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 : ruskov@nf.jinr.ru.

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.

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http://hvsys.ru/images/data/news/6_small_1368801975.pdf.

[4] TRUMP™-PCI-8k/2kPCI Format MCA Plug-In Card and Software

ORTEC MAESTRO v3.20, www.ortec-online.com,http://www.ortec-

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.

[7] S. Dushkin, V. A. Odinets, A.N. Orobinsky, Dead-time measurement, Kharkov, 2009,

(in Russian).

[8] Origin 9.1 - OriginLab©

Data Analysis and Graphing Software,

http://www.originlab.com/.

[9] D. Grozdanov, “Modernization of a 4π multi-crystal system type ROMASHKA”, Master

Thesis, University of Sofia, 2012, (in Bulgarian).