A PULSED NEUTRON GENERATOR

(UDC 621.373.2 : 539.172.84)

G. E. Murguliya and A. A, Plyutto

Translated from Atomnaya Energiya, Vol. 18, No. 4, pp. 336-342, April, 1965 Original article submitted February 8, 1964

The authors give the results of physical investigations on a neutron generator with spark-ion source, y ie ld ing pulsed-neutron fluxes from the reactions D+ D and D+ T, with mean yields ~7 .106 and

.~10 s per pulse respect ively. The total pulse length is ~100-250/jsec and the potent ial across the

accelera tor gap is ~110 kV.

A number of models of pulsed-neutron generators with spark ion sources have recent ly been developed [1, 2]. These are used in nuclear physics and its applications. However, they have not so far been subjected to deta i led

physical investigation. The working characteris t ics of a neutron generator are, of course, determined by the param- eters of the ion beam (composit ion, current strength and t ime characterist ics) , the magnitude and s tabi l i ty of the acce le ra t ing voltage, and the target properties. We have studied the influence of these factors on the working of one type of neutron generator.

D e s c r i p t i o n o f A p p a r a t u s a n d E x p e r i m e n t a l M e t h o d

We first developed a suitable method and apparatus for the problem in hand.

The construction principles of the neutron generator (Fig. l a ) are similar to those of the model described in [3]. It differs from the latter in that a grid 3 is positioned in the select ion and accelerator gap,and is connected to a capaci ta t ive potent ia l divider (C 1 and C2) and screen 7. This grid forms an in termedia te electrode; when the gen- erator is working it acquires a floating potent ia l and prevents e l ec t r i ca l breakdown. Screen 7 retains secondary e lec - trons emit ted from the target and protects the sides of porcelain chamber 4 from the deposition of a conducting layer, thus avoiding breakdown along the chamber sides. These modif icat ions made it possible to increase the d iameter of hole 2 in the l imit ing e lect rode to 16 mm, raising the ion current three or four t imes and increasing the acce lera t ing potent ia l to V o = 100-110 kV.

By introducing a large inductance, L= 300-600/iH, into the spark circuit , it was possible to s tabi l ize the evap- oration of the working substance and increase the neutron yield. The working substance was evaporated from com- bined-electrode 9 and burnt out at a depth of ~5 ram, preserving the m e t a l walls of the e lect rode c ha nne l The secondary e lect ron beam burns the working substance in the spark gap, which is situated on the axis of the spark source; for this reason, we used only the auxi l iary gaps of the standard source.

To study the ion-beam composition and energy scatter, we used a mass spectrograph and the method of parab- olae [4] (see Fig. lb) . The narrow ion beam was passed through a I - ram-d iamete r hole in the target 6 and two col- l imat ing diaphragms 13, and then traversed a region of e l ec t r i ca l and magnet ic fields 15, where it was analyzed; it f inal ly fel l on a photographic plate 14, fluorescent screen, or Type II scint i l la t ion counter. By photography with MP plates, we studied the ion-beam composit ion averaged over ~100 pulses. The photographs were subjected to photo- met ry (making a correct ion for the blackening produced by ions of different masses), and this enabled us to form some idea of the quanti tat ive beam composit ion. When necessary, visual observations were made on the screen for qual i ta t ive control of the beam composit ion in each individual pulse. The t ime characterist ics of the analyzed beam were studied by means of a Type II scint i l la t ion counter [CsJ(Ti), FEU-29].

At the same t ime we investigated the neutron yield. The mean yield for neutrons from the D+ D react ion was measured by the s i lver-act ivat ion method (see Fig. lb) . The apparatus was cal ibrated with standard Ra+ Be and Po+ Be

428

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d m n d o j .

o

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o ~ ~ 0

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o Z o ~ o

�9 ,~ . ~ 0 " 0

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�9 0

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Pig. 2. Mass spectrograms of beam composit ion (E = ion energy, kV). a) In i t i a l working stage of spark gap; b) opt imum stage; c) beam composit ion after 3" 10 5 pulses.

neutron sources. The neutron yield from the D + T re- action was measured by the copper-act ivat ion method. The t ime characterist ics of the pulse were studied with

a Type I counter, consisting of a plastic scinti l lator and an F~U-24 [photocell] . The absolute number of neu- trons in each separate pulse was determined by analyz- ing the tracks on the oscit lograms [53. A Type I counter was used to study the s tabi l i ty of the neutron yield from one pulse to another.

The total ion- induced current in the selector cir- cuit was registered by measuring the potent ial across

= Z0a . Spark gap A protects the measuring circuit from high-tension leakages occasioned by flashovers. The mean ion current to the target was determined by

the thermal method, corrections being made for cooling. The D +- ion current to the target was determined from the neutron yield.

E x p e r i m e n t a l R e s u l t s

I o n - B e a m C o m p o s i t i o n a n d I t s I n f l u - e n c e on t h e N e u t r o n Y i e l d . Mass-spectrosco- pic investigations showed that the neutron yield depends markedly on the ion-beam composit ion. The ion beam suffers defini te changes owing to the working of the ion source and the heat ing up of the working substance. This

is i l lustrated by the mass spectrograms shown in Figs. 2a, b and c. The H + and D + lines are superimposed, and their separate values are ca lcula ted from the lines of the molecular ions (HD) + and D +.

During the first thousand or so pulses, the working substance is usually evaporated ineff iciently, t4 +, H~, C + and O + ions predominate in the beam and there are few Li + ions (Fig. 2a). After a g i n g - a b o u t 103-10 a pul-

s e s - t h e source enters its opt imum working state and the working substance is evaporated intensively. D + ions then predominate , sometimes reaching 80% of the beam (see Fig. 2b). Impuri ty ions (Li + C +, O +) and molecular ions iDa ,

(HD) +, H +] are found only in smal l quantit ies. Li + ions adhere chemica l ly to the source, as they react with e le- ments in the porcelain. After about 5 �9 10 s pulses, this leads to destruction of the porcelain tube. The D + con-

tent of the ion beam remains stable at above 50% for about 105 pulses. After this, the number of impuri ty ions (H + , H2 + , Li +, C + and O +) then rises, and finally, after about 2 -3 .105 pulses, these ions become predomi- nant (see Fig. 2c).

As the beam composi t ion changes, so does the neutron yield. This is c lear ly seen from Fig. 3, which plots the re la t ive intensity of D + and Li + ions, ASD+/ASLi+ (I), and the re la t ive neutron yield, N (II), versus the number of pulses, n ( i .e . , the degree of burn-out of the working substance). The values during the optimum period are taken as unity. Curve I was obtained by photometry of a number of photographs like those in Fig. 2. In measuring curvel I , it was necessary to e l imina te the effect of target fatigue. For this purpose, the beam was directed on to the target for the short t ime necessary for the measurements, and the target was then covered with the diaphragm.

430

t A A A

O.5 ,

0 SO

A

/

I00 150 ZOO n ~ x 10 3

Fig. 3. ~SD+/ZXSLi+, the re la t ive D + content, and N, the neutron yield, versus n, the number of pulses. During the opt imum period, ZXSD+/ZXSLi+ and N are normal ized to

unity.

i/i!ii!il/i iii!i il/ iiiiiii!/il/;il ;i i ii /iiiiiiii!iiiii!i i! i i

b

II

Fig. 4. Oscillograms of ion-e lec t ron current in acce le ra t - ing gap (a) and neutron pulse (b) for reactions D+ D (I) and D + T (II). t = duration of ion-current pulse, g sec.

smal l V S is due to increase of the D + current at target 6 (see

By means of s imilar experiments , it was found that the number of pulses ,over which the neutron yield

decreases by a factor of two (by comparison to the opt imum yield) is about 1.5. l0 s, whatever the spark gap. The ove r -a l l l i fe t ime for the production model

[6] is about 106 pulses.

The working substance can be a vola t i le organic compound (e.g. , hydrocarbon) based on deuterium. This should preferably have a high deuterium content and not contain chemica l ly ac t ive substances. By this means the source l i fe t ime can be considerably increased.

T o t a l I o n C u r r e n t a n d I t s I n f l u e n c e o n t h e N e u t r o n Y i e l d . The mean ion current per pulse to the target , measured by a thermal method, increases proport ionally to the spark power and reaches

0.5-1 A when the potent ia l across the spark condenser is V S = 70 kV and the mean pulse duration is ~100gsec .

The t ime characteris t ics of the ion current were determined from oscil lograms of the to ta l ion current and the secondary-e lec t ron current in the selector cir- cuit . The ion current was corrected for the coeff ic ient of secondary emission, which in these experiments was 3.5-4 and did not vary appreciably in the acce lera t ing vol tage range V o = 60-100 kV.

Figure 4a shows oscil lograms of the to ta l ion

current: this osci l lates with the natural frequency of

the spark c i rcui t and decreases proport ionally to the osci l ia t ing current in the spark. Ion-current pulses are observed when the cathode spots fa l l on the com- bined e lec t rode 9 (see Fig. t ) .

.... 2

In general , the neutron yields from the D+ D and

D + T reactions (see Fig. 4b) are strongly correlated in t ime with the ion current. The to ta l ion current, de~ termined from the neutron yield ( taking account of the re la t ive D + content [7]) at V G = 80-100 kV, does not differ apprec iably from the value determined by the thermal method.

At constant V G, the neutron yield per pulse from the D + D react ion increases with the power of the spark discharge (Fig. 5). The spark power was increased by

increasing the in i t i a l potent ia l V S across the spark ca-

pac i tance C3; the gap K was also widened. The pulse repet i t ion frequency was kept constant by varying R 2 (see Fig. la ) . The rapid rise of the neutron yie ld for

Fig. l a ) . The bend in the curve of Fig. 5 is due to the fact that the ion beam begins to fa l l outside the edges of the target . The absence of saturation is here caused by growth of the spark-plasma density, and consequently of the ion current, with increasing V S .

At V G = 110 kV and V S = 70 kV, the maximum neutron yields from the D + D and D + T reactions per pulse of length ~250 gsec were ~7 �9 106 and ~109 respect ively.

431

71 j f ~

6 E /

4 / 3 1 2 20

\

J_A i

30 40 50 60 Vs,k V

Fig. 5. Neutron yield per pulse versus spark power at V G = 106 kV (V S = in i t ia l potent ia l across spark condenser C 3, N = number of neutrons per pulse).

Fig. 6. OsciUograms of ion-e lec t ron current in ac-

celerator gap (a) and D+ current in analyzer (b).

E n e r g y S c a t t e r o f I o n s in t h e B e a m a n d I t s I n f l u e n c e o n t h e N e u t r o n Y i e l d . The massspec t ro- grams (Fig. 2a, b, c) revea l that the ions have an energy scat-

ter of up t o - 1 5 kV, due to decrease in the acce le ra t ing voltage at large pulse currents. By increasing C z and C e , this fa i l ing-off in vol tage can be reduced, but there is then an increase in gas evolution during flashover, and this

hinders the operation of the generator. These adverse effects are espec ia l ly not iceable at low acce lera t ing voltages. At voltages )100 kV, vol tage fluctuations reduce the neutron yield only by 10-15~

In most cases, the sections of the mass-spectrogram parabolas (Fig. 2a, b, e) have a layer structure. This is

due to current oscil lations in the analyzer . Figure 6 illustrates this effect with oscillograms of the to ta l ion-e lec t ron current in the selector circui t (a) and the D +- ion current (b) in the analyzer . The beam usually enters the analyze- only during the decreasing stage of each osci l la t ion of the total ion current. Between successive entries of the beam into the analyzer , the acce le ra t ing vol tage falls off and the beam departs from the parabola, impart ing a layer struc-

ture to the blackening. In some cases the analyzer current vanishes al together, or does not at a l l correspond in am- pli tude and duration to the to ta l ion current. These deviations in the analyzer current are apparently due to fluctua- tions in the emit t ing surface of the plasma; these cause changes in the conditions for transmission of the relevant beam component through the narrow opening in the target. The lack of correspondence between the analyzer current and the total ion current might cause errors in determining the beam composition: to ensure re l iab le analysis, we therefore averaged over a large number of pulses.

W

30

20

1o , / \ 70 80 90 ~00 110

a b i f

3~

2O

I0

70 80

W

90 100110 120 70 80 90 I n

3~

2O

10[ -4,

60

Fig. 7. Histograms showing stabi l i ty of neutron yield, constructed from separate consecu- t ive series of measurements taken over ~104 pulses. I n = ampl i tude of neutron pulse, arbi- trary units; W = number of pulses, a) V G = 96 kV; b) V G = 90 kV; c) V G = 87 kV.

432

N

l �9 ~ �9 ' 0

0.5 8 16 29 32 $0 n,~IO

Fig. 8. Fat igue of TiT target versus num- bet of pulses, n. N = re la t ive neutron yield,

normal ized to unity at n= 5 .10 a pulses.

S t a b i l i t y o f N e u t r o n Y i e l d

T a r g e t F a t i g u e . The s tabi l i ty of the neutron yield depends

on the working of the ion source and on target fatigue. These two

factors were studied separately.

The neutron yield was measured, with the generator working in the opt imum state, for each of 100-150 pulses. The target was then covered with the diaphragm and a further series of measurements made over ~104 pulses, and so on. S imi lar measurements were made for a l l

six spark gaps in the product ion-model ion source. On passing from

one gap to another, the to ta l neutron yield can change by a factor of two or three. For an individual gap, the neutron yield varied by about ~ 5-10%. In the conditions of our experiments , the vacuum spark thus developed with adequate stabil i ty. Figure 7 gives typ ica l histograms of the neutron yield.

Fat igue of the target might be caused ei ther by the formation of an organic f i lm on its surface (oil from the

diffusion pump) or by sputtering from the beam.

The fatigue of TiT targets was investigated exper imenta l ly by the following method. The generator was brought into its optimum working stage, and the re la t ive neutron yield from the D + T react ion was measured, keep- ing the acce lera t ing vol tage constant at V G = 90 kV. Measurements were made over ,.,100 pulses out of each ~3 �9 10 s

pulses. The yield was measured by a moni tor based on the short- l ived induced ac t iv i ty of lead (T1/2 = 0.8 sec) [8]. A special e lect ronic circui t ensured registration of the ac t iv i ty belonging to Pb 2~ formed main ly by the react ion Pb 2~ (n, 2n)Pb 2~ Fatigue of the TiT target scarcely reduces the neutron yield during ~ 4 . t04 pulses (Pig. 8).

Our investigations of this neutron generator with spark-ion source have thus enabled us to make considerable improvements in the working parameters of the first mode l [3]. The simple construction of the generator and associ- ated e l ec t r i ca l c i rcui t make this a convenient device for laboratory investigations in nuclear physics and its appI ica- tions. In l?articular, it has been used to study short- l ived isotopes and isomers (T1/2) 1 msec) formed in (n, 2n) re- actions with various nuclei .

The authors would l ike to thank I. P. Sel inov and I. M. Rozman for their interest and valuable advice.

LITERATURE C I T E D

1.

2. 3. 4.

5. 6. 7. 8.

J. Gow and H. Pollock, Rev. Scient. Instrum., 31, 3, 235 (1960).

B. C a r l Nucleonics, 18, 75 (1960). G. E. Murguliya and A. A. Plyurto, Pribory i tekhnika 6ksperimenta, 5, 28 (1961). A. Dempster, Rev. Scient. Instrum., 7, 46 (1936). G. E. Murguliya, A. A. Plyutto, and i~. M. Rozman, Pribory i tekhnika dksperimenta, 1, 54 (1962). A. A. Plyutto, K. N. Kerval idze, and I. F. Kvartskhava, Atomnaya ~nergiya, 8, 153; (1957).

~.-G. V. Aleksandrovich and V. A. Sokovishin, Pribory i tekhnika 6ksperimenta, 5 , 7 (1961). L. Ruby and J. Rechen, Nucl. Instrum. Methods, 15, 74 (1962).

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