Characteristics of multiware proportional counters

Nuclear Instruments and Methods203 (1982) 477-482 North-Holland Publishing Company

477

CHARACTERISTICS OF MULTIWIRE PROPORTIONAL COUNTERS V.V . KUZMINOV, A.A . POMANSKY and P.S . STRIGANOV Insrirtue for Nuclear Research, The USSR Acndenn" of Sciences, hlor
In studying the aspects of constancy of cosmic rays intensity in the Earth atmosphere for the last several hundred thousand years the necessity of determing the content of "'Kr isotope in various krypton samples arises. For a complete solution of this problem it is necessary to measure: 1) the content of cosmogencous 8'Kr in the atmospheric "pre-bomb" krypton; 2) the content of 8'Kr generated through the reaction ""Kr(n,y)" Kr in a krypton sample irradiated in a reactor ; 3) the content of "Kr generated through the reactions of spallation from the heavier isotopes in a krypton sample irradiated by the proton flux in an accelerator [1,2]. The measurement conditions sharply differ from each other in all these cases. In the first case the krypton sample contains practically no interfering radioactive isotopes, and to record rare decays of "Kr a detector with an extremely low intrinsic background is needed. In the second and third cases the sample contains a large amount of "Kr isotope the background of which several thousand times exceeds the effect of " Kr decay. In these cases the detector is required to isolate relatively rare decays of "Kr in the strong interfering background conditions. All these problems are successfully solved by using the multiwire proportional counters containing in one box the main detector and a surrounding ring of guard counters [3]. The use of materials with a low content of radioaclive admixtures in manufacturing these devices enables one to achieve the allowable' background level of the main counter under low background conditions when the protective ring is active. The absence of a wall between the main and the guard detectors and the good energy resolution allow one to register "Kr decays under the conditions of a strong interfering background' from "SKr. 0167-5087/82/0000-0000/$02 .75 O 1982 North-Holland

We have used two such devices in our work . Their main characteristics aregiven in table I .

Table I P , :,meters of the counters Counter Total length of body [mntl Fiducial length of " 'dy [ntntl Outerdiameter of body [mml Inner diameter of coda Ie Outer diameter of guard ring Diameter of main counter Total volume Jet., l Volume of guard ring Icm'l Volume of main counter Icm'1 "Dead" volume lc.,] Guard ring capacity [pF] Main counter capacity [pFl Kmdof body ---

NI

N2

517

460

437

460

99.5

54.5

95 .5

50

95 .5

46

71 .5

34

3312

903

1375

347

1755

418

i82

139

123

119

17 upper

7 quartz -

478

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1. The structure A simple and reliable structure for fastening the tungsten wire wasdeveloped that allowed one to obtain the multiwire system with the required configuration inside a body made of any material . Fig. 1(b) showsthe cross-secti,on of the counter NI . The dots in the figure denote cathode wires, the crosses anode ones. The cylindrical copper box serves as an external cathode for the protective counters' ring. The protective ring consists of eighteen cells. The cathode of the main counter is formed by the protective ring cathode wires closest to the cylinder axis. The anodes are manufactured from tungsten 0.025 turn diameter wire, the cathodes from tungsten wire with a diameter of 0.050 turn. Such awire configuration is created by meansof a set of positioning rings (10 to 12) made of brass and insulating bushes (8,9, 13) made of plexiglass and teflon [fig . 1(a)]. The fixing of the wires is achieved by meansof thin slots cut in the boards of the positioning rings. The tension of a wire after winding is adjusted by screws. When turning the screws thedistance between the positir..;ng rings at the opposite ends of a counter increases by 3mm. The counter is hermetically sealed by mechanical means and with an epoxy glue. The body of the counter N2 is manufactured from quartz glass. It required introducing one more layer of cathode wires compared to the counter till . The wire fastening system in this detector is stuck to the quartz glass tube, and all connectionsare sealed with an epoxy . Thediameter of cathode and anode wires of the protec-

tive ring is 0.050 mm; the diameter of the central anode wire is 0.025 mm . In other respects the structures of both counters are similar. 2. The gas multiplication coefficient (GMC) Theabsence of a wall between the central and guard counters leads to the dependence of the electrical field in each of them on thevoltage applied to both counters . In order to reveal this dependence measurements of the GMC have been carried out with varying voltage applied to the counters (fig. 2) . Dependence I was obtained without applying high voltage to the protective ring . Dependence 2 was obtained when voltage from a single high-voltage source was appliedto both the central counterand the protective ring . Theworking mixture of a counter consisted of 39 .5% Kr, 50 .5% Ar and 10% CH4 at a pressure of 82.7 kPa. It is seen that applying high voltage to the protective ring decreases the main counter GMC. Without applying high voltage to the protective ring the same GMC is achieved at 100 to 150V lower voltage at the central counter. The voltage dependence of the GMC on a semilogarithmic scale is linear in both cases. Theproportional regime for recording quanta with the energy of 22 keV is maintained up to the GMCvalue of 3 x 10 3. Dependences 3 and4 are related to the protective ring. It is seen from these curves that the protective ring GMC does not depend in practice on the central counter voltage . For the chosen diameters of the anode wires of the central and , lard counters and for the given sizes of the devices (see table 1) it is reasonable to feed them from a single high-voltage source.

GTSC

I04 I03 102 IOI Fig . l . The wall-less multiwim proportional counter with the internal ring of guard counters. (a) Fastening structure of the multiwim systern: 1-the counter body (copper); 2-flange (copper); 3, 5-high-voltage outputs with spring contacts; 4, 6-insulators (teflon); 8, 9, 13-insulating bushes (tenon, plexiglass) ; 10, 11, 12-fixation rings with spreader screws (brass). (b) The counter cross-section. (+)-anode. (-)-cathode .

1,2 1,4 1,6 kV 1,0 Fig. 2. The voltage dependence of the gas multiplication coefficient (GMC) in the detector N2. Curve 1 : the central counter GMC when feeding the central and guard counters from a single high-voltage source. Curve 2: the central counter GMC with the protective ring switche4 off. Curve 3: the protective ring GMC with the central counter switched off. Curve 4 : the protective ring GMC when feeding the central and guard counters from asingle high-voltage source.

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X. 1,5 I,7 kV I,I 1,3 Fig. 3. The voltage dependence of GMC in the detector NI . Curve 1 : the central counter GMC when feeding the central and guard counters from asingle high-voltage source . Curve 2: the central counter GMC with the protective ring switched off . Curve 4: the protective ring GMC when feeding the main and guard counters from a single high-voltage source . Fig. 3 shows similar dependences for the detector N1 . It is filled with 90% Ar and 10% CH 4 at a pressure of 82 .7 kPa. Here we can also see the effect of the high voltage on the protective ring on the working regime of the central counter and the sensitivity of the working regime of the guard counter to the voltage on the central one. All anode wires have a diameter of 0.025 mm and the GMCin the protective ring growsmore rapidly with increasing voltage than in. the central one. At voltages higher than 1640V breakdowns begin in the protective ring . Therefore, in order to obtain at the central counter values of GMC more than 10', two high voltage sources are needed to feed the device. 3. The energy resolution The calibration of the counter N1 was performed through mica windows in its body . The spectrum of impulses from the central counter for calibration with the sSFe source (E=5 .9 keV) is shown in fig. 4. The counter is filled with the mixtureof 90% At + 10% CH ,, up to the pressure of 82.7 kPa, the operational voltage being 1606 V. Thecrosses denote the spectrum obtained without switching the gtmtt: channel to an analyzer, the dots represent the spectrumr : obtained when both the main counter and theprotecth,e one have been switched on to anticoincidences. The collecting time is the same in both cases . The energy resolution in the 5.9 keV-linc is 16.3% . Fig. 5 shows spectra for the to°Cd source. The resolution in the22 keV-line is 8.4%. One may see in the spectrum the lines of the source (EASK = 22 keV, EAßK, =25 keV), the line of the characteristic radiation of copper (EcK = 8.0 keV) and their exit peaks on argon (Et =5 .1 keV, E2 =19.2 keV).

MENEM NEI ME MEN a "l9 MEME am i SISI"

700 I 600 500 400 300 200

I0I

479

-unters

COUNTS~/CHA111EL

I00 0

20 I

40 2

60 3

80 100 120 CHANNP1. 4 5 6 keV

Fig. 4. Thess Fe soune spectrum in detector N1 (9()`i Ar + 109i Ctl p =917 kPa). I . With :-coincidences; 2. without anti, coincidences . The calibration of the counter N2 was performed directly through the quartz body with the t°'Cd source (fig .6). In this case the working mixture consisted of 39.5 Kr +50.5 % Ar+ 10% CH,, at a pressure of 82.7 kPa. The operational voltage was 1430 V. One may see in thespectra thelines of thesource and their exit peaks on krypton (Et =9.4 keV, E:= 12.3 keV). The resolution in the 22 keV-line in 8.8%. The peak in the region of 12 .5 kcV in the spectrum marked by crosses is the superposition of the exit peak (E:= 12 .3 keV) and the characteristic radiation KrK  (E=12.6 keV) coming from the protective ring. Therefore, the ,witching of the rncin and guard counters into the anticoincidence regimc leads to a stronger variation of this peak in coinparison with other ones.

cod, 1200 I000 800 600 400 200 0

MENNEN ~3-2 EMEr MMMI MEN MMMI 111111

man 2.0 4

40 8

60 12

80 16

100 CHAMiEl. 20 keV

Fig . 5 . The "Cd source spectrum in detector N1 . 1. With anticoincidences ; 2. without anticoincidences .

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4811

r7 0: r .;,P

COU1 ':` :: /CIP.:!NEL

120

3000

I00

2500 2000

80

I000

40

60

1500

20

500

0

20 40 60 80 100 120 clxv:!)EI .

4 8 I2 I6 20 24 kev Fig. 6. The n"'Cd source spectrum in detector N2 (39.5`+ Kr ; 51 .5<< Ar f- 10.4i CH,,, p =82.7 kPa). 1 . with anticoincidcuces; 2. without anticoincidences. We have examined the long-term stability of both devices . The 22 keV-line resolution in the counter N2 has changed in three weeks from 8.8% to 10.2°" 1:, and that in thecounter Nl haschanged in two weeks from 8.41i to 10 .5"1-. 4. The counter bakkground The intrinsic background of the counters has been determined in theunderground low-background laboratory [4] using extra protection consisting of 6 cm Pb + 4cm Cu +4 cm W+ 1 .5 cm of plexiglam. Fig. 7 shows background spectra forthecounter N1 containing 82 .8` Kr +7 .2%, Xe+10% CH 4 at a pressure of 139.5 kPa (the sample of "pre-bomb" krypton) . The spectrum I obtained with the anticoincidence channel switched-on OI)B'r.g/CI ;AIv .x 103.

0,20

ISO 40 4

111,1000 EMEN 80 8

120 12

160 CIIAtvNEL 16 lcev

has a distinct peak caused by decays of cosmogeneous "Kr. With the deduction of this peak the background of the counter N1 within the energy range of 4 to 20 keV was 0.215 imp/min . With the anticoincidence channel switched off the counter had a background of 1 .88 imp/min . Fig. 8 shows background spectra for the same detector installed inside the usual ground laboratory room. In the energy range of 4 to 20 keV the background of the counter with the anticoincidence channel switched off was 603 imp/min ; when the anticoincidence channel was active, the background was 76 .6 imp/min. the characteristic fluorescent radiation CuK knocked out by external background gammaquanta from the copper body of a device gen ates a peak at the energy of 8 keV which is well distinguished in spectrum 1 . The absence of such a peak in the spectrum 1 . fig .7, indicates the very low gamma-back-

O,IO

@MUNIS

0,3

//// :7

0,2

0,05

O,I 120 12

160 16

CHAiQFEL keV

Fi& 7. The detector NI background in the low-background conditions 1 . With anticoincidences: 2. without anticoincidences.

iii

0,5 0,4

0,15

80 8

in

Fig . B. The detector NI background in the laboratory. I . With anticoincidences: 2. without anticoincidences.

0,6

0,25

40 4

1 MIM M IN Mims 11111 man

COUIiTf' CIIAW.x 10'tt

0,30

0

0

ch: : .x 10 3 s

0

r4 //Rrrrr~.... 40 4

80 8

120 12

160 CRAN:iEL 16 kev

Fig.9. The detector N2 background in the low-background conditions. 1 . With anticoincidences; 2. without anticoincidences.

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COU - 'l'S/C! ;Ail .x 11?3s

481

K

100

40

80 60

30

40

20

20

r w

5 0

40 4

80 8

120 12

160 cHAr,,r4 :;-'i, 16 IteV

Fig. 10. The detector N2 background in the labor :uorv. anticoincidences; 2. without anticoincidences .

ground level inside the low-background protection. Sin, ilar spectra for the counter N2 are given in figs. 9 and 10. The working mixture of the same composition, as in detector NI, was under a pressure of 82 .7 kPa. In the low-background conditions, within the energy range of 4 to 20 keV, the counter had the background of 3.1 imp/min with the anticoincidence channel switched-off and 0.535 imp/min with the active channel on . In ti ;e laboratory room these values were 458 imp/min and 16.9 imp/min, respectively . Quartz glass was chosen as the material for the counter of quartz-composing elements, and because of the low content of natural radioisotopes in quartz. Due to the first reason. the spectrum in fig. 10 does not have any interfering lines in the -nergy range of 2 to 18 keV. The only exception is the low peak in the region of 8 to 11 keV. It corresponds to the family of tungsten L-lines induced in the tungsten COUtITS/CiWi .x 103S 300

~-1_J

0

6 8 10 12 14 IG 1 : eV 2 4 Fig. 12 . The background reduction coefficient for lite detoalor N2 ~ri,h an internal protective ring. 1 . Low-background condilion"-- 2, laboratory room ; .1 . 'hhe "' Kr source tin dtc lahoraton roomI .

wire of a inultiwire system . The background of tltc counter N2 was higher than that of the copper detector . The reason, apparently, consists of the fact that we Itused the usual quartz rather than optical quality quart, This counter was used mainly to make nleasuret ,tts with irradiated samples. Fla. I I shows spectra for the case where the ss Kr isotope was containedin theworking mixture. The other characteristics of the mixture were the same as previously . The coefficient of selection of the anticoincidence channel depends on the type of source . Fig. 12 shows selection coefficients forthe cases presented in figs . 9 to 11 . 5. The end effects

-Y

250 200

150 I00 50 . 0"

10

I . Willi

40 80 . 120 160 CHAPd1IFL 8 12 16 keV 4 Fig. 11 . The detector N2 background with "Kr in the laboratory. 1. With anticoincidences; 2. without anticoincidences .

It is known that at some distance from the ends of the counter the electrical field intensity begins to diminish. Accordingly, the gas multiplication coefficient decreases. In measurements with an internal gas-:. source this effect leads to the loss of apart of theevents which occur outside the region of a peak of full absorption of the source quanta energy. We have made direct measurements of end effects using"At isotope. As a preliminary step, the counter background was measured . Then, the radioisotope was introduced into the working mixtureand its spectrum was recordecl . The pressure was equal to 82 .7 kPa in all the cases . Fig. I1 shows the "Arspectrum in thecounterN2 with subtraction of the background. In processing the spectra the peak from ''Ar decays was approximated by Gaussian distribution (marked by crosses) . The end effect was

482

V. V. Kuznrùnoc w al. / 6fultiwim proportional coutuecr

COU :TS/CHA -.:rlEL

J1

.2

2Mî I75MI

e

150001 I 7500 5000 0

6. Conclusions The characteristics presented above indicate that the multiwire wall-less proportional counters possesses a number of advantages over detectors of other types in recording X-ray radiation from both external and internal sources.

12500

2500

effects were (10 .0 -= 0. 1)% and (4.6 =0.2)% fordetectors 1 and 2, respectively.

In conclusion the authorswish to express their gratitude to Dr. H. Loosly for critical comments and valuable discussions .

s

1114,11 20

1 A41

40 60 80 CHA",N1,L

Fig. 13. The endeffect for the detector N2. 1 . Spectrum of "Ar with the deduction of the ::ackground; 2. Gaussian distribution .

determined as the ratio of (St -S6) to So where Sf is the full area under the spectrum and SG is the area under theGaussian distribution . Afterfilling the counters with a mixture of 90% Ar+ 10% CH4 the end

References [I] H. Loosly and H. Oeschger, Earth Plan . Sco . Let. 7(1) (1969)67 . 121 I .R . Barabanov, V.N. Gavrin, A.A. Golubev and A.A . Pomansky . Izv, AN SSSR, se,phys . 37(6) (1973) 1186 (in Russian) . 13) V.V. Kuzminov and A.A . Pomansky, Radiocarbon 22(2) (1980) 311 . i41 E.L . Kovalchuk, V.V. Kuzminov, A.A. Pomansky and G.T. Zatsepin, Low-radioactivity measurements and applications Conference Proceedings, Bratislava (1977) p. 23.