The paper reports some new results on irradiation characteristics of synthetic quartz crystals and their application to radiation hardening. The present results show how the frequency shift in quartz crystals can be influenced by heat processing prior to irradiation and how this procedure can lead to radiation hardening for obtaining precise frequencies and time intervals from quartz oscillators in space

Bahadur, H.

Parshad, R.

English

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SOME NEW RESULTS ON IRRADIATION CHARACTERISTICS OF 
SYNTHETIC QUARTZ CRYSTALS AND THEIR APPLICATION 
TO RADIATION HARDENING 
HARISH BAHADUR and R. PARSHAD, 
National Physical Laboratory, 
Hillside Road, New Delhi - 110 012, 
India. 
ABSTRACT 
The paper reports some new results on irradiation 
characteristics of synthetic quartz crystals and 
their application to radiation hardening, The 
present results show how &e frequency shift in 
quartz crystals can be influenced by heat processing 
prior to irradiation and how this procedure can 
lead to radiation hardenhg for obtaining precise 
frequencies and time intervals from quartz oscil- 
l a tors  in space. 
In line with the thinking already proposed (H.Bahadur 
and R. Parshad, Proc, 12th PTTI Conference, pp.829- 
849, 1980) for explaining some of the irradiation 
characteristics of natural crystals, for the present 
work, it was assumed that the positive frequency 
shift in irradiated synthetic crystals is due to 
crystal imperfections. The crystal imperfections 
in synthetic crystals were sought to be removed by 
appropriate annealing (slow heating to 24OoC followed 
by slow cooling). Different annealed crystals on 
irradiation gave either a negative frequency shift, 
thus simulating the behavior of natural crystals 
or almost a zero frequency shift. This result woula 
be due to the positive frequency contribution of 
remanant crystal defecte matching the negative 
frequency contribution of the A-centers of the 
crystal lattice, 
The experimental results indicate a new way of 
radiation hardening. 
This work was supported in part by a Homi Bhabha 
Fellowship awarded to H, Bahadur. 
593 
https://ntrs.nasa.gov/search.jsp?R=19830027108 2020-03-21T01:12:42+00:00Z
INTRODUCTION 
By now, intense work 1-3370n irradiation characteristics of natural 
and synthetic quartz crysta 
results are the following. 
crystals, Y-and X-irradiation produce a n 
culminating in most cases t 
radiation doses. For syn 
complex and varying. For 
premium Q16, the frequency in 
crystals of the lower grade, the frequency re constant with 
irradiation dose while for the premium Q crystals, the results 
are varying (frequency increase or constant) depending upon the 
nature and technique of sweeping16 or a constancy of frequency 
followed by decrease18. 
For natural crystals, investigations in this laboratory 
showed that on irradiation, while the optically clear quartz 
gave the frequency characteristics following the literature 
results (negative frequency shift), but for the non-optically 
clear quartz the initial decrease of frequency was followed by 
an increase. This increase can be so much as to have the new 
resonance frequency greater than the starting frequency before 
irradiation. 
28,36,37 
We were led to believe that the ensuing positive frequency shift 
(after the initial negative shift) was due to the role of crystal 
defects present in the non-optically clear natural quartz, these 
defects also producing the relative opacity of the crystals. That 
crystal defects were present in such crystals was also adduced by 
results of investigations of EPR and thermoluminescence of these 
crystals38However, these studies will not be discussed in the 
present paper. 
Parallel with the role of defects in causing positive frequency 
shifts in natural crystals, it was thought that the overall 
positive frequency shift in synthetic crystals8 right from the 
time of irradiation was due to the greater density of defects 
present in such crystals (particularly in those produced at the 
time of Capone et.al.'s publication: Ref.8). The above thinking 
led us to the attempt of removing or decreasing the crystal 
defects by appropriate annealing before subjecting the crystal 
to 7-irradiation. The results have been rewarding and are det- 
ailed below. As would be seen, the results have not only funda- 
mental interest but also indicate a new way of radiation hardening 
so desirable for use of crystals in satellite-borne frequency 
standards and other associated applications. 
594 
EXPERIMENTAL RESULTS 
The various synthetic crystal AT-cut resonators st ied were drawn 
from both unswept and swept materials and oscillat in different 
frequency regions such as 1, 3.5 and 5 Mwz. Swept crystals were 
Sawyer material and their sweeping was done at Sandia Laboratories, 
Albuquerque, NM. The unswept synthetic crystal resonators were 
assembled by Andhra Electronics, India. The resonators were used 
as the frequency determining elements in a 
modified Colpitts oscillator circuit. The adi- 
ated in steps from a 6oCo source to different cumulative doses. 
At the outset, before giving results for individual crystals, it 
should be stated that all the crystals, swept and unswept, gave 
on irradiation an initial frequency instability before the frequen- 
cies took up steady values. In this paper, the experimental 
results about only the steady frequency drifts will be described 
and discussed. 
In the following are given the experimental results for the swept 
and unswept crystals separately. The studies on the swept crystals 
were done with the aim to effective1.y judge the results on the 
unswept synthetic crystals, all subjected to the same irradiation 
environment. 
Swept Synthetic Crystals 
Two swept resonators (designated as Resonators I and 11) were oper- 
ated in their third overtone in the frequency region of 3 MHz. 
Figures 1 and 2 represent the irradiation results. It can be ob- 
served that the steady frequency following the initial instability 
is unchanged as a result of irradiation. 
Unswept Synthetic Crystals 
The experimental results for unswept crystal resonators designated 
as Resonators 111, IV and V are given in tables I to 111. As can be 
seen from the tables, for all the resonators the post-irradiation 
frequencies, following the literature results, were greater than 
those of the virgin crystals, the frequencies showing a saturation 
for extended doses. At this stage, the synthetic crystals already 
irradiated were annealed. In th.e annealing procedure, the resona- 
tors 111, IV and V were gradually heated to a maximum temperature 
of 24OOC in a thermostat. They were kept at this temperature for a 
few hours (- 5 hrs) and were subsequently cooled slowly at the rate 
of about 5OC/hr to room temperature. The annealing process would, 
of course, also fully or partially bleach the crystals. It was 
interesting to observe that the resonance frequencies of the ann- 
ealed crystals were lower than those of the original virgin crys- 
tals. 
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TABLE - I 
Frequency behavior of a 5-MHz (unawept) synthetic quartz 
crystal (Resonator 111) on 7-irradiation. 
Virgin crystal frequency = 5000217 Hz. 
Dose Steady frequency Steady frequency offset 
2 5 kRads 5000253 Hz +36 Hz 
50 kRads 5000208 Hz +51. Hz 
100, kRads 5000270 Hz +53 Hz 
200 kRads 5000273 Hz +56 Hz 
At this stage the crystal was annealed. Frequency of 
annealed crystal was 4999792 Hz (-480 Hz lower than 
the frequency of the virgin crystal). Further irradiation 
showed the following results. 
Dose Steady frequency 
Steady frequency offset 
with respect to the 
frequency of the an- 
nealed c rys t a l .  
100 kRads 4999775 Hz 
200 kRads 4999765 Hz 
- 17 HZ 
- 27 HZ 
598 
TABLE - I1 
Frequency behavior of a 3.5 MHz(unswept) synthetic quartz 
crystal (Resonator IV) on ?-irradiation. 
Virgin crystal frequency = 3500125 Hz. 
Dose Steady frequency Steady frequency offset 
25 kRads 3500153 Hz +14 Hz 
50 kRads 3500158 Hz +33 Hz 
100 kRads 3500158 Hz +33 Hz 
200 kRads 3500160 Hz +35Hz 
At this stage the crystal was annealed. Frequency of 
annealed crystal was 3500079 Hz (-45 Hz lower thari 
the frequency of the virgin crystal). Further irradia- 
tion showed the following results. 
Dose Steady frequency Steady frequency offset 
with respect to the 
frequency of the an- 
nealed crystal.  
10 0 kRads 3500086 Hz +7 Hz 
200 kRads 3500084 Hz +5 Hz 
599 
TABLE - I11 
Frequency behavior of another 3.5 MHz (unswept) synthetic 
quartz crystal (Resonator V) on ^(-irradiation. 
Virgin crystal frequency = 3500087 Hz. 
Dose Steady frequency Steady frequency offset 
25 kRads 3500107 Hz 
50 kRads 3500120 Hz 
100 kRads 3500123 Hz 
2000 kRads 3500123 Hz 
~ ~ ~- - 
+20 Hz 
+33 Hz 
+36 Hz 
+36 Hz 
At this stage the crystal was annealed. Frequency of the 
annealed crystal was 3500056 Hz (-30 Hz lower than the 
frequency of the virgin crystal). Further irradiation 
showed the following results. 
Dose Steady frequency 
Steady frequency offset 
with respect to the 
frequency of the annealed 
crystal. 
100 kRads 3500062 Hz +6 Hz 
200 kRads 3500065 Hz +9 Hz 
600 
Regarding the irradiation cheract 
e present se 
visualized by us, simulated the behav 
Resonators IV and V gave post-irr 
same as they were before the second irradiation. 
DISCUSSION OF RESULTS 
The prominent results that stand out from our investigation 
that on annealing the synthetic crystals the irradiation-frequency 
behavior changed characteristics rather drastically. In one reso- 
nator the characteristic became the same as of natural crystals 
(frequency decreasing with irradiation dose). The other two reson- 
ators studied showed almost a constant frequency characteristics 
(a residual positive frequency shift). The resonator with the 
negative frequency shift validates the assumption that the positive 
frequency shift due to irradiation of unannealed crystals is a 
result of crystal defects and therefore on removing or decreasing 
the defects on annealing, the frequency behavior would simulate 
that of the natural crystals. For these crystals, the negative 
frequency shift is of-course due to the contribution of the A- 
centers formed from their precursors, the so-called [Al/M+)o - 
centers 4. 
In the context of the above, the irradiation characteristics of 
the other resonators giving almost zero or residual positive fre- 
quency shift must be due to the positive frequency contribution 
of the remanant crystal defects (remaining after non-ideal anneal- 
ing) almost matching the negative frequency shift. 
Here, it may be of interest to observe that the frequencies of the 
annealed crystals (before second irradiation) were even less than 
those of the.origina1 virgin crystals. It has been shown earlier38 
that neutron-irradiated crystals and those subjected to high enough 
temperatures ( N1OOO°C) and rapid enough cooling gave increased 
hardness and modulus of rigidity, all this leading to increase of 
resonance frequency. In the light of these results, the decrease of 
the observed resonance frequency by annealing indicates reduction 
or elimination of crystal imperfections, the premise on which the 
work of the present paper is based. 
Apart from the fundamental interest of the investigations, the 
results have the immediate application in that more controlled 
annealing procedure would yield exact radiation hardening compar- 
able to those of swept crystals (Figures 1 and 2) for satellite- 
borne frequency standards and other associated uses. 
601 
ACKNOWLEDGMENTS 
The authors appreciate the help of Dr- J.C, King of Sandia Labor- 
atories, Albuquerque, NM in providing swept crystals for the pres- 
ent, studies and wish to acknowledge the cooperation of Dr. M.M. 
Gupta, Mr. N,K.Ray and Mr. N.Kumar of Institute of Nuclear Medicine 
and Allied Sciences, New Delhi for various facets of irradiation. 
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603 
QUESTIONS AND 
None for Paper #31, Not Presented a t  PTTI 
I 
604 


Some new results on irradiation characteristics of synthetic quartz crystals and their application to radiation hardening

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