Lauritsen. An eleotrostatio defleotion system was designed to oompensate fluotuations in the beam position ~d entranoe

105- CTA

T. Lauritsen. An eleotrostatio defleotion system was designed to oompensate fluotuations in the beam position ~d entranoe

angle to the elec,trostatio analyzer. It was hoped that 'such oorreotions w,ould make frequent reca1ibration of the e1ectro- statio analyzer unnecessary. The defleotors oonsisted of two sets of parallel plates aoting in the same direotion as the energy-analyzing plates whioh oould simultaneously adjust the position and entrance angle for greater, orbit reproducibility and one set of plates in the orthogonal direotion which would produoe the same results when used in oonjunotion with the mass-separating magnet. Variable five-kilovolt power supplies

and a remote-control panel were designed and built by John

Domingo in a manner designed to maintain orthogonality between

-118-

position and angular displacements of the beam. When instal- led in the base of the accelerator column, the deflectors performed their function adequately but produced an unfor- tunate electron loading which reduced the ter.m1nal voltage by an undesirable amount. The deflectors were then removed, redesigned, and rebuilt in a more compact geometry which would locate the top of the deflector system well below the bottom grounded ring of the accelerator column. The rebuilt deflec- tors were not installed because a satisfactory method of ad-

justing the position of the energy analyzer was devised by Bardin

~

al. (44,45).

The position of the beam in the entrance tube is deter- mined by a pair of slits which could be rotated remotely at

a viewing station loc'ated by the deflector control panel.

For viewing the beam position from the images of the slits, a periscope and lens system was designed to focus on a remotely insertable quartz located beneath the second of the two slits.

The operation of this part of the system has'been quite satis- factory.

A major problem was encountered in counting the reaction products after momentum analysis in the magnetic spectrometer.

As the reaction products inclUde all the isotopes of H and He, pulse height analysis was required to separate the alpha

yield. The conventional apparatus used for particle detec-

tion at the spectrometer exit has been CsI crystals mounted

on a photomultiplier tube. The problem arose from the fact

-119-

that .alpha particles or twice the energy of deuterons make an only slightly larger pulse height in CsI. For the ener- getics used in the present experiment, elastically scattered deuterons were present in great abundance rrom the (HD) com- ponent of the singly-charged mass-three beam. Prori1e yields rrom thick target scattering indicated that. (HD) / He 3 = ^10-3

in the incident beam. This large amount of deuterium is prob- able evidence or a leak in the rour-way valve system allowing small amounts of deuterium to leak into the ionization bottle.

The elastic scattering yields of D were round to be at least an order of magnitude greater than the alpha yields from the reactions. The

~et

effect ror Csl detection was that the alpha-particle pulse height spectrum appeared as a small bump .

on the high-energy side or the deuterium pulse-height spectrum.

This situation ruined the statistics or the alpha counting.

To correct this problem a new detection system was designed for the spectrometer exit. A solid-state counter with a

counting area of 1/2 cm2 .was mounted in a cylindrical housing that is rapidly interchangeable with the Csl and photomulti- plier tube. .The counter is a p-n junction with a counting layer depth equal to the range or 4 Mev protons. The counter pulse-height resolution was round to be better than 1% for

4 Mev alpha particles. The pulses from the counter went

through conventional amp1irication and were stored and analy-

zed in a 100-channel pulse-height analyzer. Since the response

of the counter is accurately linear in energy, protons and

-120-

alphas make a pulse height twice as

1a~ge

as do

deute~ons

arter'momentum selection. A 10-4 cm nickel toil was placed in front of the

counte~ su~face

to

lowe~

the alpha energy to a point intermediate to that of the deuterons and

p~otons.

Figure 22

demonst~ates

a typical pulse-height spectrum thus obtained.

b. Targets

The nitrogen targets used in this experiment were pre- pared by Hebbard (46). Titanium was evaporated onto a nickel backing, and the resultant layer was nitrided by induction heating to a red-heat ror a short time in an

atmosphe~e

ot dry ammonia. The resulting TiN compound has been found to be stable under bombardment. The ammonia for the N15 target was prepared from ammonium nitrate, supplied by the Eastman . Kodak Company, with the ammonium radical enriched in N15 to 67~. The N14 target was prepared by the same procedure using natural ammonia.

The thickness of the TiN layer to 429 kev

p~otons

was original.ly measured by

Hebba~d

by observing the yield of the 4.43 Mev gamma rays from N15 (p,ay) C12 near the 429 kev reso- nance for that reaction as a function of proton energy. The result was a 7 kev thickn,ess for 429 kev

p~otons.

In the

~resent

experiment the thickness of eaoh target spot used was

calculated from the integrated alpha yield ot N15 (p, a ) C1 2,

which measures the total numbe~ ofN15 atoms in the target.

-121-

The known

che~c~l

composition of the target and the titanium and nitrogen, stopping cross sections were then employed to calculate the thIckness of the target, which agreed with Hebbard's value. The amount of N15 present was found to be 1.9 x 1017 'atom cm-2 with variations of as nmch as 25% for various target spots. The amount of N 1 4 ^{in the N14 target}

surface layer was found to be 2.0 x 1017 cm- 2 by again using the N15 (p,

^{(J )}

012. yield and the natural composition of ni tro- gen. A belatedly discovered major difference in the two tar- gets will be mentioned later.

c. Calibrations and Errors

I

had the advantage of perfor.ming this ,experiment at the conclusion of accurate Q-value measurements by Bardin et

~.

(44, 45), who devised a method for electrostatic analyzer alignment capable of reproducing the Li7 (p,n) threshold to better than one part in eight thousand. The analyzer has also been found to be linear over its range to better than

0.1~.

At the beginning of each run the position of the 1210 kev reso- nance i~ N15 (p, a ) 012 'Was found to agree with Bardin's cali-

,

bration to better than

0.1~.

For these reasons the incident He3 energy was computed to be 2.763 Mev from the calibrations of Bardin et ale with a probable error of 0.1% or 3 kev. - -

^.

The magnetic spectrometer is not so accurately reproduce-

able because of a combination of hysteresis effects and jar-

ring of the cone-bearing fluxmeter mounts as described by

-122- '

Bardin (45). For this reason the spectrometer was calibrated against the incident particle energy at the beginning ot each run. This calibration was made in several ways which all

agreed tor a given run: (a) the elastic scattering ot He 3 from a nickel blank, (b) the energy ot the alphas from

N15 (p, a, )C12 at the 1210 kev resonance, and

(0)

the energy ot the protons to the 3.945 level in N14 trom 012 (He 3, p) N14.

This last reaction was found to be the most satisfactory for the to1lowing reasons: (a) 012 was always present as a thin layer on the front surtace of the target and as such needed 'no surtace

l~yer

correction; (b) the protons possessed energies

at both 90

⁰

and 150

⁰

that placed them in the middle ot the spectrometer energy range under investigation; and (c) the Q-val~e ot 834 ± 4 kev measured in the c'ourse ot the experi-

m~nt'

tor this reaction agrees exactly with the latest tabu- lation of Ajzenberg-Se10ve and Lauritsen (47), the probable

error not contributing

appreciab1~

to the tinal probable error of the experiment. This Q-value was calculated using the

calibration of the spectrometer based on a clean elastic scattering profile from a nickel blank.

It is appropriate at this point to stress the advantages of calibrating E2 against,El as done in this experiment. The present case will serve as a fine

^exrump1e~

Suppose that a

known Q-value such as C12 (He), p) i's used to calibrate the

spectrometer against an incident energy El. If the incident

energy were 'actu811y in error by an wmount

~

El • then E2 would

-12.3-

be in error by an amount A E2 = (bE ^{2 ). A} E:J.. It' the Q-value

. ' bEl

C

of a second reaction is now measured at the same El and very near the same E2 , that Q-value will be in error by an amount

Consider a numerical example at 90°. For C12 (He 3, p) the.

quantity

(bE2/b~)c

• 0.7.3. For the reaction being studied, N15 (He 3,

^{(J ) ,}

the quanti ties bQ/bEl and bQjbE2 are respec- tively -0.78 and 1.28. By' inserting these values, one finds

. AQ = ^{[-.78 + 1.28} (.7.3)JA~ ^: .15A~ ^•

The above equation means that an estimated error

o~

.3 kev in EI is almost completely compensated by the calibration procedure. An exactly analogous argument applies to small uncertainties in the spectrometer angle and will not be '

explicitly presented here.

I~

these. were the only sources

o~

error, the uncertainty in the measured Q-value would roughly equal the uncertainty of' the

Q-valu~

against which the calibration was made.

The major uncertainty in the Q-values of' this experiment comes ~rom a dit't'erent source. For the (He3 , a) reactions on nitrogen, the incident particle energy must be reduced by the energy thickness of the surface carbon layer plus one-half

the energy thickness

o~

the TiN layer

~or

the incident particle

Mi

^o~

energy El , and the observed E2 must be increased by the

-124-

analogous quantity for particle M2 and energy E2. The lion's share of this uncertainty comes from the surface carbon layer.

Using the integrated yield of C 12 (He 3, p) and the 90

⁰

differ- ential cross section of 0.8 rob/ster. given by Bromley eJ;,&. (48) the amount of carbon on the surface was found to be commonly

as high as 8 x 1017 atom cm- 2 • This value changed from spot to spot and also changed continuously with running time. This amount of carbon corresponds to a 16 kev thickness for 2.8 Mev

++ .

He 3 ions. By monitoring this yield, the author feels that