**3. NPL active media on transitions of atoms and atomic ions**

#### **3.1. IR lasers operating on transitions of Xe, Kr, and Ar**

a compact spectrometer through the fiber, and the recorded spectrum is displayed on a

computer.

172 174High Energy and Short Pulse Lasers

**Figure 4.** View of the experimental setup on the DC-60 accelerator.

top of the chamber (b).

**Figure 5.** 400 Torr helium (a) and 600 Torr neon (b) luminescence induced by argon ions.

radioactive isotopes [36, 56–58] and pulsed nuclear reactors [59–61].

Beam from the narrow ion-excited region is reflected on the separating flange (a, b) and on the

Studies on luminescence spectra of gaseous media were also conducted with the use of

Nuclear-pumped lasers operating on IRtransitions ofXe, Kr, and Ar were investigated in detail and have maximum output parameters for NPL. The research by VNIIEF in 1972 during the first experiments was performed with the VIR-2 reactor; the output power of xenon laser with an optimal pressure and composition of the He-Xe was 25 W with efficiency ~0.5%, but the results were not published that time [62]. In 1974, obtained laser action on He-Xe composi‐ tion (λ = 3.51 μm) with excitation by uranium fission fragments [63]. It was one of the first publications on the achievement of lasing under direct nuclear pumping [63, 64].

Most of the lines with laser action refer to nd-(n + 1)p transitions of Xe, Kr, and Ar atoms (n = 5, 4, 3 for Xe, Kr, Ar, respectively). Xenon laser (λ = 1.7–3.5 μm) has received the most studies, as it has the highest output parameters. He, Ar, Kr, and compounds thereof served as buffer gases. The first NPL xenon laser has the maximum achieved energy parameters:


Xenon laser also has the lowest lasing threshold: 1.5°1012 n/cm2 s in Ar-Xe mixture (λ = 2.03 μm) [7, 68]. These advantages, as well as the absence of degradation of the laser mixture as a result of radiation and chemical reactions, join the lasers operating on IR transitions of Xe, Kr, Ar the ranks of most promising in terms of reactor-laser creation [18].

Upper location of laser levels suggests a weak temperature dependence of its output param‐ eters. However, in the case of lasers operating on IR transitions of inert gases, the output power is halved at the temperature of the mixture 350–550 K [3, 69]. The reasons for this are still not fully understood and are the matter of discussions. The most probable reasons were consid‐ ered:


Although lasers operating on IR transitions of inert gases are studied for over 40 years and considered to be the most promising, there is still no clarity with mechanism of upper nd-levels population [3, 73]. The processes of deactivation of lower (n + 1)p-levels can be considered well established; it is a collisional quenching in collisions with atoms of active medium, and with electrons at high-power pumping. The main problem in determining nd-levels population mechanism is associated with complexities of IR radiation registration within the reactor experiment.

The main presently discussed mechanisms of levels population (B—Xe, Ar, Kr; A—buffer gas atom) [73] are as follows:


Widely held hypothesis implies for levels population in processes (3) of dissociative recom‐ bination of heteronuclear ionic molecules with electrons [29, 74]. In works [3, 73, 75], the main channel of population is considered to be the process (2) of electron-ion recombination of B2 + ions. In recombination mechanism of Xe levels' population, the lasing failure with addition of 5 Torr of uranium hexafluoride toAr-Xe mixture [16] can be explained not only by quenching xenon levels by UF6 molecules, and also by electron attachment to UF6, by recombination of xenon ions with negative ions in the mixture.

According to [33, 76], dissociative electron-ion recombination of molecular ions of inert gases cannot be the main process of nd-levels population ofinert gas atoms. It is expected to populate levels by direct excitation of secondary electrons from the ground state of atom, as well as transfer of excitation from buffer gas atom [33].

#### **3.2. Visible-range lasers operating on Ne atom transitions**

Atomic neon laser created in 1961 by Javan A. and others is the first laser with active gas medium. Therefore, the first proposals [1] and first experiments [2] on NPL creation were associated with well-known transitions of He-Ne laser with a wavelength of 632.8 nm and 1.15, 3.39 μm. In 1980, it was reported [77] on laser action of 5s′[1/2]<sup>1</sup> 0 –3p′[3/2]<sup>2</sup> neon atom transi‐ tion (λ = 632.8 nm, **Figure 6**) after excitation of <sup>3</sup> He–Ne mixture by products of nuclearreaction 3 He(n,p)T in stationary nuclear reactor. At the same time, laser efficiency was approximately 0.03% and the lasing threshold was reached at a very low thermal neutron flux *F* = 2°10<sup>11</sup> n/ cm2 s. High gain in a mixture of 3 He-Ne (1.7°10−2 cm−1 at *F* = 2°10<sup>12</sup> n/cm<sup>2</sup> s) was also measured in work of Chinese authors [49]. The results of these studies are questionable and subject to discussion [78, 79]. Simple estimates [78] show that in these conditions [77] population cannot be obtained even in the extreme case, when all power deposited in gas is fully transferred to the upper laser level, and the level is not quenched in collisions with atoms. In our experi‐ ments, the neutron flux density was gradually changed from 1011 to 1014 n/cm<sup>2</sup> s; however, the lasing threshold in <sup>3</sup> He:Ne = 5:1 mixture was not reached [53, 79]. In addition, the lumines‐ cence spectrum of He-Ne mixtures at ionized radiation pumping has no 632.8 nm line (**Figure 7**).

Nuclear-Induced Plasmas of Gas Mixtures and Nuclear-Pumped Lasers http://dx.doi.org/10.5772/63823 175 177

electrons at high-power pumping. The main problem in determining nd-levels population mechanism is associated with complexities of IR radiation registration within the reactor

The main presently discussed mechanisms of levels population (B—Xe, Ar, Kr; A—buffer gas

Widely held hypothesis implies for levels population in processes (3) of dissociative recom‐ bination of heteronuclear ionic molecules with electrons [29, 74]. In works [3, 73, 75], the main channel of population is considered to be the process (2) of electron-ion recombination of B2

ions. In recombination mechanism of Xe levels' population, the lasing failure with addition of 5 Torr of uranium hexafluoride toAr-Xe mixture [16] can be explained not only by quenching xenon levels by UF6 molecules, and also by electron attachment to UF6, by recombination of

According to [33, 76], dissociative electron-ion recombination of molecular ions of inert gases cannot be the main process of nd-levels population ofinert gas atoms. It is expected to populate levels by direct excitation of secondary electrons from the ground state of atom, as well as

Atomic neon laser created in 1961 by Javan A. and others is the first laser with active gas medium. Therefore, the first proposals [1] and first experiments [2] on NPL creation were associated with well-known transitions of He-Ne laser with a wavelength of 632.8 nm and 1.15,

He(n,p)T in stationary nuclear reactor. At the same time, laser efficiency was approximately 0.03% and the lasing threshold was reached at a very low thermal neutron flux *F* = 2°10<sup>11</sup> n/

in work of Chinese authors [49]. The results of these studies are questionable and subject to discussion [78, 79]. Simple estimates [78] show that in these conditions [77] population cannot be obtained even in the extreme case, when all power deposited in gas is fully transferred to the upper laser level, and the level is not quenched in collisions with atoms. In our experi‐

cence spectrum of He-Ne mixtures at ionized radiation pumping has no 632.8 nm line (**Figure**

ments, the neutron flux density was gradually changed from 1011 to 1014 n/cm<sup>2</sup>

He-Ne (1.7°10−2 cm−1 at *F* = 2°10<sup>12</sup> n/cm<sup>2</sup>

He:Ne = 5:1 mixture was not reached [53, 79]. In addition, the lumines‐

+e→B(nd)+e.

+

**4.** Transfer of excitation in inelastic collisions: Ar\*+Xe→Xe(5d)+Ar.

**3.** Recombination of heteronuclear ionic molecules: AB+

+e+e (M)→B(nd)+e(M), M— third particle.

+e→B(nd)+A.

0

He–Ne mixture by products of nuclearreaction

–3p′[3/2]<sup>2</sup> neon atom transi‐

s) was also measured

s; however, the

+

experiment.

atom) [73] are as follows:

174 176High Energy and Short Pulse Lasers

**1.** Shock-radiative recombination: B+

**5.** Step excitation: Xe(6s, 6s′)+e→Xe(5d)+e.

xenon ions with negative ions in the mixture.

transfer of excitation from buffer gas atom [33].

tion (λ = 632.8 nm, **Figure 6**) after excitation of <sup>3</sup>

s. High gain in a mixture of 3

lasing threshold in <sup>3</sup>

3

cm2

**7**).

**3.2. Visible-range lasers operating on Ne atom transitions**

3.39 μm. In 1980, it was reported [77] on laser action of 5s′[1/2]<sup>1</sup>

**2.** Electron-ion recombination: B2

**Figure 6.** Scheme of laser transitions in neon. The wavelengths of laser and resonant transitions are indicated in nm.

**Figure 7.** Emission spectrum of neon at a pressure of 605 Torr under ion beam excitation in the 570–900 nm region. The vertical green line indicates the wavelength of 632.8 nm.

3p levels of neon atoms are efficiently populating during excitation by the products of nuclear reactions of neon and its mixtures [57, 80] (see **Figure 7**). To create laser operating on 3p–3s transitions of neon, it is necessary to solve the problem of lower s-levels deactivation. These levels are metastable or resonantly coupled to the main level and have trapped radiation at neon pressures important for nuclear pumping radiation. Rapid depletion of lowerlaserlevels at a relatively low concentration of quenching particles can be achieved in the processes occurring with Coulomb cross-sections. In Ref. [20], authors show the possibility of deactiva‐ tion of excited states in the Penning process. The required selectivity of deactivation of the upper and lower levels can be achieved using the lower levels states as resonantly coupled to the main, for which the ionization cross-section of quenching by additives is particularly high [81].

Quasi-continuous lasing in allowed neon atom transition 3p′[1/2]0–3s′[1/2]<sup>1</sup> <sup>0</sup> (λ = 585.3 nm) was first observed in the afterglow of discharge [82], and then at pumping by powerful electron beam of Ne-H2, Ne-Ar, Ne-Ar-He mixtures [83]. In [84] by reducing the concentration of neon and quenching the lower level of additives by an order compared to [83], and pumping power by electron beam ~100 kW/cm<sup>3</sup> with He:Ne:Ar = 96:3:1 mixtures at a pressure of 3 atm at the same transition obtained laser efficiency of 1–2%. The laser efficiency with ionizing pump‐ ing at λ = 585 nm in the opinion of other authors cannot exceed 0.5% [85]. The electron beam pumping provided for laser action in a row of 3p–3s neon transitions in the spectrum red region [86] (see **Figure 6**).

Progress in creating efficient lasers of visible range operating on neon at the electron beam pumping power 10–100 W/cm<sup>3</sup> stimulated work on nuclear-pumped lasers operating on 3p– 3s transitions of neon atom. Laser action operating on 3p–3s neon transitions at pumping by uranium fission fragments was obtained in 1985 at VNIIEF on VIR-2 reactor. These results were published in 1990 [3, 87]. In 1985, we have also conducted experiments on WWR-K stationary nuclear reactor for excitation of triple mixtures of argon or krypton, neon, and 3 He, which has led to a negative result [53, 79].

Obtained within the experiment efficiency values for NPL operating on neon (to 0.1%) are much lower than at pumping by electron beam [84, 85]. Threshold values of thermal neu‐ tron flux density are significantly higher than 1014 n/cm<sup>2</sup> s [87–93]. Report [90] on NPL lasing in <sup>3</sup> He-Ne-H2 mixture with a lasing threshold of 1014 n/cm<sup>2</sup> s is doubtful, as hydrogen at a pressure of 0.57 bars must be considerably quenching the upper laser level. Unlike lasers operating on IR transitions of Xe, Kr, Ar, the lasing mechanism in lasers on 3p–3s transitions of neon was considered reliably established [39, 87, 94]. Already the first works on neon lasers pumped by an electron beam or nuclear radiation contain roughly the same ideas about the lasing mechanism. The lower 3s-states are deactivated in the processes of:


The main process of 3p levels population is considered to be dissociative recombination of Ne2 + ions with electrons; most studies assumed that all of these levels are populated in recombina‐ tion of molecular ions in the main vibrational state. In [80, 84] to explain changes in the efficiency of various levels pumping with the neon pressure, was made a conclusion on population of 3p′[1/2]0-level in processes of recombination of vibrationally excited levels of Ne2 + . Population efficiency of level 3p′[1/2]<sup>0</sup> increases in triple mixtures with high helium content, which is explained in [80] by the formation of vibrationally excited levels of Ne2 <sup>+</sup> in the following processes:

$$\text{Fe}^+ + 2\text{He} \rightarrow \text{HeNe}^+ + \text{He} \tag{15}$$

$$\text{ } \text{HeNe}^\* + \text{Ne} \rightarrow \text{Ne}\_2^\* \text{ } \text{(v>0)} + \text{He} \tag{16}$$

Another mechanism for neon levels' population when excited by a hard ionizer is proposed in [79]. Based on the study of spectral-temporal characteristics of pure neon and He-Ne mixtures pumped by heavy charged particles, the conclusion was made that the population of neon levels occurs in direct excitation by nuclear particles and secondary delta electrons, and in He-Ne mixtures also in the process of excitation transfer from metastable helium atoms (Hem):

$$\text{Fe}^{\text{m}} + \text{Ne} + \text{He} \rightarrow \text{Ne} \left( \text{3p} \right) + 2\text{He} \tag{17}$$

However, this work did not receive recognition: In the review article [18], it was not men‐ tioned, and in monograph [3], it was questioned with reference to [95]. During the study of luminescence of He-Ne mixtures with quenching additives [30], we have obtained results confirming the main conclusion in [79]—the principal and clearly dominant channels of neon 3p levels population in pumping by a hard ionizer are the processes unrelated to dissociative recombination of molecular ions of neon.

#### **3.3. Metal vapor lasers**

transitions of neon, it is necessary to solve the problem of lower s-levels deactivation. These levels are metastable or resonantly coupled to the main level and have trapped radiation at neon pressures important for nuclear pumping radiation. Rapid depletion of lowerlaserlevels at a relatively low concentration of quenching particles can be achieved in the processes occurring with Coulomb cross-sections. In Ref. [20], authors show the possibility of deactiva‐ tion of excited states in the Penning process. The required selectivity of deactivation of the upper and lower levels can be achieved using the lower levels states as resonantly coupled to the main, for which the ionization cross-section of quenching by additives is particularly

first observed in the afterglow of discharge [82], and then at pumping by powerful electron beam of Ne-H2, Ne-Ar, Ne-Ar-He mixtures [83]. In [84] by reducing the concentration of neon and quenching the lower level of additives by an order compared to [83], and pumping power

same transition obtained laser efficiency of 1–2%. The laser efficiency with ionizing pump‐ ing at λ = 585 nm in the opinion of other authors cannot exceed 0.5% [85]. The electron beam pumping provided for laser action in a row of 3p–3s neon transitions in the spectrum red

Progress in creating efficient lasers of visible range operating on neon at the electron beam

3s transitions of neon atom. Laser action operating on 3p–3s neon transitions at pumping by uranium fission fragments was obtained in 1985 at VNIIEF on VIR-2 reactor. These results were published in 1990 [3, 87]. In 1985, we have also conducted experiments on WWR-K stationary nuclear reactor for excitation of triple mixtures of argon or krypton, neon, and 3

Obtained within the experiment efficiency values for NPL operating on neon (to 0.1%) are much lower than at pumping by electron beam [84, 85]. Threshold values of thermal neu‐

pressure of 0.57 bars must be considerably quenching the upper laser level. Unlike lasers operating on IR transitions of Xe, Kr, Ar, the lasing mechanism in lasers on 3p–3s transitions of neon was considered reliably established [39, 87, 94]. Already the first works on neon lasers pumped by an electron beam or nuclear radiation contain roughly the same ideas about the

, 3s′[1/2]<sup>0</sup>

The main process of 3p levels population is considered to be dissociative recombination of Ne2

ions with electrons; most studies assumed that all of these levels are populated in recombina‐ tion of molecular ions in the main vibrational state. In [80, 84] to explain changes in the efficiency of various levels pumping with the neon pressure, was made a conclusion on population of 3p′[1/2]0-level in processes of recombination of vibrationally excited levels of

0 [20],

0

, 3s′[1/2]<sup>1</sup>

0 [81].

with He:Ne:Ar = 96:3:1 mixtures at a pressure of 3 atm at the

stimulated work on nuclear-pumped lasers operating on 3p–

s [87–93]. Report [90] on NPL lasing

s is doubtful, as hydrogen at a

<sup>0</sup> (λ = 585.3 nm) was

He,

+

Quasi-continuous lasing in allowed neon atom transition 3p′[1/2]0–3s′[1/2]<sup>1</sup>

high [81].

176 178High Energy and Short Pulse Lasers

in <sup>3</sup>

by electron beam ~100 kW/cm<sup>3</sup>

pumping power 10–100 W/cm<sup>3</sup>

which has led to a negative result [53, 79].

**-** Penning for metastable states 3s[3/2]<sup>2</sup>

**-** release of an electron for resonance states 3s[3/2]<sup>1</sup>

tron flux density are significantly higher than 1014 n/cm<sup>2</sup>

He-Ne-H2 mixture with a lasing threshold of 1014 n/cm<sup>2</sup>

lasing mechanism. The lower 3s-states are deactivated in the processes of:

0

region [86] (see **Figure 6**).

#### *3.3.1. Mercury vapor lasers*

**HgII laser**. The first report on the lasing of ion NPL operating on mercury vapor appeared in 1970 [96]. This work presents results obtained at <sup>3</sup> He(350 Torr)+Hg(3 Torr) mixture pumping on IIN pulsed reactor with neutrons flux up to 5°1016 n/cm<sup>2</sup> s. According to authors, the registered lights power ~10 mW was associated with laser action of mercury ion transition 72 P3/2–72 S1/2 (λ = 615.0 nm). Further on, these results were questioned, and in <sup>4</sup> He(600 Torr) +Hg (2.5–10 mTorr) mixture pumping by 10B(n,α)<sup>7</sup> Li reaction products has proved lasing with a wavelength of 615 nm [97]. The conditions [97] had no lasing at partial pressure of mercu‐ ry vapor ~3 Torr.

Pumping mechanism at the transition of 72 P3/2–72 S1/2 mercury ion at ionizing pumping was considered in [53, 98, 99]. Discrepancy between the results of [96, 97] is possibly due to the difference in mechanisms of upper laser level population at low density and high density of mercury vapor [53, 98]. Calculations [99] indicate a low energy laser characteristics at λ = 615 nm (0.04% maximum efficiency) even with optimal parameters.

**HgI laser**. The emission spectra of mixtures of <sup>3</sup> He + Hg and <sup>3</sup> He + Hg + Kr in excitation by products of nuclear reaction <sup>3</sup> He(n,p)T studied in [100]. It was concluded that the excitation of low-lying HgI levels by electron impact is predominant in high-pressure plasma:

$$\text{Hg}(\text{6}^{\text{l}}\text{S}\_{0}) + \text{e} \rightarrow \text{Hg}(\text{6}^{\text{3}}\text{P}\_{\text{l}}) + \text{e} \tag{18}$$

$$\text{Hg}(6^3 \text{P}\_{0,1,2}) + \text{e} \rightarrow \text{Hg}(\text{nx}) + \text{e} \tag{19}$$

Continuous laser λ = 546.1 nm with optical pumping [101] uses similar scheme of excitation (electron replaced with photon): a beam with a wavelength of 253.7 nm, corresponding the mercury resonance line, excites 6<sup>3</sup> P1 mercury level and beam λ = 435.8 nm transmits excita‐ tion to 7<sup>3</sup> S1-level. However, an attempt to ensure deactivation of 6<sup>3</sup> P2 state by N2 molecules according to the scheme used in optically pumped laser at ionizing radiation pumping was not successful [102].

We have proposed another scheme of creating inverse population in laser on mercury triplet lines [54]. Our works [36, 98] have shown that population of 7<sup>3</sup> S1-level of mercury atom occurs in the process of dissociative recombination of molecular ions and not in direct or stepwise excitation by electrons. It is proposed to use H2 to destruct the lower level at the transition of 73 S1–6<sup>3</sup> P2; H2, D2—on the transition of 7<sup>3</sup> S1–6<sup>3</sup> P1. As the pumping is carried out through ion channel, xenon should be used as buffer gas and charge exchange from xenon to hydrogen is slow. The use of krypton is less justified due to low value of rate constant of Kr2 <sup>+</sup> ion re‐ charge on mercury atoms. High selectivity of dissociative recombination of Hg2 <sup>+</sup> is largely driven by relatively low temperatures involved in recombination of electrons. Therefore, by increasing the pumping powerto the levelrequiredforlaser operation, itisusefulto use helium to cool the secondary electrons. Thus, the optimal gas mixture of laser operating on mercury triplet must be four-component He-Xe-Hg-H2 [54].

Quasi-continuous lasing at 7<sup>3</sup> S1–6<sup>3</sup> P2 transition of mercury atoms using this scheme was obtained on pulsed nuclear reactor EBR-L in VNIITF [103]. Kinetic model of He-Xe-Hg-H2 nuclear-pumped laser based on VNIITF experiments was developed in [104]. Externally similar scheme is implemented in excitation of mercury mixtures with inert gases by elec‐ tron beam [105]. This work uses a mixture of He+Ne+Ar as a buffer gas at a total pressure of 2300 Torr. With reference to the paper later than [36, 98], recombination of Hg2 <sup>+</sup> as the main population channel of 7<sup>3</sup> S1 has been specified. An attempt to use H2 for 6<sup>3</sup> P2 level population was unsuccessful, and at hydrogen pressure of 20 Torr laser action failed. What was also interesting in this study was the absence of molecular additives quenching the lower level. Apparently, the de-excitation of lower laser level took place in formation of excimer mole‐ cules (HgR)\*:

$$\rm{Hg(6}^{3}\rm{P}\_{2}) + \rm{R} + \rm{M} \rightarrow \rm{(HgR)} \rm{\*} + \rm{M} \tag{20}$$

where R—Ne or Ar.

**HgI laser**. The emission spectra of mixtures of <sup>3</sup>

products of nuclear reaction <sup>3</sup>

178 180High Energy and Short Pulse Lasers

mercury resonance line, excites 6<sup>3</sup>

tion to 7<sup>3</sup>

73 S1–6<sup>3</sup>

not successful [102].

He + Hg and <sup>3</sup>

low-lying HgI levels by electron impact is predominant in high-pressure plasma:

S1-level. However, an attempt to ensure deactivation of 6<sup>3</sup>

lines [54]. Our works [36, 98] have shown that population of 7<sup>3</sup>

P2; H2, D2—on the transition of 7<sup>3</sup>

triplet must be four-component He-Xe-Hg-H2 [54].

S1–6<sup>3</sup>

Quasi-continuous lasing at 7<sup>3</sup>

population channel of 7<sup>3</sup>

cules (HgR)\*:

Continuous laser λ = 546.1 nm with optical pumping [101] uses similar scheme of excitation (electron replaced with photon): a beam with a wavelength of 253.7 nm, corresponding the

according to the scheme used in optically pumped laser at ionizing radiation pumping was

We have proposed another scheme of creating inverse population in laser on mercury triplet

in the process of dissociative recombination of molecular ions and not in direct or stepwise excitation by electrons. It is proposed to use H2 to destruct the lower level at the transition of

channel, xenon should be used as buffer gas and charge exchange from xenon to hydrogen is

driven by relatively low temperatures involved in recombination of electrons. Therefore, by increasing the pumping powerto the levelrequiredforlaser operation, itisusefulto use helium to cool the secondary electrons. Thus, the optimal gas mixture of laser operating on mercury

obtained on pulsed nuclear reactor EBR-L in VNIITF [103]. Kinetic model of He-Xe-Hg-H2 nuclear-pumped laser based on VNIITF experiments was developed in [104]. Externally similar scheme is implemented in excitation of mercury mixtures with inert gases by elec‐ tron beam [105]. This work uses a mixture of He+Ne+Ar as a buffer gas at a total pressure of

S1 has been specified. An attempt to use H2 for 6<sup>3</sup>

was unsuccessful, and at hydrogen pressure of 20 Torr laser action failed. What was also interesting in this study was the absence of molecular additives quenching the lower level. Apparently, the de-excitation of lower laser level took place in formation of excimer mole‐

S1–6<sup>3</sup>

slow. The use of krypton is less justified due to low value of rate constant of Kr2

charge on mercury atoms. High selectivity of dissociative recombination of Hg2

2300 Torr. With reference to the paper later than [36, 98], recombination of Hg2

He(n,p)T studied in [100]. It was concluded that the excitation of

1 3 Hg 6 S0 1 ( )+® + e Hg 6 P( ) e (18)

( ) <sup>3</sup> Hg 6 P0,1,2 ( ) +® + e Hg nx e (19)

P1 mercury level and beam λ = 435.8 nm transmits excita‐

He + Hg + Kr in excitation by

P2 state by N2 molecules

<sup>+</sup> ion re‐

<sup>+</sup> is largely

<sup>+</sup> as the main

P2 level population

S1-level of mercury atom occurs

P1. As the pumping is carried out through ion

P2 transition of mercury atoms using this scheme was

( ) <sup>3</sup> Hg 6 P2 ( ) ++ ® + R M HgR \* M (20)

#### *3.3.2. Cadmium and zinc vapor lasers*

The first nuclear pumping of visible range laser operating on cadmium vapor was carried out by MEPhI researches on BARS pulse reactor [106]. In <sup>3</sup> He-116Cd mixture was obtained laser action on 4f 2 F0 5/2,7/2–5d2 D3/2,5/2 transitions of ion Cd+ (λ = 533.7 and 537.8 nm), and later on transition from λ = 441.6 nm [107]. The first successful experiments on laser pumping on metal vapor (mixture of He-116Cd, λ = 441.6; 533.7 and 537.8 nm) by uranium fission fragments were carried out in 1982 by researches of VNIIEF and VNIITF on EBR-L reactor [3]. Maximum parameters of Cd+ -laser with nuclear pumping is also obtained in this reactor: 1000 W at an efficiency of 0.4% on the blue line and 470 W on the green lines [44].

At present, the basic processes of population of upper laser levels of Cd+ are considered to be established:

4f2 F0 5/2,7/2 upperlevels oflasertransitions from λ = 533.7 to 537.8 nm are populated due to charge exchange processes

$$\text{+ He}^+ + \text{Cd} \rightarrow (\text{Cd}^+)^\* + \text{He} \tag{21}$$

forming higher-lying levels of 6f, 6g, 8d, 9s [108] and subsequent cascade transitions in 4f state.

5s22D3/2,5/2 upper levels for transitions with λ = 441.6 and 325 nm are populated in Penning and charge exchange processes

$$\text{'} \text{He}\_2\text{}^+ + \text{Cd} \rightarrow (\text{Cd}^+) \text{\*} + \text{He} \tag{22}$$

$$\text{+ He\*}\\
\text{(2S)} + \text{Cd} \rightarrow (\text{Cd}^\*)^\* + \text{He} \tag{23}$$

Laser action at 325 nm was achieved only when pumped by an electron beam, although the threshold pumping power was only 10 W/cm3 [109].

Deactivation of lower levels can occur as a result of radiative transitions, quenching by electrons, conversion processes of atomic ions into molecular ions

$$(\text{Cd}^\*)^\* + \text{Cd} + \text{He} \rightarrow \text{Cd}\_2{}^\* + \text{He} \tag{24}$$

and for the levels lying above 62 S1/2, in Penning process on its own atom

$$(\text{Cd}^\*)^\* + \text{Cd} \rightarrow \text{Cd}^\* + \text{Cd}^\* + \text{e} \tag{25}$$

Lasing mechanisms of NPLs on ion transitions of cadmium and zinc are very similar. Laser action by pumping <sup>3</sup> He-Zn mixture observed on transition of 4s22D5/2–4p2 P3/2 (λ = 747.9 nm) [110], 60 W output power obtained during pumping by uranium fission fragments at this transition [44].

Information about cadmium-vapor atomic lasers with ionizing pumping is scarce. During experiments was registered laser action (1–2 W) with a threshold density of 1016 n/cm2 s neutron flux on the lines of 1.648 and 1.433 μm at pumping He-Cd mixture by uranium fission fragments [44]. When pumping He-Cd mixture by an electron beam was obtained laser action on the line 361 nm of cadmium atom [111]. Kinetic model [112] included processes involving excited cadmium atoms and attempted to calculate some laser characteristics for 1.648 μm line.

Luminescence of <sup>3</sup> He-Cd and <sup>3</sup> He-Xe-Cd mixtures in the radiation region of stationary nuclear reactor investigated in [113]. Measured value of rate constant of Xe2 <sup>+</sup> charge exchange on cadmium atoms is small (~10−13 cm3 s−1), in contrast to the constant of charge exchange on mercury atoms. A sufficiently high density of cadmium vapor (~3 × 1018 cm−3) was establish‐ ed at a temperature of about 700°C, and such density of cadmium requires consideration of quenching 63 S1 state by its own atoms. Perhaps krypton ions charge exchange on Cd will be faster. In addition, cadmium atoms in krypton can be ionized in the Penning processes.
