A solenoidal β-ray spectrometer for coincidence experiments

NUCLEAR

INSTRUMENTS

AND METHODS

22 (1963) 93--100, N O R T H - H O L L A N D

PUBLISHING

CO.

A SOLENOIDAL II-RAY SPECTROMETER FOR COINCIDENCE EXPERIMENTS H. V E R H E U L , J. J. VAN R O O I J E N , T I t . S. I~APTEYN and J. B L O K

Na~uurkundig Laboratorium der Vrije Universiteit, A msterdam Received 1 October 1962

To perform (e-, 2J)-coincidence experiments we have built a solenoidal fl-ray spectrometer with a maximal solid angle of 10%. The construction of this spectrometer arid the e x -

perimen±al tests are described. Its figures of merit are compared with those of other solenoidal spectrometers.

1. Introduction To perform m e a s u r e m e n t s on complicated decay schemes of radioactive nuclei we h a v e construct e d a m a g n e t i c t - r a y s p e c t r o m e t e r suitable for (e-, ~)-coincidence experiments. To m a x i m i z e the n u m b e r of true coincidences in these e x p e r i m e n t s t h e solid angle co Of such a s p e c t r o m e t e r h a s t o b e as large as possible a t a given resolution r/. Therefore a solenoidal s p e c t r o m e t e r was chosen. A detailed t r e a t m e n t of t h i s t y p e of spectrometers was g i v e n b y J. W. M. D u m o n d (ref.t)), who cons t r u c t e d a v e r y precise i n s t r u m e n t t h a t satisfied his conditions. To produce a v e r y homogeneous magnetic field he c o n s t r u c t e d a n ellipsoidal coil, t h e c o n s t r u c t i o n of w h i c h was very expensive a n d took a lot of time. T h e m a x i m a l solid angle was 9 % a n d t h e m a x i m a l focusable energy 2.5 MeV. I t was impossible to focus electrons w i t h h i g h e r energy, because t h e c u r r e n t should n o t exceed 31 A as a consequence of t h e c a p a c i t y of t h e cooling system. We h a v e c o n s t r u c t e d a m u c h simpler a p p a r a t u s w i t h a m a x i m u m solid angle of l0 % a n d a m a x i m a l focusable e n e r g y of 3.5 MeV, whereas its figures of merit are almost t h e same as those of D u m o n d ' s instrument.

these t h r e e effects, h a v e to be t a k e n equal. Ill t h a t case t h e following relations b e t w e e n t h e p a r a m e t e r s of t h e i n s t r u m e n t hold (cf. ref.2)). A(~I) ~o2 A(~) co~" (1) t/ = 1 . 6 9 ~ t g 2 ~ ; p = ½Rlsin2cq.tg20 q w i t h A ( ~ ) = 3 + 3 tg 2 cq + ¢I tg4cq ; ~itg 2cq + t g i t = 0 . ~1 is the emission angle of t h e central r a y a n d R~ the m a x i m a l radial d e p a r t u r e of the central r a y from t h e axis of the i n s t r u m e n t . W e w a n t to give a~ t h e value t h a t minimizes ~ at a given value of co, so we h a v e to minimize A(~I) / 2 t g 2 a t. The m i n i m u m of this function occurs for at = 42°10 ', b u t t h e r e is only a small v a r i a t i o n b e t w e e n a~ = 30 ° a n d cq -- 60 °. F o r technical reasons we h a d to take the i n n e r radius of the solenoid < 18 cm. Moreover the maxim u m available c u r r e n t is 40 A. As a consequence of these facts a n d t h e demands, m e n t i o n e d i n the int r o d u c t i o n we h a d to take ~t = 34° a n d R t = 13.30 cra. I n this case the distance L t between t h e source a n d t h e detector is 62.00 cln. I n our i n s t r u m e n t we m a d e variable t h e places of the two e n t r a n c e slit baffles, which define the solid angle to a n d t h e emission angel as, the places of the two resolving slit baffles a n d t h e places of t h e source a n d t h e detector (see fig. 3). So it is possible to search

2. The Values of the Parameters As a consequence of t h e finite range of emission angles, t h e finite w i d t h of t h e resolving slit a n d t h e finite radius p of t h e d i s k - s h a p e d source t h e r e will be a c e r t a i n m o m e n t u m d i s t r i b u t i o n of t h e focused electrons. U n d e r o p t i m u m conditions t h e b a s e - w i d t h s of t h e t h r e e m o m e n t u m d i s t r i b u t i o n s caused b y

x) J . W . M . Dumond, Ann. Phys. 2 0957) 283. 2) H. Verheul, Ph. D. Thesis, Vrije Universiteit, Amsterdam (1962). 93

94

et al.

H. V E R H E U L

experimentally the best position of the detector, to v a r y the solid angle and so' the resolution of the spectrometer and to define another emission angle for the central ray. This angle can be chosen closer to the optimum value of 42 °, when the desired solid angle is smaller than 10%. For ~1 = 42° the maxim u m adjustable solid angle is 3.5 %. I n table 1 and table 2 values of ~/, the source area 0 and the luminosity L calculated with the formulae (1) are given for some values of ~o in the case of ~1 = 34° resp. 42 °. To compare our instrument with other solenoidal spectrometers in fig. 1 is plotted the resolution ~/ versus the luminosity L over the square of the distance L1 between the source and the detector for several instruments as was suggested by E. Persico and C. Geoffrion3). '

'

......

i

' ' .......

i

....

TAJSLE 1

(=1 = 34°) ~(%)

~(%)

p(em)

O ( m m ~)

/(ram~)

2.0 3.9 5.9 7.8 9.8

0.4 1.6 3.5 6.2 9.7

0.10 0.40 0.87 1.55 2.43

2.96 47.3 239 755 1850

0.058 1.84 14.1 58.9 182

O ( m m ~)

L(mm 2)

TABLE 2 (~1 = 42°)

~(%) 1.2 2.3 3.5

o.,.ii7i i, 0.53 1.2

0.19 3.08 15.6

0,002 0,072 0.55

,,,-i

3. T h e C o n s t r u c t i o n o f t h e S p e c t r o m e t e r 10

13

2

0.~

u

0:1

n

ullnlpl

I

n

r

3.1. T H E S O L E N O I D

~o:

n ~',l

1.0

S"

,

10

~

~,,,i

i

100

. it rT07 Fig. I. C o m p a r i s o n of several solenoidal s p e c t r o m e t e r s . The p o i n t s refer to t h e calculated or e x p e r i m e n t a l l y d e t e r m i n e d v a l u e s of t h e i n s t r u m e n t s of t h e following a u t h o r s : 1. 2. 3. 4.

(exp.) (calc.) (exp.) (calc.)

C. M. W i t c h e r , P h y s . R e v . 6 0 (1941) 31. E. l-laggstrom, P h y s . R e v . 62 (1942) 144. S. J h a n a n a n d a , P h y s . R e v . 71 (1947) 321. L. F e l d m a n and C. S. W u , P h y s . R e v . 76 (1949)

180. 5. (cale.) H . de W a a r d , P h . D . Thesis, U n i v e r s i t y of Gron i n g e n (1954)6. (exp.) J, G. S i e k m a n , P h . D . Thesis, U n i v e r s i t y of Gron i n g e n (1959). 7. (ealc.) P. ]3retonneau a n d J. Moreau, J. P h y s . R a d i u m 14 (1953) 25. 8. (cale.) P. S c h m i d t , R e v . Sci. I n s t r . 23 (1952) 361. 9. (calc.) J . W . M. D u m o n d , ref.1). 10. (exp.) K. A. D o l m a t o v a a n d V. M. K e l m a n , Nucl. I n s t r . a n d Meth. 5 (1959) 269. 11. (ealc.) O u r i n s t r u m e n t ~1 = 34° ref'2) ' 12. (ealc.) O u r i n s t r u m e n t c*x = 42 ° ref.2).

For the production of the magnetic field of the spectrometer we constructed a coil, consisting of 26 disks, each of which is built up out of two sections wound of copper wire (12.5 x 1 ram2). The disks are cooled by means of a 2 m m copper plate, along the outer edge of which copper pipes with an internal diameter of 5 m m are soldered. These copper pipes are connected with each other. This cooling system appeared to be quite sufficient, at a current of 40 A the temperature of the coil was only about 4 degrees above room temperature. To produce a pure homogeneous magnetic field the medium radii of the sections have to lie on an ellipsoid of revolution and the number of turns per cm has to be constant along the axis of the ellipsoid. J. W. M. Dumond I) reached with such a construction a dexdation of the homogeneity of 0.17%. But as a consequence of the accuracy of the formulae mentioned in section 2, a deviation of the homogeneity of about 2 % will be sufficient. Therefore we constructed a much simpler coil (cf. fig. 2). The inner radius of the disks in the middle of the solenoid is 18.0 cm. At both ends they are smaller, the smallest has an inner radius of 6.0 cm. For most sections the number of turns is 36, their height is s) E. Persico a n d C. Geoffrion, R e v . Sci. I n s t r . 21 (1950) 945.

A S O L E N O I D A L ~-RAY S P E C T R O M E T E R FOR C O I N C I D E N C E E X P E R I M E N T S

3.96 cm and their length 2.56 cm. The number of turns per cm of a disk built up out of these sections is 26.08 c m - 1 . Some sections have 48 windings (see fig. 2), their height is 5.28 cm and the number of

95

added. For our solenoid this computation yields: Bz(M) = 27.594 i gauss, wherein

Bz(M ) is the

field intensity in the direction

096~ S

M

0

Fig. 2. Shape of the solenoid. The interrupted line represents the calculated shape of the magnetic field, the full line the shape of the field determined experimentally.

turns per cm is 34.77 c m - 1. Some disks are placed at a certain distance from each other as can be seeI1 ill fig. 2. For each disk we cMculated the intensity of the magnetic field in the direction of the axis for an arbitrary point Oil the axis as a function of the strength of the current i through the windings. To calculate the field intensity due to all the disks the field intensities due to the single disks have to be

of the axis in the centre of the solenoid. In fig. 2 the calculated ratio Bz(z)/B~(M ) for the whole solenoid is given for points on the axis of the instrument. Between the source and the detector the calculated m a x i m u m difference of Bz(z ) and B~(M) is 1.7% (cf. section 4). Because this solenoid is iron-free, the field intensity is linearly related to the current. Therefore it is possible to measure the field intensity by

Fig. 3. Schematic drawing of the solenoidal spectrometer.

96

H. V E R H E U L

m e a s u r i n g t h e s t r e n g t h of t h e c u r r e n t (see section 4). T h e t o t a l resistance of t h e coil is 3.07 ~2, its t o t a l length 79.0 cm. 3,2. T H E DI A P H R A G M S Y S T E M

T h e disks are p u s h e d on a v a c u u m c h a m b e r . This v a c u u m c h a m b e r has t h e form of a cylinder w i t h on b o t h sides a t r u n c a t e d cone. These cones are locked up b y m e a n s of t h e lightguide (see fig. 3) respective t h e source holder. I n t h e v a c u u m seals O-rings are used. The v a c u u m c h a m b e r is e x h a u s t e d b y m e a n s of a r o t a t i n g p u m p a n d a m e r c u r y diffusion p u m p i n a b o u t one h o u r to 5 x 1 0 - 6 m m H g . The pressure ill t h e v a c u u m c h a m b e r is m e a s u r e d w i t h a McLeod m a n o m e t e r . B e t w e e n t h e source alld t h e detector a lead cylinder w i t h a d i a m e t e r of 6 cm a n d a l e n g t h of 1 2 c m h a s b e e n placed on t h e axis of t h e i n s t r u m e n t to p r e v e n t g a m m a rays from t h e source to reach t h e d e t e c t o r directly. This lead cylinder is fastened to t h e innerside of t h e v a c u u m c h a m b e r b y m e a n s of six brass rods. To this lead cylinder two disk-shaped baffles of 4 m m thickness each are connected i n such a m a n n e r t h a t t h e i r positions on t h e axis of t h e i n s t r u m e n t c a n be a d j u s t e d from t h e outside of t h e i n s t r u m e n t . T h i s c a n be done w i t h a n accuracy of 0.03 ram, w i t h o u t b r e a k i n g up t h e v a c u u m . T h e o t h e r two d i a p h r a g m s are realized b y means of a cylinder of a l u m i n i u m a n d a r i n g - s h a p e d a h m i n i n m plate, b o t h connected to t h e v a c u u m chamber. These baffles h a v e a t h i c k n e s s of 4 m m a n d can be a d j u s t e d from t h e outside of t h e i n s t r u m e n t too, w i t h a n accuracy of 0.03 ram. T h e places of t h e detector a n d t h e source can be a d j u s t e d w i t h an accuracy of 0.01 m m from t h e outside of t h e ins t r u m e n t w i t h o u t b r e a k i n g up t h e v a c u u m . So in the d o m a i n of t h e i n s t r u m e n t where t h e electrons will pass t h e r e are 6 rods for t h e lead cylinder a n d 2 rods for t h e e s t a b l i s h m e n t of t h e d i a p h r a g m s . These rods h a v e a d i a m e t e r of 6.5 m m each. This will cause some s c a t t e r i n g of the.electrons, b u t it was n o t possible to realize t h e diap h r a g m s y s t e m w i t h less material. I n fig. 3 a s c h e m a t i c drawing of t h e s p e c t r o m e t e r is given. a) P. S. D u b b e l d a m , Ph. D. Thesis, Vrije U n i v e r s i t e i t A m s t e r d a m (1959).

et al.

3.3. T H E C U R R E N T T H R O U G H T H E S O L E N O I D

T h e available c u r r e n t source for t h e spectrom e t e r was a 4.8 k W direct currer/t generator. T h e o u t p u t voltage was a d j u s t a b l e b y regulating the c u r r e n t for t h e field of t h e auxiliary generator w h i c h p r o v i d e d t h e c u r r e n t for t h e field of t h e m a i n generator. T h e c u r r e n t t h r o u g h t h e solenoid h a d to be stabilized, because of c u r r e n t fluctuations due to changes i n t h e o u t p u t voltage of t h e generator a n d to changes i n t h e resistance of t h e solenoid. As m e n t i o n e d in 3.1 we d e t e r m i n e d the i n t e n s i t y of the field, a n d so t h e (Bp)-value of t h e focused electrons, b y m e a s u r i n g t h e s t r e n g t h of t h e current. To measure t h e field w i t h a n accuracy of 0.1% a n d to reach a resolution of 0.5 % we w a n t t h e fluctuations i n t h e c u r r e n t to be = 0.05%. T h e generator o u t p u t showed a low frequency ripple of a b o u t 10 %. To stabilize t h e o u t p u t voltage of t h e generator against t h i s fluctuation t h e current s u p p l y for t h e field of t h e auxiliary generator was regulated b y t h e s i g n a l of a difference amplifier, w h i c h compares a p a r t of t h e voltage across a

RL Fig. 4. Block scheme of t h e c u r r e n t stabilizatio n circuit.

resistance parallel to tile load w i t h a c o n s t a n t voltage (see ref.*)). W i t h this circuit a stabilization of a b o u t 3 % was reached. This Was sufficient for a safe operation of t h e transistorized c u r r e n t stabilization circuit described below. T h e block scheme of t h e c u r r e n t stabilization circuit is given ill fig. 4. The current stabilization is b a s e d on t h e following principle: Variations A[ in t h e c u r r e n t I, due to variations AE i n t h e supply voltage E a n d to variations AR z in t h e resistance RL will cause a v a r i a t i o n AE R in the voltage drop ER across a c o n s t a n t reference resistance R R. The voltage drop E R is c o m p a r e d to a c o n s t a n t voltage

A SOLENOIDAL

If-RAY SPECTROMETER

in a difference amplifier. The amplified difference signal regulates inversely the current I. W h e n the amplification factor of the circuit is #, we find for the current fluctuation AI due to the voltage fluctuation A E AI I

- -

1

=

1 +

(1 +

ziE Ji)RR/R

L

E L

'

wherein E L is the voltage drop across the resistance R L. For the fluctuation A I caused by the resistance fluctuation ARz we find: AI

- -

1

=

I

AR L

1 + (1 + g)RR/R L R L"

Hence it follows that the quality of the current stabilization circtzit is determined b y the ratioRR/R L and b y F, which have to be as large as possible. To make RR/R L large the supply voltage has to he a much larger voltage source t h a n actually needed by the load. We used a transistorized voltage stabilization circuit. This circuit stabilizes a voltage drop of 22-35V across a constant manganin resistance. Manganin is used because of its low resistance change with temperature. This resistance was cooled b y air, it was not necessary to cool it with oil. The scheme of the circuit is given in fig. 5. The entire current for the solenoid passed through 40 transistors in parallel. These transistors (OC 16) are mounted on a copper plate of 50 × 50 cm 2. To prevent the transistors from being overheated this plate was coole d by water through copper pipes with all internal diameter of 3 m m soldered on the plate. 8x

0C72

/,Ox

2~oc,6. If ~ 5x

~.TK

7K

2x0C71

4,7K

1N

~

RR

220R

to the

EXPERIMENTS

97

Each transistor is combined with a series bulb, which serves as protection of the transistor. For safe operation of the transistors the voltage drop across them has to lie between l l and 15 V. This voltage drop Call be established by varying the output voltage of the generator. A part of the voltage across a resistance parallel to the manganin resistance R R is compared with the constant voltage of a Zener diode (18V). The difference signal is amplified and inversely fed back to the base of the parallel transistors. The voltage drop E R across the resistance RR can be adj asted between 22 and 35 V bj( means of the potentiometer P in fig. 5. To be able to establish every value of the current between 0 and 40 A, the resistance R R is divided into steps. The resistance is variable in steps from 0.1 up to 30 ~: B y estaMishing RR, regulating P and adjusting the voltage drop across the parallel transistors by means of the supply of the generator the current is adjustaMe. The stability of the current stabilized with this circuit was about 0.03 %, which was sufficient for our measurements. The current was determined by measuring the voltage across a 0.05 ~2manganin resistance, cooled with oil, with a compensation circuit. As a consequence of this stabilization circuit part of the supply voltage is needed for the stabilization. In our case the m a x i m u m current through the solenoid is 32 A when the generator was used. But for focusing electrons with an energy of 3.5 MeV a current of 40 A was necessary (cf. section 2). To adjust currents of 32-40 A a Se-rectifier is available. 3.4. T H E D E T E C T O R

1

H

~_~

FOR COINCIDENCE

solenoid

Fig. 5. T r a n s i s t o r i z e d c u r r e n t stabilization circuit.

As detector for the electron-spectrometer a plastic scintillation counter was chosen, because this detector permits very fast counting, useful for coincidence measurements. We used a disk-shaped plastic detector (NE 102) w£th a diameter of 5 cm and 2 m m thickness. W h e n a large solid angle of the spectrometer is used, not all electrons passing through the opening diaphragms will he focused on the detector as a consequence of this diameter. To use a larger diameter however, is disadvantageous because the background will increase with the square of this diameter. This effect will cause a

98

H. VERI-IEUL ~ al.

reduction of the effective solid angle. Calculations showed this effect to be 0 for co <_ 3 %. For r~ = 8 % the effect is about 10%. The photomultiplier tube which has to be combined with this detector is sensitive to the magnetic field and has to be placed at a sufficient distance outside the solenoid so that the magnetic field of the latter does not influence its operation seriously. Therefore we constructed a lucite light guide with a length of 40 cm (cf. ref.2)). T h e n the photocathode of the photomultiplier tube is placed in a point where the field intensity is 5 % of the value ill the centre of the solenoid. We have tested several types of photomultiplier tubes for the use in the spectrometer. This was performed with a NaI(T1)-crystal and the 660 keV g a m m a radiation of laTCs. The height of the pulses of the photomultiplier tube due to these g a m m a rays seemed to be a function of the intensity of the magnetic field on the place of the photocathode. The smallest effect was found for an EMI 9514 AS. Using a / l - m e t a l cylinder which was 5 cm longer than the used photomultiplier tube the effect of the magnetic field was even nihil. 3.5. T H E GAMMA R A Y S P E C T R O M E T E R

To measure (e-, ?)-coincidences we used a gamma ray scintillation spectrometer consisting of a 3" x 3"-NaI(T1)-crystal, a lucite light guide and a D u m o n d 6363 photomultiplier tube shielded with #-metal (Harshaw match-window unit). The diameter of the detector was chosen as large as possible (in our case 3"), to maximize the solid angle co~ of the g a m m a ray detector and the ratio of the photoefficiency and the Compton efficiency of the crystal. The diameter of the truncated cone of the spectrometer however does not permit us to use a shaped light guide in this case. Therefore we took a cylindrical lucite light guide with a length of 50.0 cm and a diameter of 8.0 cm. The intensity of the light is about 45 % for such a light guide. The light guide is mounted in an aluminium housing. The intensity of the magnetic field on the place of the photocathode is about 2 % of the value in the centre of the solenoid. No effect of this magnetic field in the response of the photomultiplier tube was found. The source bolder of the spectrometer is constructed in such a manner that the maximal

available solid angle is 13.5%. This solid angle is limited because no material has to be placed just behind the source in the spectrometer to avoid scattering of the electrons. I n our case the minimum distance between source and detector is 4 cm. To measure coincidences between the pulses of the B-detector and of the ?-detector we used a fastslow coincidence circuitS). The fast coincidence circuit is of the type due to B. Rossi6). The resolving t i m e , of our circuit is 3.5 x 10 - s sec. The value o f , is determined b y counting the number of chance coincidences using two different sources for the fl- and ?-detector. To diminish the number of chance coincidences during the experiments the low voltage pulses of the fl-detector due to the noise of the photomultiplier tube are discriminated by means of a discriminator (see section 4). The gamma ray spectrum is recorded with a RCL 256 channel pulse height analyser. I n coincidence experiments this multichannel is triggered b y the pulses of the slow coincidence circuit.

4. The Experimental Tests of the Apparatus The magnetic field in the: solenoid was measured by using the Hall effect. The experimentally determ i n e d intensity of the field along the axis is given in fig. 2. In a direction perpendicular to the axis the deviations were < 0.5%. We also determined the variations of the intensity of the magnetic field along the trace of the central ray. These variations are < 2.3 ~o. The intensity of the magnetic field for a point on the axis of the solenoid has been measured several times as a function of the current through the windings. No deviations from a straight line were found (cf. fig. 6). This ensures that there are no magnetic disturbances influencing the magnetic field seriously. The calculated intensity of the magnetic field in the centre of the solenoid for i = 10 A is 276 gauss. The experimental value was 280 gauss. The baffle system was tested b y means of a 137Cs source. For several establishments of the baffle system we measured the K-conversion line of the transition of 660 keV and determined the resolution s) K. Siegbahn, fl- and y - r a y spectroscopy (North-Holland Publishing Company, Amsterdam, 1955). e) B. Rossi, Nature 125 (1930) 636.

A SOLENOIDAL

~-RAY SPECTROMETER

~. The results, compared with the calculated values are given in fig. 7. The deviations between ~/¢.1~ and t/~,p for smaller values of o~ were caused by the area of the source (see table 1) and the mutual influence of the three effects mentioned in section 2, which were assumed to be independently in the derivation of the formulae (1). For the light guide used in combination with the plastic scintillator (see section 3.4) we measured the loss of light intensity with a NaI(T1)-crystal in the following manner. The height of the pulses of the photomultiplier tube due to the gamma radiation of 660 keV

of a lSTCs source

was

measured

FOR COINCIDENCE

99

500

-~ ~ 300 ¢o

200

/t

when

t

I

2

the crystal was coupled directly to the photomultipier tube and with the light guide in between.

EXPERIMENTS

+ i

ii0

I

+

i

118

I

in Amp;~re

Fig. 6. The intensity of the magnetic l~eld as a function of the current through the windings.

20

20

'G •~

0

15

:g 10

o, _c

5

//

= 13 mm z

Iq.cat = 16 °/o

rt

~S

10

+

105

I

106

107

0

A

1.04

1.05

c

u~ •":- 1 5

0 : 1~#mm z ,,~ : 3 %

15

0 =13mm z Lo = 15O:o

c:

t0 Z

rLQt= 025 %

~

10

v

~a ~=

m

c:

=,

C

20

=.

,.o

1,07

1

m

~

1.06

in Arn p;~re

I' in A m p e r e

i0

i¢: O~ %

%=060 %0

c

0 104

0 =13ram z

-'~'-- 15

91°"

%

lO

m

5

1.04

1.05

i

in A m p e r e

1.06

1.07

B

I

I

I

l

104

1,05

1.06

1,07

i

in A m p e r e

D

Fig. 7. The K-conversion line of the 660 keV transition of lS~Cs measured at several esf~blishments of the diaphragm system.

100

H. W E R I - I E U L 6t a[.

The high voltage o n t h e photomultiplier tube was precisely the same in both cases. We made this measurement several times and conccuded the in tensity loss to be 49 %. This result agrees with that of other authors (for instance T. R. GerholmV)). The signal to noise ratio of the fi-detector was measured for several values of the energy of the incoming electrons, focused by the spectrometer and for several voltages on the photomultiplier tube, A typical example of the spectrum of the

The coincidence circuit was tested by means of coincidence experiments on the decay of 198Au and lZTCs. In the first case the conversion line on the fl-spectrum was found iI1 a single measurement hut 2O

m

5000

c m 100

300

500

keV

3ooo

Fig. 9. Single fl-ray s p e c t r u m of ~37Cs.

510

I

I 150 Pulse

I

I 250

height

Fig, 8. Pulse h e i g h t distribution of the plastic scintillator (combined with t h e light guide) w h e n electrons of 73 k e V are detected. The high v o l t a g e is 1700 37.

pulses of the fl-detector measured with the multichannel is given in fig. 8. I t was important to do this because the bias level of a discriminator (see section 3) has to be adjustedin such a Iflanner that the Background is small but t h a t no electron pulses are lost, when electrons of low energy are detected (cf. also ref.9)). Using a high voltage of 1700 V and adjusting the discriminator as is indicated with the arrow ill fig. 8, we measured the single beta ray spectrum Of 137Cs (cf. fig. 9), The calculated Kurie plot, after correcting the spectrum for background, is straight above 40 keV. This means that no pulses due to electrons of this energy are lost in the discriminator. The background was 26 counts per sec.

not when the fl-spectrum was measured in coincidence with the g a m m a ray. In the second case the single gamma ray spectrum consists of the spect r u m due to the y-transition of 660 keV and the K-R6ntgen line of the converted transitions. I n the ?-spectrum measured in coincidence with the conversion electrons, only the K-R6ntgen peak was found. W i t h these instruments we have performed (e-, ?)-coincidence measurements on the electroncapture decay of 176Ta to the even-even nucleus 176Hf (see ref.8)).

Acknowledgements The authors wish to express their gratitude to Drs. M. C. Lelie and Drs. F. W. A. Habermann for valuable discussions on the subject, to Ir. K. van Suchtelen and Mr. J. W. M. van Overbeek of the " N a t u u r k u n d i g Laboratorium Van de N.V. Philips" at Eindhoven, Holland for their suggestions on the current stabilization problem and to Messrs. W. Jongsma and L. Walinga for their technical assistance. 7) T. R. Gerholm, l~ev. Sci. ]nstr. 26 (1955) 1071. s) H. Verheul et al., Nucl. Phys. to be published. 9) D. Bererlyi and Gy. M~th6, Nucl. Instr. a n d Meth. 13 (1961) 161.