13.99
R 4651930 7753.2 600 521 13.09
Table 5.
Source Maestro 2k
MCA
N
(imp)
n
(imp/s)
(s)
(s)
Real
(s)
Live
(s)
DTL
(%)
R1 Fitting of the
photo-peak
with Origin
Gauss area
function
110 -240
1572510 2620.9
0.955 8.5 0.8
600 587 2.24
R2 1668530 2780.9 600 586 2.37
R1 + R2 3241040 5401.7 600 572 4.61
R 3168030 5280.1 600 573 4.51
Table 6.
Source Parsek 8k
MCA
N
(imp)
n
(imp/s)
(s)
(s)
Real
(s)
Live
(s)
DTL
(%)
R1 Fitting of the
photo-peak
with Origin
Gauss area
function
1300 - 2600
1092430 1820.7
0.929 17.4 0.6
600 581 3.17
R2 1437780 2396.3 600 575 4.17
R1 + R2 2530210 4217.0 600 556 7.33
R 2442390 4070.7 600 558 7.08
Conclusions
Two calibration sources method was used for determination of the non-paralyzed
effective dead-time of two DAQs, one of which has instrumental dead-time correction
implemented.
The average effective dead-time of the chain with the plug-in Ortec 2k MCA board,
determined by 2 calibration sources, was found to be eff = (9.0±0.2)s.
The experimentally determine effective dead-time is in agreement with the expected
effective one (8s ORTEC ADC + 1s Amplifier Time Constant).
For the first time was determined the effective dead-time of Parsek 8k USB connected
Desktop ADC board. It was found to be eff = (16.9±0.2)s. The corresponding effective dead-
time losses were in the range of ~ 313%, depends on the counting rate [9].
Acknowledgements
The authors would like to thank N.A. Gundorin and S.S. Pavlov for submitting the
OSGI calibration sources, N. Koyumdjieva and V.N. Shvetsov for their permanent interest in
their work.
References
[1] N.Chikov, N. Janeva, G. Rashkov, N.Vassilev, 4 Multidetector System ROMASHKA for
Low Gamma Activity Measurements, INRNE - JRC Conference - Informational Days,
18.02.2003, Sofia,
http://www.beo.inrne.bas.bg/BEOBAL/BEOBAL_Presentations/2422_NJ.pdf,
http://www.beo.inrne.bas.bg/BEOBAL/BEOBAL_Presentations/2423_NV.pdf.
[2] G. V. Muradyan et al., Preprint 2634, Kurchatov Atomic Energy Institute, Moscow, 1976.
[3] Multi-channel high-voltage power systems, HVSys, Dubna, http://hvsys.ru/en/,
http://hvsys.ru/images/data/news/6_small_1368801975.pdf.
[4] TRUMP™-PCI-8k/2kPCI Format MCA Plug-In Card and Software
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online.com/download/ORTEC-Software-File-Structure-Manual.pdf.
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http://www.parsek.ru/en/products.html,
http://www.parsek.ru/en/products/4KSACPUSB.html.
[6] G. F. Knoll, Radiation detection and measurement, Wiley, 2000.
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(in Russian).
[8] Origin 9.1 - OriginLab©
Data Analysis and Graphing Software,
http://www.originlab.com/.
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Thesis, University of Sofia, 2012, (in Bulgarian).