A magnetic spectrometer for neutron capture experiments

NUCLEAR

INSTRI'MENTS

AND

METHODS

16 (1662)

199-213;

NORTH-tIOLLAND

PUBLISHING

CO.

A MAGNETIC SPECTROMETER FOR NEUTRON CAPTURE EXPERIMENTS G. B-~-CNSTR0-'XI, A. B.ACKL1N a n d N. E. HOI..MBEIC-G

Institute o~ Physics, Uppsala and K. E. B E R G K V I S T

Nobel Institute o~ Physics, Stockholm Received 3 ~'Iarch 1962

A 5 0 c m radius double f(musing s p e c t r o m e t e r is described, which is used fox" m e a s u r i n g internal conversion s p e c t r a following n e u t r o n c a p t u r e . A collimated n e u t r o n b e a m i n t e r a c t s w i t h a t a r g e t inside t h e s p e c t r o r a e t e r . A t a r g e t a r e a of 20 c m 2 m a y be used a t a line h a l f - w i d t h of 0.18 °~, d u e to a special

a r r a n g e m e n t i n v o l v i n g electrostatic acceleration. T h e m a g n e t i c field is r e g u l a t e d to a few p a r t s in I(Y~, a n d d a t a are t a k e n a u t o m a t i c a l l y . P r e l i m i n a r y results a r e shown. The i n s t r u m e n t m a y also be used for g a m m a - r a y experintents, with eitt~er C o m p t o n electrons or ptxoto-eleetrons.

1, Introduction important still, it is desirable to be able to record After the capture of slow neutrons by nuclei the internal conversion spectra, which in conjunction binding energy of the neutrons is liberated as suc- with galnma ray intensities may be used for nmlticessive gamma quanta, which m a y possibly be polarity assignments. A double focusing spcctroineter is evidently a converted. The energy range of this g a m m a radiation extends to 2-12 MeV. Successfully inter- suitable analyzer for the purposes mentioned, but preted, the radiation data provide valuable in- the iron components of the reactor and the usually limited space around it does not permit one to use formation about the level structure of the product nucleus, and in particular one expects a fairly the iron-free variety. Thus, we dediced to accept complete population of the low energy levels. The the disadvantages of an iron yoke instrmnent but experimental methods of this field of research have to utilize the inherent accuracy to the limit by carebeen reviewed by one of nst), and a comprehensive ful construction of the auxiliary equipment. There discussion of the energy measurements in particular existed at the institute a double focusing spectrometer of 50 cm radius, which had been designed for has more recently been given by Bartholomew, Knowles, and Lee-Wlliting2). use with radioactivity and which was able to focus The choice of instrument was governed by tile electrons of up to 4 3IeV energy. Fortunately, the requirements of comparatively high resolution and iron core of this same instruinent permitted much accuracy but also of a certain versatility. An in- higher flux to be applied without producing apstrument which is to cover the full energy span of preciable saturation effects. In fact, it was found the g a m m a radiation has to be based on the analysis that the magnet only needed water-cooling to of secondary electrons in a magnet. While it seemed permit operation at 12 MeV electron energy. The that at high energies the Compton coincidence new iron poles could thus be machined to tile same method 3) would yield the best results, it also ap- contour as the e ~ s t i n g poles. peared that the merits of this method would 1) G. B h c k s t r 6 m . Nucl. I n s t r a n d Moth 4(1959) 5. rapidly decrease towards lower energies and that ~) G. A. B a r t h o l o m e w . [, W. Knowles anti (;. E. l.eephoto-electron conversion might prove more ad- Vfhiting, R e p o r t s on P r o g r e s s in P h y s i c s (1960) 453. vantageous at this end of the spectrum. More z) I[. T. Motz, P h y s . Rev. 104 (1956) 1353. 199

200

G. B X C K S T R ( J M

et al.

proportional to the flux density. A n o t h e r similar coil rotates some distance away, r i ~ d l y a t t a c h e d to the first coil b y means of a shaft, a n d is located in the field of an iron free m a g n e t coil. The c u r r e n t t h r o u g h this coil is a d j u s t e d to make the induced voltages equal, which is d e t e r m i n e d b y adding the voltages ~'ith opposite phase a n d observing cancellation. W h e n cancellation occurs, the current supplied to the m a g n e t coil is a measure of the s p e c t r o m e t e r field. This a r r a n g e m e n t is still in use and has proved to be accurate, when required, to ~ i t h i n a few p a r t s in 10 ~. It is to be noted, however, t h a t the m e a s u r e m e n t of the magnetic field is t a k e n at one point only, a n d t h a t the value obtained need not correspond exactly to the proper average of the field at the electron orbits. Although the probe coil is located at the same distance from the centre of the m a g n e t as are the electron paths, it is obvious t h a t the electron m o m e n t a remain proportional to the measured flux density only if

A s!;ectrometer to be used w i t h a nuclear reactor should permit c o n t i n u o u s operation for weeks a n d the tedious m a n u a l operation obviously needs to be s u b s t i t u t e d b y some k i n d of a u t o m a t i c d a t a recording i n s t r u m e n t , wtffch also m a y be m a d e more efficient a n d accurate t h a n the m a n u a l one. To make a compromise between tiffs m a n u a l operation of the i n s t r u m e n t on the one h a n d a n d the labor required to keep complex control circuits in working order on the other, it was decided to construct a n i n s t r u m e n t which could a c c u m u l a t e d a t a witho u t m a n u a l aid, b u t which h a d t h e simplest possible p r o g r a m m i n g facilities. At a certain stage of the construction it was realized t h a t while m a n y research groups were s t u d y i n g g a m m a r a y spectra, less effort h a d been d e v o t e d to the i n t e r n a l conversion aspect t h a n it seemed to deserve, and therefore internal conversion studies became the first task of this spectrometer. MA6k~T

SPECTROMETER

~IRI; cKELV1N -VARLEY

~ TE,HPERATURE COMPENSATION SI~JNT 1

AI.,IPI.ITLJOE I PI,~,_cESENCJTII~ DC ERROR RECTIFIER. SY3NAL Fig. 1. Black s c h e m e of

the

2. Magnetic Field Measurement A speciaI t y p e of r o t a t i n g coil m a g n e t o m e t e r has previously been used with this type of double; focusing spectrometer, a n d a detailed account has been given4). The principle of the m e t h o d is to h a v e a r o t a t i n g probe coil at a chosen point of the field to be measured, so as to produce a n induced voltage

magnetometer apparatus.

the relative field d i s t r i b u t i o n is c o n s t a n t a t all times. I t is clear t h a t tB.is sets t h e limits of the accuracy t h a t the m e t h o d can t)roduce. I n practice it has been found t h a t the u n c e r t a i n t y is app r o x i m a t e l y 2 p a r t s in 10 4 ir~ case the m o m e n t a c o m p a r e d differ b y a factor of two, a n d it m a y be 4) (;. B/ickstr6In, Nucl. I n s t r . I (1957) 253.

A MAGNETIC S P E ( ' T R O M E T E R FOR NEUTRON CAPTURE E X P E R I M E N T S

expected to be higher or lower than this value depending on the m o m e n t u m ratio. Adjacent lines, whether they are produced in the same source or not, m a y thus be compared with an accuracy essentially set by the line widths, say, to within -2~ of a half-width. Since the half-widths obtained in the neutron capture experiments with this instrument will scarcely be smaller than 1 : 103, it seems sufficient to have an accuracy of better than 5:105 in the regulation and measurement of the magnetic field. In the present case, where automatic operation is desired, the above arrangement is inconvenient, since it requires the accurate regulation of two currents. If, on the other hand, a constant reference field is used, i.e. the iron free coil is replaced by a permanent magnet, the same idea m a y still be used, but the actual operation will be simpler. The apparatus is shown symbolically in fig. 1. The voltage induced by the permanent field is now divided by a precision attenuator in order to match the unknown voltage. The difference between the two alternating voltages is fed to the tuned amplifier at the right, which operates with a very small input. The amplified signal goes to a phase sensitive detector, the output of which directly shows the difference in amphtude of the two AC voltages. The procedure for obtaining the desired spectrometer field is thus to set the attenuator bridge to the proper value and to adjust the magnet current so that the error signal vanishes. The obvious advantage of the permanent field arrangement is the simple settings of the field, which is achieved in two operations. The reliability of the reference field is probably also better, since the permanent field may easily be made high enough to reduce to negligible values the perturbations caused by ambient fields. There exists in addition the possibility of shielding the magnet with soft iron. The control of the temperature also inwfives less problem, since there is no heat dissipation in the permanent magnet. The disadvantage with a permanent field arrangement, and the reason why it was avoided in the earlier construction, were due to the problems arising in connection with the voltage divider and the load which it applies to the generating coil.

201

The voltage generated may be written E = Bs~o e i~t, where B is the flux density, s the total area presented by the coil. and co the angular velocity. This area is temperature dependent: s = s o (1 + 2).~), where ,~ is the linear expansion coefficient and T the temperature. Considering tile left circuit in fig. l, one finds the following expression for the voltage developed over the attenuator R:

RBso(1 + 2/i~)we i°'t ER = ~r~+ R¢ + Ri0(l + pT)

where the temperature dependence of the coil resistance, R i = Ri0(1 + pz), is included. Re is the collector contact resistance For copper we have 2). = 3.2 × 10-s/°C and consequently sor, le precaution has to be taken to control the temperature dependence of E R. This m a y be achieved by stabilizing the ambient temperature of the coils and chosing a high R so that the power dissipation in the coil remains negligible and the influence of R i becomes small at the same time. However, we found it expedient to make use of the fact that the two temperature dependent factors m a y be made to cancel by choosing a suitable value of the ratio R / R v The value at which cancellation occurs is R

Rio

p -- 2).(1 + RJR~o ) 2).

p -

=

23.

=

120.

Apart from fulfilling t2..e above condition, R has to be sufficiently high to make any normal change of R¢ influence E R negligibly. With our present collector arrangement, consisting of silver-carbon brushes and beryllium-copper slip-rings, Re was measured to be about 0.1 ohm during rotation, and any change with time was less than half this value. In this case, a choice of R = 5000 ~2 would introduce an error of only 1:10 s into the measurements. The 5000 ~ load on the coil was not found to induce any measurable rise in the temperature of the coil, and furthermore, a rise of a few degrees would have been unimportant in view of the cancellation. Other temperature effects exist which m a y introduce an error in the m o m e n t u m value. The shaft is longitudinally fixed by means of a ball bearing approximately half-way between the two ceils. The thermal expansion of the aluminium shaft wil!

202

(;. BXCKSTR().XI et al.

therefore disphtce b o t h coils, which, of course, will change the induced voltage, if there is a g r a d i e n t in the field. The p e r m a n e n t m a g n e t field is inhomogenous, but symmetrical, a n d since the coil is located at the centre the, effect on the voltage should vanish.. The gradient in the spectrometer, however, is a b o u t l % / c m in the region of the probe coil, a n d this causes a n increase in the field reading of 1.6 x 10-5/:C. The effect m a y he c o u n t e r a c t e d to solne e x t e n t b y the expansion of the s u p p o r t of the apparatus. A n o t h e r effect, working in the opposite direction, is the ext)ansion of the v a c u u n l t a n k a n d h nee, of the m e a n orbital radius. One should control or measure the t e m p e r a t u r e s involved, w h e n e v e r extreme accuracy is needed. There are reasons for not m a k i n g R v e r y high. Obviously, the insulation resistance to g r o u n d of the right part of the circuit including the collectors, cables, arid the coil, should ideally lm R x 105 or more, in order to not influence the accuracy of the a t t e n u a t o r . W i t h R = 5000 f/, a n insulation of 500 .XI~? would t h u s be acceptable, a n d is b y no means difficult to realize. I t is to be n o t e d t h a t t h e amphfier n o r m a l l y operates at zero i n p u t a n d hence need not be considered as a load on the a t t e n u a t o r . The mechanical angle between the r o t a t i n g coils has to be fixed to a v~due which makes the induced voltages add with opposite phases as accurately as possible. As explained more closely in rcf. I), the phase deviation (5 from 180 ° t h a t can be, in practice, tolerated depends on the electronic a r r a n g e m e n t a n d on t h e desired accuracy of field controh In the present a p p a r a t u s , the phase angle deviation should be kept below 3 x 10 -4 radians if a regulation accuracy of 3:105 is to be obtaincd. The phase angle is however influenced b y o t h e r titan mechanical factors (fig. 1). The u n a v o i d a b l e s t r a y capacity C to g r o u n d introduces a phase shift between the i n p u t and o u t p u t of the a t t e n u a t o r , a n d this shift depends on the setting of the bridge, being zero at full output. In order t h a t the angle de~iation m a y be kept small at all times, w i t h o u t a d j u s t m e n t of fi, the t o t a l capacitive phase shift m u s t not exceed 3 x 1 0 - 4 r a d i a n s . At a typical capacitance of 500 p F a n d at the frequency used (21 c/s), a n att e n u a t o r resistance of 5000 ~1 makes the phase shift a b o u t 10-4 a n d is accordingly sufficiently low.

Needless to say, the a t t e n u a t o r itself must be constructed so as to introduce negligible phase shift. A voltage divider of tile Kelvin-Var]ey type with an i n p u t resistance of 5000 ~/was specially ordered for this i n s t r u m e n t . 111 view of tile a u t o m a t i c operation, this divider was constructed as a closed resistor set with 12 o u t i m t lugs from each decade, to which relay switches couid be connected w i t h o u t i m p a i r i n g the accuracy of the precision resistors. T h e overall accuracy of the bridge is a b o u t 2 parts in 105, referred to the output, in the region of att e n u a t i o n 1.0 to 0.1. If the a t t e m l a t o r were to operate over the whole d y n a m i c region of the spectrometer, i.e. a b o u t 100, this would lead to an i n c o n v e n i e n t l y low o u t p u t voltage a t low m a g n e t i c fields. F o r this reason, a n d in order to allow for sx~dtch contact resistance, it was preferred to h a v e two different o u t p u t s of tile probe coil in the spectrometer, so t h a t the bridge would not generally h a v e to be set at values lower t h a n 0.1. The bridge consists of five decades, a n d consequently t h e m i n i m u m field i n c r e m e n t becomes 1 part in 104 or less, depending on the a t t e n u a t i o n . A m o n g the a m b i e n t magnetic fields against which the p e r m a n e n t m a g n e t h a s to be shielded, the s t r a y field of the spectrometer is the most serious one, not because of its strength, which is of the order of the e a r t h ' s magnetic field, b u t because it is not constant. To reduce the p e n e t r a t i o n of outside fields into the air gap, the reference magnet was c o n s t r u c t e d x~dth a closed, rectangtflar, soft iron yoke, which se~a'es as the r e t u r n circuit of the two ticonal blocks, m o u n t e d on the inside. Some shielding is also provided b y the soft iron tank, which serves as a c o n t a i n e r for the oil into which the m a g n e t is immersed. F u r t h e r m o r e , the reference field was oriented in a direction perpendicular to the s t r a y field of the s p e c t r o m e t e r a n d the interference is accordingly reduced to a small change of direction of the reference field r a t h e r t h a n a change of magnitude. Since this change of direction, .which ,,'as found to be 3 x I 0 - 3 radians at m a x i m u m s p e c t r o m e t e r field, is equivalent to a phase change of the induced voltage, some means h a d to be found to reduce the effect b y an order of m a g n i t u d e , in order to facilitate the a u t o m a t i c operation. The r e m e d y was to introduce a small m a g n e t coil into

A MAGNETIC

SPECTRO3IETER

FOR

NEUTRON

CAPTtJRE

EXPERIMENTS

203

the permanent magnet, such as to compensate for magnet, is limited by the available pole gap and by the perpendictflar stray field in the air gap The the condition that the resistance shonld not exceed 5000/120ohms in order to permit temperature correct field strength was obtained by passing part of the spectrometer current through this com- compensation. With the present apparatus, about 20 V output m a y be obtained. This implies that the pensation coih The coil also serves for manual voltages to be compared have the magnitude 2-20V. setting of the phase angle, since a purely mechamcal adjustmen'c of this angle would be inconvenient to An error signal of 1 part in 104 should be amplified well into the volt region, and hence an make. The effect of the stray field of the permanent magnet on the field m the spectrometer was com- amplification of about 104 is desirable. The amplipensated by means of an additional, unshielded tier consists of two similar stages, tuned to 20.5 cps and 21.5 cps respectively, each yielding a voltage permanent magnet. Measurements on the permanent magnet in use amplification of 40. This circuit is shown in fig. 2. show a temperature coefficient of 3 parts in 104 per The selective part is preceded by an ordinary RCdegree centigrade. The whole magnet is immersed coupled a m p l i f e r stage, with a voltage amplification of 50. With an E(7C83 as the first tube, the in an oil bath, the temperature of which is kept constant to within + 0.2¢C, and since the oil noise was as low as 2 #V referred to the int)ut. The temperature fluctuations are quite rapid, the con- transistorized version shown to the right in the staney of the magoetic field may be considerably figure was later tried, and pr,~ved to yield an amplibetter than +_ 3 parts in I05. A test o~'er a period fication of 100 per stage whereas the noise was 17 #V. of one week showed a long time drift of less than The output of the tuned amplifier only reflects 1 part in 104. The rotating shaft is supported at either end and the magnitude of field devmtton and not the sign, in the middle by bearings. The bearing inside the and the appropriate error signal has to be produced by means of a phase sensitive rectifier. The spectrometer has to be non-magnetic and was made of oil-filled sintered nylon ("Nylasint") with an axle principle is most easily explained by ~,. vector diagram, fig. 3. The subtraction of the voltage E~ and of chrome-plated brass. With the proper mttgnetic field in the spectro- E2 yields AE, which is the input of the amplifier. meter, there is cancellation of the voltages at the The phase sensitive detector, in this case a ring deinput of the amplifier to the right in fig. 1. How- modulator, has the property oI giving the magniever, unless great (:are is taken to make the refer- tude and sign of the component of the input voltage ence field very uniform, there will be harmonic which is in phase with a certain reference signal. frequencies present which do not cancel, since there Thus, if AE is used as the input and either E~ or E2 is no corresponding voltage from the opposite (:oil. is taken to be the reference signal, one obtains i/"-, - ] E , ' at the output. However, since AE For this reason, and also in order to suppress general noise, the amplifier has to be frequency must be amplified before entering the rectifier, selective. The first harmonic should x anish m view there is in general a phase difference A~0 between of tl~e s y m m e t r y of the field, but the second har- AAE and AE which nmst be corrected before the monic amounting to a few percent, had in this case signal is applied to the detector. If this is not done, to be specially suppressed by ~neans of a twin T a false error reading is obtained. The practical filter. A fundamental frequency of 21 c/s was chosen criterion for Aq3 = 0 is that the error reacting is to compromise between the disadvantage of having independent of the phase angle 6. The phase shill: of low output voltage and large coupling condensers selective amplifiers depends sensitively on the and the adwmtages of operating far from the 50 c/s frequency, however, and therefore a similar tuned hum frequency. Mechanical vibration of the shaft amplifier was included in the reference signal channel in order to ensure stable operation. also puts an upper limit on the frequency. The over-all accuracy of the magnetometer apThe number of wire turns on the rotating co11, paratus described above is about +_ 3 x l0 - s , i.e. and hence the output voltage from the permanent

204

G. BXCKSTR()M el al.

the flux density at the point of the probe coil is measured with this precision, if one neglects the ageing effect of the reference magnet, which seems to be 10 -5 per day or less. It is also assumed that

current should be stable to within a few parts in l0 s over a period of !0-20 sec without feed-back from the magnetometer; with the signal from the magnetometer applied, the power supply should be

.250V

-20 v STAB~tZED 05pF I IO~k

lOOk

681 ~0C75

T-FILTER C

C=0.25pF

O=5~F

g. G47M[

L I

i

---o

-..~-lV

*20 V 5TABP";ED

Fig. 2. Basic circuits used in the selective amplifier. Two alternatives are shown.

the vacuum tank and the rotating shaft keep a reasonably constant temperature, say _+ I~C during the time of the measurement.

E1

E

ANGLE ,~

]

t

ii i

Fig. 3. Vector diagram illustrating the principle of the phasesensitive detector.

3. Spectrometer Current Supply The current supply had to fulfill the following requirements: It should be capab!e of delivering from 0.I to 13 A into the 7 ohm magnet coil; the

capable of correcting the magnetic field with a time constant of a few seconds. If the power supply were constructed with an ordinary rectifier delivering about 100 V DC and a set of series transistors, then these evidently had to dissipate a maximum of 300 W and stand a tension of ahnost 100V. Alth,mgh this arrangement is feasible it was found impractical, and it was preferred to derive the power from a motor-generator set, tile output of which could be easily varied to the voltage required. This voltage source yie!ds only about 0.5°.0 of ripple and m a y be adequately filtered ~-ith a 32 ~F capacitor. The regulator circuit is silox~.'n in fig. 4. Because of the slow and uniform heating of the magnet c o i l sufficient short time stability of current was obtained by stabilization of the voltage drop over the coil, or over the coil plus a series resistance, depending on the current required. The voltage is controlled by means of three parallel power transistors (De!co 2N277), which derive their base current from a current amplifier also using 2N277. The base circuit of this stage only requires a few mA, which are supplied by a symmetrical voltage amplifier. The input of this amplifier senses the difterencc between a constant reference voltage and a chosen fraction of the output of the power supply

A 3I:\GNE'I'I(~ SPECTROMETER

o b t a i n e d t h r o u g h a helipot. The voll.age of t h e power transistors p r o x i m a t e l y 6 V, i n d e p e n d e n t of voltage, b y means of feed-back

FOR N E U T R O N

average collector was kept at apthe actual o u t p u t to the g e n e r a t o r

EXPERIMENTS

CAPTURE

205

resistors in series with the m a g n e t coil are used in order to produce sufficient voltage drop at low current. Tim power amplifier constructed to drive the -10v

.: .

.

.

.

•

Li

~c7~

--t

i ~nEEO~,Ca ........

2:~OV

1,0,1

!

""

L_ _ . / [

........

J ',~'a.l a C,E

OlZ

O~JT~T~>~_t 201~

A~PlJF~

• ,oo~

33k !

i

C;OOE INPF_~¢~Ck

Fig. 4. Block scheme of the regulated power supply. The reference v o l t a g e u n i t is silown in detail to the right.

field. T h e excitation current for the g e n e r a t o r is t a k e n from a 85 V supply, a n d the current is m a d e variable b y tneans of a 2N1022 power transistor d r i v e n b y a one-stage amplifier (OC74). The reference voltage supply is also shown in fig. 4. It consists essentially of the Zener diode OAZ201, which stabilizes its own current. The Zener voltage of this particular diode has a v a n i s h i n g t e m p e r a t u r e coefftcient at a current of a b o u t 7 mA, a n d a d v a n t a g e was t a k e n of tMs fact. The outptlt voltage (5.6 V) was found to be c o n s t a n t to within + 2 x i0 - s for m i n u t e s u n d e r n o r m a l room t e m p e r a t u r e conditions. An o r d i n a r y 10-turn helipot of I0 k~2 was originally used as the a t t e n u a t o r across the output, b u t because of its finite resolution it was replaced b y a corresponding fihn helipot~ when this product a p p e a r e d on the market. T h e hclipot m a y be r o t a t e d m a n u a l l y in s t a r t i n g up the a p p a r a t u s and, b y means of a servo motor, to c o m p e n s a t e for c u r r e n t drift a n d to step the m a g n e t i c field from one s e t t i n g to the next. The two last stages of the gear are friction coupled in order to reduce backlash. Since the helipot could not be c o n v e n i e n t l y used over the full range of the i n s t r u m e n t , an additional helipot was connected in series to be set at the beginning of an experiment. Fixed C o m p u t e r Cnntrols Ltd., London.

3V DC servo m o t o r is operated directly on the o u t l m t of tile phase sensitive detector, fig. 5. It consists of a voltage difference amplif.er a n d a following three-stage current amplifier, which is floating w i t h respect to the first part and is operated on 8 V AC. Because of the time lag be.tween the m a g n e t o m e t e r error signal and tile correction t h a t the servo m o t o r applies to the magnetic fiekt, the system is prone to oscillation. This t e n d e n c y was eliminated b y introducing a phase advance filter between the two p a r t s of the amplifier. A signal proportional to the m a g n i t u d e of the field error, which was needed to control the c o u n t i n g a n d t i m i n g circuits, is obtained from the difference amplifier b y means of diodes. The s t a b i l i t y of the servo system is not limited b y the gain, nor by the stability of the reference voltage, b u t is set b y the i n p u t circuit of the voltage amplifier, which is connected to the helipot. The base current of the first transistor flows t h r o u g h the i m p e d a n c e of the hclipot, a n d the fluctuations of this current caused b y t e m p e r a t u r e changes influence the a t t e n u a t i o n of the helipot a n d hence the constancy of t h e o u t p u t voltage. The remedies consist of choosing as low a helipot impedance as possible, ~ising a sma!l-signal silicon t r a n s i s t o r at the input, a n d protecting it from air turbulence.

206

G. BXCI,:STR{J.M e g a l .

4. Automation

Tim automatic operation of the spectrometer m a y be understood from fig. 6. A certain magnetic field, B, is obtained by setting

This had been delivered by the factory with soldering h:gs, so that switches cotfld be attached externally without impairing the accuracy of the divider. The telephone relays were later replaced by

-2_0V

e>

2B

.....

L ./ Fig. 5. P o w e r a m p l i f i e r for tile DC s e r v o motor.

the precision AC voltage divider, after which the magnetometer system feeds an error signal LIB into the power supply, which starts correcting the error. In a practical case, this regulation system achieves the desired magnetic field x~a,thin a few seconds. The voltage divider is a 5000 f2 KelvinA:arley bridge, consisting of five decades. Each of :hcse

4AGN ETOMEI~~_~ ACVOLTAGE DIVIOER 5DECADESI ~oI ROTARY

SWITCiEi~S--~I

Ledcx rotary sx~dtches, which proved to have. much smaller contact resistance (about 5 m #2). The electromeelq, anical switches were interconnected in such a way that each time the setting of a certain decade is transferred from 9 to 0 (by way of positions 10 and 11) a pulse is passed o~l to the next decade to make its setting increase by one unit.

SPECTROMETER SIJt:I:tY

POWER

0

I

DAB I

IIMER MONT IOR SCALER

I RELAYPULSER ,

iI'

Fig. 6

C~NTS STARTCOMMANO MOMENTUM

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l}|ock d i a g r a m illustrating t h e a u t o m a t i c operation.

decades requires a pair of 1 i-position switcl-e~ for the setting of the input to OUtlmt ratio. For this purpose we originally used ordinary teiel)hone relays, connected by short cables to the resistor set.

The magnelic tMd .may thus be increased between the desired values to be used in a measuremerit simply by applying a suitable number of voltage pulses to the decade switches. These pulses

A MAGNETIC

SPEC'IRO.METER

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are taken from a "relay pulser", which essentially is another 11-position rotary switch, connected to be free-running for all positions except the zero position. Thus, when a pulse is applied to its coil, this switch is released from its stable position and makes a full turn, stopping again at the zero position. l:or every step of this motion a voltage pulse appears over the coil of the "pulser", and, evident ly, by using a certain number of these pulses for driving a chosen decade switch, one can make the bridge setting change in a pre-determined manner. A certain programme is chosen by selecting the decade in which the setting is to be changed. After this, one decides by setting another switch how m a n y pulses are to be transferred to the decade switch for each cycle of the "pulser". hi the machine, this is achieved by breaking the train of pulses by means of a relay, when the "pulser" has reached the chosen position. The "pulser", however, ahvays completes a full cycle for each point, except in the trivial eases where a change of 0 or 1 unit is desired, when the "pulser" is not energdzed at all. The scaler for the electron detector and the timer were built as two identical units, each assembled from six Hewlett-Packard decade blocks. The two scalers work with a common reset circuit and use synchronous input gates. The timer takes a 100 c/s signal from the mains or m a y alternatively be used with a beam monitor. Data are recorded by means of a IIewlettPackard printer.The first five digits give the setting of the voltage divider, obtained from the rotary switches by means of the usual voltage code of the printer, and hence essentially yield the m o m e n t u m value. The following six digits give the mlmber of counts measured by the scaler. A recorder connected to the analogue output of the printer traces the spectrum. The total programme of the machine accordingly proceeds as follows: The scaler counts during a preset time interval or during the time it takes to accumulate a certain number of monitor pulses. The timer then closes the inlmt gates of both timer and scaler and at the same time gives a print command to the printer. The printing event closes a contact which momentarily energizes the magnet

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of the "pulser", and a relay short-circuits the input of the magnetometer. The magnetometer thus simulates zero error and the voltage from the power supply remains unchanged. When the rotary switches have all come to a stop, the magnetometer input is nnshorted, and a second later timer and scaler are reset. As long as the spectrometer power supply is still correcting the error, the rectified error signal, acting on a threshold amplifier, keeps the input gates of the sealers in the "off" position. As soon as the magnetic field error has become smaller than a pre-set value, the gates open again, and the automatic cycle repeats. Clearly, this apparatus giw's the simplest kind of programme: the field is increased by constant amounts and the counting time per point remains fixed during automatic operation. In many applications of a spectrometer it would no doubt be desirable to have some variation of step length and counting time with respect to the counting rate, or with respect to a previous knowledge of the spectrum. This would require additional apparatus, which could be used with the existing system practically without Inodification. During the year the apparatus has been used at the reactor, however, the limitations of the simple programme has been found to be of little disadvantage in practice. Since meast, rements usually have been taken with 1000 sec per point at line half-widths of between 0.25 and 0.6°0, and the spectrum thus is scanned very slowly, it is quite convenient to make the few necessary modifications of the programme during the daily check of the operation of the machine. The advantages of a more detailed programme does not, at the present stage, seem worth the additiona! complexity it involves.

5. Internal Conversion Experiments With a double focusing spectrometer of thc x.~2 type, where the source has to be kept within the magnetic field, internal conversion electrons from neutron reactions can be studied only by extracting a neutron beam from the reactor and passing it through tile spectrometer. Furthermore, if ttle beam entrance and exit x~d.ndow are to be the same as those used for the g a m m a ray experiments, the beam has to be perpendicular to the radius at

208

G. II"/,CKSTRi)M et

the source position. There are evidently two possibilities: the magnet m a y analyze the electrons going in the direction of tile neutrons or those proceeding in the opi;osite direction. A h h o u g h the "irst alternative wmtld allow the same geomet W as t h a t used for the g a m m a ray experiments, the second one is actually preferred. This is in order to avoid the very large n u m b e r of Comoton electrons enlitted from the source and its backing, which are caused b y tile strong g a m m a radiation accompanying the neutrons from the interior of the pile. The total a r r a n g e m e n t is show,'~ schematically in fig. 7. In tt}e e x p e r i m e n t s m a d e so far, the neutrons were obtained from a through chamlel of reactor R 1 of AB Atomencrgi, Stockhohn. The slow neutron flux available in the channel is 2 x 1012/cm 2 sec. at the m a x i m u m and tile d i a m e t e r of the tube is 55 ram. A neutron scazterer, consisting of a 50 cm long graphite plug, was introduced near the centre of the reactor. F u r t h e r out against the wall of the reactor an aluminium cylinder of 100 cm hmgth

al.

apparatus, when the reactor is on. i n the part of the tube going throt,gh the concrete there is "m :.ron collimator of 120 cm length with cadmitun clad ends. The hole of this collimator was so machined t h a t the whole surface of the graphite plug may just be seen from any point of the source..lust outside the concrete titere is a lead co!limator, mainly used for supl)ressing cadmium g a m m a rays. Much of the neutron shielding outside the reactor consists of tin boxes filled with a mixture of boron and paraffin. It was realized that in order to reduce the background at the detector it was necessary to use neutron s h M d i n g even very close to as well as within the spectrometer. Therefore, two big tanks were constructed, through which the neutron beam could pass by way of a suitably sized hole, and which were filled with a saturated solution of boric acid. There is one such tank at the entrance and one at the exit window. The windows, which had to w i t h s t a n d atmospheric pressure, were 12 x 12 cm 2 and were made of 0.1 mm aluminium foil. Sheets of boron plastic are used within tile vacuum where

~

BOE)N• I~,~FFIN

EII]] BORON PLASTIC Fig. 7. Schematic d r a w i n g of tile internal conversion e x p e r i m e n t :act up at reactor R I.

was litte(t into the cb.anneh This cylinder m a y bc pneumatically filled with paraffme oil which attenuates the beam sufficiently to allow a I)erson to approach and make necessary modifications in the.

t h e y are needed to absorb stray neutrons. At a distance of about ! :neter from the exit window the b e a m is caught by layers of lead and boronparaffme shielding.

A MAGNETIC

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The instrument includes two detectors,which may be used alternatively: a plastic scintillator 10 by 50 mm, and a twin Geiger tube, which may be operated in coincidence. The latter detector was used mostly in tile experiments performed so far. The background of this coincidence Geiger tube is 3 counts/min when the reactor is not running; this background increases to about 6 counts/min when the beam is on and the counter slit is closed so that no electrons are a d m i t t e d into the detector. The contribution to the background caused by the reactor was, in the very first experiments, 50 times as large and only after much trial and analysis of the factors was it possible to achieve the present favourable figure. As is seen from fig. 7 the detector is surrounded by a lead shield, about 15 cm thick. It was found that considerable background was caused by slow neutrons coming from neighbouring experiments or scattered out of the main beam. Most of the counting rate so produced appeared to be caused by neutron capture g a m m a rays from the lead shield. It was thus necessary to encase the lead shielding in a box made of boron loaded plastic. Although this change drastically reduced the background, there still remained one component which could be proved to be neither g a m m a rays nor thermal neutrons, and which must have been caused by inelastic scattering of fast neutrons in the detector and the lead and probably also by thermalization in the shielding and capture as before. Most of the fast neutrons were found to be emitted from the beam catcher and the tunnel in front of it. To improve conditions, as much moderator material as possible was put in at the proper places so as to thermalize the neutrons before they entered the boron plastic box. This finally brought the background down to the present level. Substitution of some lead by bismuth would give additional improvement. The background from electrons m a y sometimes exceed b y far the contribution from other radiation. One part of this component, which consists of beta rays from radioactivity induced in the source, depends, of course, on the weight of the source and also on time. Another part of the electron background depends directly on the weight of the source and is caused by the scattering of g a m m a rays at

NEUTRON

CAPTURE

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209

various parts of the spectrometer, i.e. the inside of the wall, the source holder, and the baffles, producing secondary electrons. Still another continuous distribution of the same character is caused by pair formation, which, however, was found to be less prominent, in the case studied. In the conversion experiments performed with Cd 1~*, measurements were made with a total background as low as 10 counts/min, whereas other measurements, especially those involving the multi-source arrangement, had to be made at 60 counts/min. There may be ways of reducing the electron background, apart from increasing the resolution of the instrument, but the attempts made so far have not brought much improvement. A suitable source thickness for the present apparatus is roughly 1 mg/cm 2 per MeV. Since it is rather unusual that self-supporting foils of such thickness may be prepared, the target generally has to be made by electroplation or evaporation in vacuo on to some suitable backing foil. Such a backing foil should be made of a material having a low neutron cross section, and the products should if possible not be negatron emitters, or else have a long half-life. The foils should furthermore be strong, non-ferromag, netic and electrically conducting. One of tile best materials for this purpose is beryllium, which fulfills the above conditions, although it may be difficult to obtain in very thin foils. One of the advantages of the interual conversion technique For studying neutron capture reactions is the fact that enriched isotopes may be used in most cases. The use of enriched isotopes, however, makes the process of electrodeposition preferable to thermal evaporation, since the risk of waisting the material is considerable in the latter case. Therefore, the backing material also has to be suitable for electrodeposition, in the case of Cd 11, it proved impomible to obtain a satisfactory source by electroplating on alumiuium, whereas the same procedure was successful when copper was used as a backing. Copper has a neutron cross section as high as 4 barns and could be used as a backing only because of the unusually high cross section of the target isotope which was 20000 barns. Two source arrangements have been used with

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A MAGNETIC

SPECTROMETER

t'OR NEUTRON

this i n s t r u m e n t . The first one uses a simple source strip, 50 m m long b y 2-8 m m wide, fixed diametrically across an a l u m i n i u m ring. The ring fits closely into a circular i n d e n t a t i o n a t the tip of the source

CAPTUItE

EXPERIMENTS

211

nosity of the i n s t r u m e n t , so t h a t a m u c h wider source could be used w i t h o u t impairing the resolution. This m e t h o d is used in a high luminosity fls p e c t r o m e t e r s) a n d consists in applying a voltage

: Fig. 9. View of t h e m u l t i - s t r i p source.

holder rod. In this m a n n e r the radial position of the ring m a y be reproduced very accurately, p r o b a b l y w i t h i n I0 -2 ram. Obviously the centre of the act i v i t y d i s t r i b u t i o n need not, however, coincide with t h a t of the ring. A quite accurate m e t h o d of comp e n s a t i n g for this discrepancy is to m a k e two recordings of t h e same electron line, reversing the source ring between the runs a n d taking the average value of the m o m e n t u m readings o b t a i n e d in the two positions. This average m o m e n t u m reading is t h u s the one t h a t would be observed in the ideal s i t u a t i o n t h a t the a c t i v i t y were accurately centered. This tectlnique was i n v a r i a b l y applied in energy m e a s u r e m e n t s against s t a n d a r d lines. Portions of the spectra o b t a i n e d are shown in fig. 8. I t is evident from the results t h a t even with a source of high cross section, the flux of this reactor does not b y far p e r m i t the full utilization of the resolution of the i n s t r u m e n t . It is p l a n n e d t h a t the whole a p p a r a t u s be m o v e d to reactor R2, which will produce an e x t e r n a l flux 100 times higher. Meanwhile, an a t t e m p t was m a d e to increase the lumi-

gradient in the radial direction across the source. This will introduce a differential acceleration of the electrons such t h a t electrons e m i t t e d at different radii b u t with the same energy will be b r o u g h t to a common radial focus. Fig. 9 shows the source arrangement. The source is divided into 20 narrow strips, insulated from each other a n d m o u n t e d on a perspex frame. One strip is connected electrically to the v a c u u m t a n k a n d each of the others is connected to the a p p r o p r i a t e o u t p u t of a linear voltage dixdder, so t h a t , going from one edge of tile source to the o t h e r the potential is increa_aed b y a c o n s t a n t a m o u n t from one strip to the next. A grounded baffle is placed in front of the source. The voltage divider consists of a n a r r a y of similar, stable radio resistors (22 _~IO, l W), which are kept in v a c u u m r a t h e r close to the source. In this w a y there is only one high voltage inlet into tile tank, a n d a phosphorbronze spring from this inlet a u t o m a t i c a l l y makes contact with tile source holder when this bolder is pushed t h r o u g h t h e v a c u u m lock into position. The s) K. E. B e r g k v i s t , to be p u b l i s h e d .

212

G. BACKSTR()M gt al.

high voltage is derived from 50 kV stabilized power supply~. In a 1/~¢~rtype of field, focusing takes place after an angle of n V~2, but this result strictly holds only if source and focus are on the same radius. In working with a very wide source, it is important to note that the focusing angle will depend on the radial position of the source strip and on the position of the entrance baffle (Bergkvist, loc. cit), which in the present case is situated 90 ° from the source. F r o m an experimental investigation it was found that the best focusing in this case would be achieved by letting the source make an angle of approximately 40" with the radius of the spectrometer, in good agreement with a theoretical value of 42 °. One fortunate result of this inclination is that it lowers the voltage gradient that has to be applied for a given electron energy. The source array has a total width of 5.5 cm and, because of the 40 ° angle, a radial extension of 4.2 cm, which corresponds to the full width of the neutron beam at this point. The voltage required to achieve best focusing is typically 16kV at 500 keV electron energ3, and 50 at 2 MeV. The practical procedure adopted in setting the high voltage is to adjust experimentally for m a x i m u m resolution, using some strong conversion line, and then make use of the formula E = electron energy

v=¼e

l+i~

)-

~ =z/,,~ ~

to determine the voltage variation versus momentum to maintain focusing. The voltage should be adjusted and maintained with an accuracy of about ½%. Preferably, one should have automatic regulation of the high voltage V so that the above formula is always fulfilled. In the experiments done so far, the measuring time per point has been 1000 sec, and since the resolution with this multi-strip arrangement has been between 0.20 and 0.30%, the voltage has only to be changed twice a day, and this could, of course, be done manu~flly. When a higher flux eventually becomes available, the situation m a y be different, and a system for automatic setting of the voltage has been planned. This m a y t Made by Brandenburg Ltd., England.

be done by forming an analogue DC voltage proportional to the magnetic field, i.e. to momentum, and by applying the necessary unlinear correction to this signal, after which it may be used as a reference voltage regulating the high voltage source. Fortunately, the deviation from linearity is not strong, in the energy region of immediate interest, and a polygon approximation may be used. Fig. 8 shows portions of spectra obtained with the multi-strip source. The results are quite interesting to compare with those obtained ~ i t h an ordinary single source (upper part of same figure). The external neutron flux was the same in both cases, roughly 6 × 106/cm 2 s. It is noticed that not only has the line width been reduced to less than one hMf of the previous value, but the counting rate is also several times higher. The resolution with the multi-strip source, when thc voltage is properly adjusted, is practically the same as that which would be obtained with one of the strips only. It is evident that an error in m o m e n t u m m a y be introduced unless the high voltage is accurately controlled. Actually, energy calibrations were ahvays made by the ordinary source arrangement. Intensity measurements, however, are not influenced by a deviation of the voltage from the correct value since the total line area simply is the sum of the line areas from the individual sources, no m a t t e r if the images coincide or not, provided the voltage does not change during the time the line is recorded. The useful focal length at a resolution of 0.2% is several cm in this instrument, and thus it would be possible to increase the efficiency of the instrument by another order of magnitude by having an array of detectors side by side, preferably solid state detectors, which are compact and may be placed close together. Such an arangement will later be introduced.

6. Gamma Ray Measurements As mentioned earlier, the instrument was planned for g a m m a ray experiments as well, although the measurements have so far been confined to conversion electrons. Fig. 10 shows how the spectrometer m a y be used in gamma ray work When measuring Compton eiectrons, which is the

A MAGNETIC

SPECTROMETER

FOR NEUTRON

best known method for g a m m a rays of a few MeV or more, a narrow beam of g a m m a rays is taken from the source, located near the centre of the reactor. The g a m m a rays are directed against a foil

Fig. 10. A r r a n g e m e n t for t h e m e a s u r e m e n t of g a m m a rays.

of low atomic number, e.g. beryllium, from which a cone of Compton electrons emitted approximately in the direction of the g a m m a ray is selected, by means of the baffle, and analyzed. As explained in ref.1), higher resolution is obtained if only those electrons are counted which are in time-coincidence with secondary g a m m a rays scattered from the foil within a backward cone. For this purpose two NaI crystals are introduced into the vacuum tank, on either side of the beam. Since the intention is to collect baekscattered g a m m a rays, the crystals are to be brought as close to the beam as is compatible with a reasonable background of chance coincidences. At a given g a m m a energy resolution, the backscattering cone has to be narrower the lower the energy, and therefore, in order to utilize the same scintillators, it should be made possible to locate these at different distances from the foil. Mternatively, g a m m a ray measurements m a y be performed by analyzing photo-electrons emitted from a uranium foil. Compton electrons will, however, still be present, forming a high and rapidly varying background. The background may be suppressed, and the spectrum considerably simplified, if coincidences are taken between electrons and the associated uranium X-rays from the K shell. The solid angle subtended by the X-ray detectors should, of course, be as large as possible. Compton q u a n t a will also enter these crystals, but with suitable location of the crystals, the corresponding

CAPTURE

EXPERIMENTS

213

electrons are stopped by the baffle, and energy discrimination will also tend to reduce this contribution from the Compton effect. With a suitable bucking coil to reduce the magnetic field in the photomultipliers to a tolerable value, the mlfltipliers may be kept in the vacuum, but it would certainly be difficult to avoid disturbing the field within the electron bundle or at the probe coil, which would make the magnetometer unreliable. In any case, this arrangement would strongly limit the space available for lead shielding. The light from one crystal may be brought to the photo-mlfltiplier through horisontal light guides in the s y m m e t r y plane of tile instrument, but in that case only the outer crystal can be used because of the space taken up by the beam. These considerations leave only the alternative of introducing vertical light guides through 55 mm bores in the iron plates, which also permits the photo-multipliers to be kept in the relatively field-free regions above or below the spectrometer. Since several crystal locations had to be allowed for, there is actually a series of holes in the iron plates, and all holes not needed are filled with iron plugs of the same type of iron. With 50 mm light guides of 30 cm length, which collected 50% of the light, and with a 5 mm thick iron tube around the photomultiplier in addition to the p-metal shield, the magnet could be set to focus 12 MeV electrons without noticeable effect on the pulse-height. The holes through the iron plates are evidently small enough not to influence the focusing properties, since the resolution has been found to be no worse than in corresponding magnets without these holes.

Acknowledgements The authors wish to express their thanks to Prof. Kai Siegbalm for initiating the project and for following its progress with such interest. Thanks are also due to the staff of AB Atomenergi for their enthousiastic cooperation, and in particular to Dr. K. E. Larsson for much useful advice. The work was made possible by a grant from Statens R~d f6r Atomforskning.